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post #1 of (permalink) Old 03-04-2005, 01:51 PM
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Megasquirt part list any other help.

I have decided to take the plunge and fi my jeep.i currently have a 1986 4.2 in my 1980 cj-7. iwll soon be building a 4.6l for my 86 cj-7 possibly megasquirt it if all goes well with the "practice run".

To hopefully clear up any confusion from my last statment i am asking about using a megasquirt controller and a gm tbi on the 258.

I have decided to go with a gm tbi. what i need to know from the guys who have done it is what parts you used? sensor? what parts from megasquirt? fuel pump? actual part numbers or what vehicle the parts originally installed on would be appreciated. Any info from the experienced crowd would really be great.
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post #2 of (permalink) Old 03-04-2005, 01:56 PM
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Re: Megasquirt part list any other help.

Give me your email and I'll send you a write up I did on it. I used parts from a 305 chevy TBI, but a 350 or 4.3L would do. Bought a Carter Universal fuel pump. Kinda pricey and noisy, but its been dependable. Two year and no problems.
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post #3 of (permalink) Old 03-05-2005, 11:28 AM
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Re: Megasquirt part list any other help.

Megasquirt Fuel Injection on a 258 AMC
(Version Jan 26/03)

This will describe how I built a Throttle Body Injection (TBI) system for an AMC 258 using a Megasquirt (MS) Electronic Control Unit (ECU).

Why would you want to build a Megasquirt Fuel Injection system for your jeep when you can buy a system sold commercially? I can think of a number of reasons:

1. It will satisfy that urge to become more knowledgeable in how electronic fuel injection works.
2. You will have insight into how to repair your system if necessary (a reason many avoid EFI).
3. If you enjoy building things, you can count your cost of time spent as entertainment, and build this system for a fraction of what a commercially available system would cost.
4. This system is fully tunable. This is especially important if your engine is modified.

The MS system and its assembly is well described on the MS Home Site on the Web ( and the Yahoo MS group ( Be sure to read the FAQ at ( My objectives were to build a simple, reliable and low cost fuel injection system that could be retrofitted onto a jeep inline six and provide the low end torque and fuel economy that this motor is noted for. I hoped for improved performance and found the completed system not only improved the power generated by the motor - especially noticeable at highway speeds - but improved the throttle response under all conditions. I’ve achieved 21.4 mpg fuel economy on highway travel. I have found no downside this system compared to the previous carburetor setup. Cold starting and running is excellent.

The cost of building the system was approximately $500.00. That low cost included:

1) TBI, sensors and harness at a local wrecker $20.00
2) Bosch Auxiliary Air Device from wrecker $5.00
3) Electric fuel pump $150.00
4) Wire, solder, terminal blocks, fuses $50.00
5) Fuel line, hoses and clamps, $50.00
6) Fuel Filter (GF1580K) $10.00
7) Megasquirt ECU $110.00
8) Power / Relay Board kit $55.00
9) Stimulator kit $35.00
10) Aluminum for adapter plate $20.00

Although not essential, I would recommend getting both the MS Power / Relay Board kit and the MS stimulator. The Power / Relay kit provides a unit that will sit in the engine compartment and house all fuses and relays. It has a terminal strip for attaching the wiring for all sensors, injectors, power and ground, and has a DB 37 connector that will attach with a cable to the Megasquirt ECU, which should be mounted inside the passenger compartment to protect it from heat. The stimulator is a test unit having a connector on it that will plug directly into the MS ECU. It provides all necessary inputs and power to the ECU for testing the ECU at various stages of completion and for getting experience with using the software on your PC that will be used to configure and monitor the ECU.

Theory of Operation

The TBI unit acts like an electronic version of a carburetor. Rather than relying on the flow of air to create vacuum in the venturi of a carburetor, the TBI has a smooth bore and fuel is injected under pressure (approximately 13-14 psi) from the two injectors above the throttle plates. The ECU and sensors measure various data and calculate air flow and required fuel. As more fuel is required, the injectors are held open longer during each cycle by a precisely timed electrical pulse. At idle the injectors receive a pulsewidth of approximately 2-3 milliseconds. At WOT the injectors will remain open for up to 17 milliseconds or more to deliver the necessary fuel. As RPM’s increase, so does the frequency of these pulses. MS is a speed density (SD) fuel injection system, which means that the ECU calculates air and fuel flow based on the input of primarily the rpm signal from the distributor and the manifold air pressure (MAP) sensor. This is as opposed to a Mass Air Flow design which attempts to measure actual air flow by means of a sensor through which the air flows. Both methods work equally well and are used by OEM’s. MAF systems are popular because they are more forgiving of performance improvements without having to reprogram the ECU. MS overcomes this by making the ECU fully user programmable. Various sensors such as temperature, oxygen sensor and TPS provide enrichments corresponding to the conditions that may change during operation. MAP is measured in Kpa and measures air pressure, which varies from approximately 100 KPa at atmospheric pressure and down to zero at absolute vacuum. A normally aspirated engine will experience MAP pressures varying from around 28 KPa at idle (and even more during deccelleration) to close to 100 Kpa at wide open throttle (WOT). The MAP pressure is measured from a source beneath the throttle plates and somewhat corresponds to load experienced by the engine. A higher MAP reading at a given RPM means the engine cylinders are more fully filling with air. SD systems require the ECU to be programmed with values of volumetric efficiency (VE) at various MAP and rpm values for an individual engine. If the engine is modified such that it’s VE changes, so must the data be changed in the ECU’s program. On the MS system, we have an 8x8 grid to fill in with VE values at 8 different MAP and RPM values. A VE of 100 means the cylinder has filled with air to 100% of its displacement. VE increases as the throttle opens further, and this corresponds to increasing MAP values. VE also peaks at the rpm at which torque peaks. For a stock 258 motor this is around 1800 rpms.

My VE table looks something like this:

20 38 38 38 38 38 38 33 33
30 38 38 47 50 49 48 47 45
40 48 48 52 52 52 48 47 45
50 52 53 54 54 54 48 48 48
60 63 63 63 63 63 63 57 57
75 79 79 79 79 78 78 73 73
90 88 90 90 90 90 84 78 78
100 94 97 98 100 100 90 84 84
500 1000 1500 2000 2500 3000 3500 4000

Values along the left are MAP values in Kpa, values along the bottom are in RPM.

These numbers are based on using two 50 lb/hr injectors and a Reqfuel of 11.3/16.9. Injector open time of 1.0 ms, battery voltage correction of 0.2, PWM mode inactive by setting PWM threshold to 25.4, 2 injections per cycle, injector staging simultaneous.

Reqfuel is the time in milliseconds the injectors needs to open to deliver the correct quantity of fuel for 100% VE for all four strokes (720 degrees of crank rotation). The second value indicates the actual open time taking into account the staging options. Don’t worry, you don’t have to calculate this by hand, the Megatune software will calcuate this from the number of cylinders, displacement and size of your injectors. As well, your VE table need only be close enough to get the motor running and from there you fine tune. This table works well on my engine but can be tuned further. Once the engine reaches operating temperature and is above 1300 rpms, the O2 sensor will provide correction data to the ECU. This 02 feedback is known as Closed Loop operation. I found that having both injectors squirt simultaneously (injector staging simultaneous) produced the smoothest idle.

In a fuel injection system, fuel typically travels a continuous path from fuel tank to fuel pump to the injectors with excess fuel returning through the regulator then back to the fuel tank. The fuel pump provides fuel flow and pressure beyond the requirements of the motor at WOT. The fuel pressure regulator internal to the TBI unit creates back pressure on the injector side of the fuel circuit to regulate it to around 14 psi. Excess fuel returns to the fuel tank via the 5/16 return fitting on the TBI. If you use the stock return line on your jeep, as I did, it is ¼” diameter. A fresh piece of 5/16” EFI hose will fit tightly over this 1/4 “ line.

Fuel injectors are typically rated in lbs/hr delivery of fuel at a rated pressure. A rough rule of thumb is that you can make 2 hp per lbs/hr, so that two 50 lb/hr injectors can supply fuel to make 200 horsepower when operating at 100% duty cycle. Injectors should be sized such that duty cycle is less than 100% (80% would be a good margin). GM TBI’s from 4.3, 305 and 350 engines are similar except for the size of the injectors. Thus a TBI from a 4.3, 305 or 350 will work fine on the 258. Unless you expect your 4.2 to make more horsepower than a GM 4.3, the 4.3’s TBI and injectors will suffice. The only constraint is that if you have too large of injectors, fine control of fuel flow at idle may not be possible as the pulse width at idle may be too short to control. The 305 TBI I am using works well. The GM TBI injectors vary in capacity, apparently even within a given TBI such as the 4.3. Those in my 305 TBI were both Green / White marked. If you remove the wiring connector on top of the injector, the color code should be visible on the exposed tip just beneath the conductor. If you are wrong in determining the exact size of the injectors the results are not catastrophic. It just means your Reqfuel calculation will be wrong and your VE table will have either lower or higher numbers in it when correctly tuned.

Here is the information I have been able to gather about GM TBI injector sizing. I have seen conflicting opinions, so don’t take this as gospel:

4.3L TBI typically uses (2) 45 #/hr
5.0L TBI uses (2) 50 #/hr injectors
5.7L TBI uses (2) 61 #/hr injectors.

My General Parts List:

1. 1984 AMC 258 Inline 6 with stock aluminum intake manifold and cast iron exhaust having an 02 sensor port;
2. GM HEI distributor (not required);
3. Carter (Federal-Mogul) Inline Electric Fuel P5001 (9-22 psi, 50 gph @ 20 psi):
4. GM TBI (from 1989 305 V8);
5. GM TBI wiring harness from junkyard;
6. Megasquirt ECU kit;
7. Megasquirt Power / Relay kit;
8. Megasquirt Stimulator kit;
9. 3/8 steel fuel line to run from the fuel pump to the engine bay;
10. 3/8 EFI hose;
11. 3/8 EFI fuel line with barbed end (sold as line repair piece / 16mm Saginaw fitting connected to 18" of 3/8"OD steel tube Dorman part number 800-153);
12. 3/8 cable hangers
13. 5/16 EFI hose;
14. 5/16 EFI fuel line with bared end (sold as fuel line repair piece / 14mm Saginaw fitting connected to 18" of 5/16"OD steel tube Dorman part number 800-151);
15. EFI fuel hose clamps;
16. Fuel Filter (GF1580K)
17. 18 gauge wire;
18. 16 gauge wire;
19. coaxial cable
20. GM throttle position sensor (TPS);
21. GM coolant temperature sensor (CTS) GM #12146312
Wells SU107
AC DELCO 213-310
22. GM intake air temperature sensor (IAT) GM #25036979
Wells SU109
GP SORENSEN 779-19001
23. 1.0 ohm 25 watt ceramic power resisters (qty. 2);
24. 4.7 ohm 25 watt ceramic power resisters (qty. 2);
25. automotive terminal block (qty 2);
26. Bosch Auxiliary Air Device AAD (source Volvo, VW mid 80’s);
27. 5/8 emissions hose;
28. 5/8 pcv elbow (Help /Motormite 247044 /24-5115-2)
29. 5/8 pcv grommet (Help / Motormite 242303 / 24-5144-2)
30. aluminum plate to fabricate adapter from GM TBI to intake manifold
31. throttle linkage ball stud to fit jeep throttle cable
32. Optional: 3 wire (heated) oxygen sensor from 95 Astro Van with a 4.3L. Sensor# 025312185, harness # 012117025. Use these part numbers. Purple wire is the sensor, other two wires are 12v in run position.

Tools necessary to complete the project

1. Typical hand tools for mechanics work (wrenches, sockets, etc.);
2. Fine tipped soldering iron (I used a Weller WP25) and sidecutters;
3. 1 ½” diameter metal cutting hole saw to make adapter plate;
4. 5/16 – 18 tap and drill for TBI mounting bolts
5. die grinder and aluminum shaping burr
6. double flaring tool for 3/8 steel fuel line

Tasks to be performed:

1) The MS system is a completely unassembled printed circuit board (PCB), many discrete components and several Integrated Circuits. You will have to identify all the small components, install them in the correct locations and polarities and solder them in place. As stated in the MS assembly guide, it is best to start with the P/R board as it give you practice on larger components. See the assembly guide for all the details. You will also need to perform minor machining of the aluminum case ends for installation of LED’s, and cable connectors.
2) Fuel Lines and Hoses: Install a new 3/8 steel fuel supply line along passenger side frame rail and mount inline electric fuel pump. Line double flared at back end with 3/8 flare fitting for fuel pump and a shallow flare at engine bay end to retain fuel hose. Secured to inner frame rail with 3/8 cable hangers. If you mount the inline pump on the frame rail just ahead of the rear wheel well, you will need to make up a short piece of steel line to run back to the tank. Be sure to bend sufficient clearances for the rear shock body. The GM TBI unit has metric O-ring fittings on it. Be sure to get the steel adapters that screw into the TBI castings when you get your TBI unit. These fittings are not tapered pipe thread. You can get 3/8 and 5/16 fuel repair lines from your auto parts store in various lengths. These fittings will have a the correct metric O-ring fitting on one end and a hose retaining bead crimped on the other. This is the least expensive way I found to attach fuel hose to the TBI. I used lines that were about 12” long and bent them with a tubing bender to get them to the desired locations for attaching the hose. Be sure to use high pressure fuel injection hose. A fuel injection hose clamp is available that doesn’t cut into the hose and clamps more evenly than a worm gear type hose clamp. Don’t scrimp on safety or cut corners with the fuel fittings anywhere. The electric fuel pump can deliver large volumes of fuel under significant pressure and you will want a system that would hold together in a crash or some sort of failure of the pressure regulation. Ordinary fuel line is not capable of withstanding the pressure the pump can generate. Preferring to have less connections under pressure, I mounted the fuel filter on the inlet side of the fuel pump. The outlet side of the fuel pump I used accepts a 3/8 flared fitting on the steel line. I purchased a GM fuel filter that was made for EFI and had flared 3/8 ends and a mounting tab. A short length of EFI hose connects the filter to the pump inlet.
3) Fabricate adapter plate to mount GM TBI on intake manifold. Here you can get creative. A gasket for the TBI and one for the old carburetor help in laying out the pattern on the aluminum. Minimum thickness for the adapter plate ¾” aluminum. Maximum thickness based on hood clearance for air cleaner. You will need to drill and tap holes for the 3 TBI mounting bolts and drill though and countersink 4 holes for mounting the adapter to the manifold. I used allen head bolts to mount the adapter to the manifold. Note: GM TBI is much shorter in height than carburetor being replaced, so this does give you some additional under hood clearance. I used 2 1/4 “ thick aluminum for the adapter. This allowed me to very gently adapt the larger diameter of the TBI unit (1 11/16”) to the smaller diameter of the manifold (1 ½”). Not only are the holes in the manifold smaller, but they are much closer together. To maintain a consistent cross sectional area throughout the plenum, I removed a significant amount of metal from the outer edge of the plenum bores to within 1/8” of the edge of the manifold. I continued this down as far as I could to the bottom of the plenum bores then gradually blending into the plenum. I figure there is no point in having a larger diameter TBI than the carburetor throttle plates and not giving it full access to the plenum. Don’t forget there is a water passage in the intake manifold that you don’t want to cut into! Be sure to remove the electric heater from the bottom of the intake manifold so that you can clean out any metal chips and get a good view from both sides. You will get a new gasket and O-ring for it with the Fel-Pro gasket set. I cut the holes in the adapter plate with a 1 ½” hole saw (a thick plate will have to be cut from both sides), and centered the saw halfway between the centers of the manifold and the TBI openings. I then enlarged the hole in the top of the adapter to match the tbi unit and removed metal from the inner surface of the adapter bores to line up with the closer spacing of the manifold openings. Once finished, I used epoxy between the adapter plate and the intake manifold as a sealant. As there will be exposed bolt holes in the adapter that hold it down to the jeep intake, the stock TBI gasket will not work on top of the adapter. I used 1/16” aluminum to make a thin plate, identical to but slightly larger than the TBI gasket. I sealed the plate down with Permatex sealer over the adapter plate and put the stock TBI gasket on top. Ideally, the cross sectional area should change as little as possible throughout the adapter, despite the bores angling towards each other at the bottom end. I have found that throttle response is crisp under all conditions and top end power is increased, so my approach seems to have worked.
4) Fabricate throttle cable retainer to mount to intake manifold. This may not be necessary if a bracket is obtained from a Chevy S-10. If your TBI was so equipped, the S-10 bracket bolts directly to the side of the TBI unit and will accept some jeep throttle cables. Note that I mounted the TBI unit with the fuel lines facing forward and the throttle arm located on the passenger side. I used a piece of an existing jeep throttle cable bracket to get the square opening that the outer cable sleeve snaps into. I made a bracket to mount to the top of the intake manifold, rearward of the TBI. There are two bolt holes on top of the intake manifold located there. The bracket is made from 2” x ¼” bar stock and is L shaped. I welded the piece of cable bracket I had cut off to that bracket mount the cable. Make sure the inner cable has some slack or else you will not be able to close the throttle fully at idle.
5) TBI unit: The TBI unit needs to have a ball stud for the jeep throttle cable mounted to it. The old cylindrical gm stud can be removed by grinding the backside away, then driving it out with a pin punch. The ball stud obtained from the parts store was too large in diameter and had to be filed down to the correct diameter by mounting it in a drill press and using it like a mini-lathe to cut it down to the right size with a file, then emery cloth was used to polish the surface. A thread locker compound was used on the threads to retain the nut. A washer is needed behind the nut as the old hole is much larger than needed. The TBI unit has a throttle stop screw that has it’s head concealed beneath a sheet metal plug. This plug can be drilled out to give access to this adjustment. A torx driver is used to adjust the throttle stop. Ported vacuum for your distributor is located just beneath the pcv fitting, and two manifold vacuum sources are located on each side as well.
6) Fast Idle Control. Currently, MS does not support control of the GM IAC unit. Some experimenters have adapted the code and hardware to control a GM IAC. The GM IAC uses a bi-polar stepper motor to control idle speed. This requires a more complex electronic system to drive it and more software to control it. Instead, MS provides for operation of an idle speed solenoid that will be de-activated once a set temperature is met. This allows you to use either a throttle stop solenoid or an air valve solenoid to allow extra air into the manifold to increase idle speed. The MAP sensor detects the extra air being brought into the system and it delivers an increased volume of fuel at idle to maintain the correct mixture. On a GM TBI, the throttle plates are never fully closed. A small amount of opening allows the fuel striking the throttle plates at idle to be drawn into the manifold. I elected to use a method that allows for a progressive fast idle. Bosch makes a small device called an Auxiliary Air Device (AAD) that acts as a valve. It has two hose fitting and an electrical connector. It is commonly found on fuel injected VW’s and Volvo’s. The AAD is a valve that is open fully when cold and progressively closes it as it warms up, just like a choke, only opposite in direction. It not only responds to ambient temperatures, but has a 12 volt heater coil that warms it up once the engine has started. The AAD requires 12 volts and should be wired up to the fuel pump relay. That way even with the key on, the AAD will not begin to open unless the motor is running. In keeping with my desire to keep the costs of this project down, I went to the local pick a part yard. Every old (80’s) fuel injected Volvo I looked at had one on the side of the aluminum valve cover, held down by two allen head screws. It is attached to the air cleaner assembly and to the intake manifold by 5/8 hoses. In order to use the same port as the GM IAC used to introduce added air to the system at idle, I gutted my GM IAC by drilling out the 3 roll pins. I pushed a PCV grommet inside the IAC housing and a 5/8 pcv elbow into the grommet (all available in the “Help” line of products). I then attached the pcv fitting to the AAD with 5/8 pcv hose and used a pcv air filter for the inlet on the AAD. I mounted the AAD to one of the bolts on the intake manifold that holds down my throttle cable bracket. I find this system is working well, and provides about 300 additional rpms above warm idle speed, during a cold start. The AAD is adjustable. There is a small nut and lockwasher located on one side of the AAD. The nut is on a small stud in a short slot (which you can’t see), and moving the nut and shaft upward or downward will open or close the valve more. If you decide to use the GM IAC port as I did, you will need to block off the passage in the TBI that leads to the air cleaner from the IAC port. I simply used epoxy to close the 3/8 opening that the tip of the IAC used to plug. I tried using the pcv port and other manifold sources on the TBI for introducing extra air for a fast idle, but they were too restrictive to flow the added volume of air necessary. A connection to the manifold itself, as used on MPI systems could cause problems with mixture distribution as the manifold on a TBI system flows both air and fuel, unlike a MPI system that only flows air through the manifold.
7) Construct DB 37 cable to connect MS ECU to MS Power / Relay unit. The Megasquirt home page has complete instructions you can print out on how to do this. You will need to determine the length of cable needed. I mounted the Power / Relay unit on the passenger fender well, just above the starter relay. I mounted the MS ECU inside the passenger compartment glovebox. Wherever you mount the ECU, you will need access to the ECU to connect the serial cable to your laptop computer for tuning. The jeep ECU from the BBD stepper carb has now been completely removed from my jeep. The grommet on the firewall was removed from the old wiring harness by cutting through one side with a hacksaw and the grommet re-used for the DB 37 cable. When constructing the DB 37 cable, I found that 18 awg wire was the largest wire that could be soldered to the pins on the DB 37 cable plug. 16 awg can be used for the common grounds, as it can be “forked” at the ends and soldered to two adjacent pins on the connector. I use a small diameter coax cable for the tachometer lead in order to reduce the chance of interference.
8) Clean up old wiring harness. You can remove much of the old wiring harness from the passenger side of the jeep if you previously had a Carter BBD with stepper motor. The Jeep ECU will no longer be needed and serves no role. If you are running the original Motorcraft Duraspark ignition system, you will need to perform the “Nutter By-Pass” to remove the Jeep ECU from the loop between the distributor, Duraspark Module and ignition coil. I am running a GM HEI distributor from a GM Inline Six motor and it only requires a key on power source. Remove the old Jeep ECU and it’s harness from behind the glove box. You will need to retain the wiring to the oil pressure sender unit, alternator, starter, and distributor. Remove the corrugated cable protector from your harness to trace the wires back to the connector on the drivers side firewall. Note there are two large red wires. One is red / white and is key on power, and the other red wire is connected to the battery at all times. This key on power wire is handy for connecting your GM HEI distributor and key on power to the MS Power / Relay board.
9) Vacuum line: A small diameter vacuum line is needed from the manifold to the MS ECU. I used 1/8 vacuum line and attached one end to the TBI unit fitting beneath the pcv port on the TBI and the other to a hose adapter right at the MS ECU. This allowed me to mount a short length of 1/8 line to the MAP sensor on the ECU and remove the ECU from the vehicle easily when needed. Black hose is often conductive and needs to be shielded from the internal parts on the ECU. I used heat shrink tubing over the line inside the ECU. This both insulated the hose and helped retain it on the internal MAP sensor. The small diameter of the line doesn’t seem to have an appreciable effect on the accuracy or timing of the MAP readings as my throttle response if quite crisp.
10) Air Cleaner Assembly. The GM TBI uses a 5 ¼” base air cleaner. You will experience interference between a large air cleaner and the brake assembly. That is why jeep used an offset air cleaner. A 10” diameter air cleaner may fit. I am using an offset snorkel that I fabricated out of fiberglass that connects to a reusable K & N filter. Don’t forget that the engine will rock towards the brake assembly during engine braking.
11) Wiring: You will need a good ground, battery connection and key on power source to the Power / Relay board. This is a good time to put new battery cables on and make secure connections to these sources at the battery and to the heavy red / white stripe wire on the main harness (key on power). The engine block, frame and body all need to be well grounded in order to avoid problems. A brass bolt or terminal on the firewall can be wired to the ground to provide a common mounting point for the ground connections you will need to make. I obtained all sensors and weatherpack connectors from the donor vehicle. To my surprise, none of the sensors were faulty and needed replacement. When experimenting with the completed ECU and stimulator you will get to see the values generated by the sensors to spot any bad sensors. You may need to lengthen some of the wires from the old harness. Soldered connections and heat shrink tubing will make for secure, lasting connections. The wiring you will need to fabricate will consist of the following:
a) Fuel pump to P/R board. I preferred to run a ground wire all the way back to the battery terminal. 16 gauge wire is the heaviest that will fit into the terminal block on the Power / Relay board. I used 14 gauge wire for the fuel pump and trimmed back some strands at the end of the positive cable where it went into the terminal block.
b) Oxygen sensor to P/R board. Only a light gauge wire is necessary. Jeeps use a single wire 02 sensor that is heated by exhaust gasses. This relies on good grounding of the block to get accurate signals. You can use a 3 wire 02 sensor that is electrically heated, or a 4 wire 02 sensor that is heated and has a shield. If you use the 3 or 4 wire version, you will need to run wiring to key on power source.
c) Injectors to P/R board. The harness I obtained from the wrecker was long enough for making this connection with a wiring harness running in front of the valve cover. Each injector has two wires. Polarity does not seem to be important. One wire on each injector gets +12 volts from the P/R board terminal and the other wire goes to an Injector bank terminal on the P/R board. Use the two banks for the two injectors, that will give you the ability to alternate squirts. GM TBI’s use low impedence injectors (approximately 1.4 ohms each). This means that they cannot run off 12 volt power without some current limiting method. High impedence injectors (>10 ohms) don’t require any special treatment. You can either use resisters to lower the current draw of low impedence injectors, or Pulse Width Modulation (PWM) in the MS software setup. I chose to use resisters to limit current to the injectors, rather than PWM current limiting. PWM current limiting requires a little more initial tinkering to get the right values. It is deactivated by selecting PWM current limit threshold of 25.4 milliseconds. For the resisters, I mounted two terminal blocks to a sheet of 1/16” alumimun, parallel to each other, about 5” apart. Between the terminals I ran the 25 watt power resisters. You will need to wire the 1 ohm and 4.7 ohm resisters in series, in line with the 12 volt power wire to each injector. Each injector thus has it’s own power line that has 5.7ohms of resistance between it and the driver transistor. This added resistance prevents the injectors from overheating from too much current flow. You could substitute two or three equal values resisters with 25 watt ratings to get a more even heat dissipation. I have found the resisters don’t even warm up during operation of the system, so this likely isn’t necessary.
d) CTS to P/R board. I installed the GM CTS on the drivers side of the engine block in place of one of the coolant temperature switches previously mounted there. The thread on the GM sensor (3/8 NPT) is the same as the old coolant temperature switch. Once again I re-used the weatherpack connector from the old harness, but had to add length to it.
e) IAT to P/R board. The GM “open element” IAT sensor is originally located inside the air cleaner housing, but outside the filter element. Reportedly this sensor can be harmed by moisture and is prone to disintegration. Do not mount the sensor such that if it falls apart it will be drawn into the intake manifold. I mounted mine just outside the K & N open element filter. The sensor needs to measure the temperature of the air being drawn into the intake manifold to adjust for changes in air density with temperature.
f) TPS to P/R board. There are three wires on the TPS. One receives +5 volts and is called Vref on the P/R board. One wire is grounded at the P/R board. The TSP sensor lead is attached to the wiper on the internal potentiometer, and experiences increasing voltage with throttle opening as it approaches the +5 volts end of the potentiometer’s internal resistance. You can verify your identification of the terminals with a multimeter set to resistance mode. The TPS line will vary in resistance as the throttle is opened and closed. It will have the least resistance to the return line when the throttle is closed and the least resistance to the Vref line when the throttle is fully open. TPS operation is important for acceleration enrichment and triggers the electronic equivalent of an accelerator pump when moved quickly in the opening direction.

I want to re-emphasize the importance of making safe and secure fuel lines as the fuel is under pressure for a considerable length of line and the pump is capable of delivering a substantial quantity of fuel. As well, the quality of your wiring will determine the reliability of this system. Use good quality wire, solder all connections securely, use heat shrink tubing to protect the connection from mechanical fatigue and corrosion, and protect all wires from heat and abrasion. Your safety and the reliability of this system depends upon this.

Tim Martin
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Re: Megasquirt part list any other help.

Thanks Tim, [img]images/graemlins/yourock.gif[/img]
Awesome write up. I'm sure you don't mind if I copy and paste your post into a Word document and save it on my hard drive. [img]images/graemlins/cool.gif[/img]
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Re: Megasquirt part list any other help.

I got my MS and was thinking of putting a 4.0 head on my 4.2 and use the MPI that came with the head instead of putting on the TBI. Any problems you can think of doing it this way ?
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post #6 of (permalink) Old 03-05-2005, 12:57 PM
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Re: Megasquirt part list any other help.

I got my MS and was thinking of putting a 4.0 head on my 4.2 and use the MPI that came with the head instead of putting on the TBI. Any problems you can think of doing it this way ?

[/ QUOTE ]

IMHO... it's the best you can get. The 4.2L block is better and you get the SMPI from the 4.2. However since you've got the SMPI with the 4.0... are you trying to integrate the MegaSquirt to the SMPI? I didn't think they've got that worked out yet and it was only working for the TBI.
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post #7 of (permalink) Old 03-05-2005, 01:18 PM
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Re: Megasquirt part list any other help.

You won't get sequential multi port injection using the megasquirt. You can fire all the injectors off at once or in two banks alternating (to reduce pressure fluctuations). Many of the guys that are doing megasquirt are using it on multiport systems, either factory, or their own conversions to multipoint. I'm not talking about jeeps here, but performance enthusiasts. They report it works well. I'm told that sequential injection shouldn't be considered a necessity, but that it can improve emissions (at idle?). Many cars came from the factory with non sequential multiport systems, so I speculate it should work on the jeep as well.
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post #8 of (permalink) Old 03-05-2005, 04:18 PM
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Re: Megasquirt part list any other help.


what a great reply tim. Thanks a ton. if anybody else has anymore to add please do.(if its possible to add more) the more experiences of others i can draw from the better.
post #9 of (permalink) Old 03-05-2005, 09:18 PM
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Re: Megasquirt part list any other help.

Yeah, I was thinking about firing it in two bank of 3 to keep it simple. We did that on a Merikur a couple of months ago. REaly looking forwad to this as soon as it warms up a bit.

I just might rebiuld my spare motor and set it all up on that one. IT will cut my down time quite a bit.
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post #10 of (permalink) Old 03-06-2005, 06:57 AM
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Re: Megasquirt part list any other help.

[img]images/graemlins/goodpost.gif[/img] [img]images/graemlins/applause.gif[/img] [img]images/graemlins/40BEER.gif[/img]

Terrific writeup! Good work!
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