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post #1 of (permalink) Old 06-10-2007, 04:06 PM Thread Starter
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Can I Blame The Cpu?

You guys know about the problems i have had with my 97 TJ SPORT over the past few months. The thing will turn over and turn over but wont start. It is getting fuel. I was have problems with what i thought was water screwing with the CPU after a wheeling trip. If the CPU finally gave up on me then it wouldnt start. I have decided to just find a 97 TJ CPU and just plug th esuccker in and try to start it. if it starts then i guess i can blame the CPU. anybody know where i can get a brain for a 97 TJ Sport for cheap ( less than 300 dollars ). Anybody in the charleston Sc area that may have a CPU that i could borrow for a day to test it? thanks RYAN

97 TJ SPORT...2" SKYJACKER LIFT KIT, 30X9.50X15 BFG ALL TERRAINS, BLACK STEELIES, SAIL CLOTH SOFT TOP, SONY MARINE GRADE BOAT STEREO.

97 CHEROKEE SPORT
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post #2 of (permalink) Old 06-10-2007, 05:02 PM
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Quote:
anybody know where i can get a brain for a 97 TJ Sport for cheap ( less than 300 dollars )
Under $300, that is a good one. My brothers, friends, uncle's wife, once knew someone selling hens teeth. I think they also sold CPU's that cheep.

If it is getting fuel, is it squirting out of the test valve on the fuel rail when cranking or is it just dribbling out? Can you hear the fuel pump run when you turn the key on? If the fuel pump is not running can you hot wire it at the PDU (Power Distribution Unit), and make it run? Are you getting spark? Are all the relays in the PDU good? Are the proper contacts in the PDU getting power with key on, how about during cranking? If yes then the Ignition switch and wiring is ok. Are the Proper contacts in the PDU being grounded by the computer with the key on, how about during cranking? If not then the CPU of wiring to it is bad. Have you tried unplugging the CPU and cleaning the contacts with a good electric contact cleaner? Have you check the Crank Position Sensor CPS and its connections? These are know to go out and give similar problems. Is the computer giving any codes? If you turn the key on (without turning to crank) then off 3 times and then leave it on the computer should flash the check engine light. If every thing is okay it should flash five times and then pause and flash 5 more. Code=55. If it does not flash then do the other idiot lights work? If it flashes more then just the 55 what are those codes?

If you don't have a wiring diagram and multi meter get one.

Power for much of the engines electric systems, for example the fuel pump or ignition coil, flows as fallows. The actual power comes from the battery to the PDU, than trough a relay in the PDU, to the pump or coil. Power then grounded back to the battery through the frame on the case of the pump, or through the CPU in the case of the Coil.

Okay so what controls the Relay. Power for the relay comes from the battery through the ignition switch, to the PDU, it then flow through the coil on the relay and out to the CPU, the CPU than grounds the connection to complete the loop. So both the Ignition switch and the CPU need to be working And the CPU needs to want the pump to work, for the fuel pump or ignition coil to have power.

CPU's do go bad. The one in my Friends 97 Dodge went bad as determined by me, however given the cost we took it to a professional who confirmed the CPU was Bad. His was about $1000. To keep the EPA happy the CPU needs to come from the same model and year. So they are hard to find and often expansive if fallowing all the rules. The point I am trying to make is that there are many other parts the CPU works with that will make the CPU look bad, however those other parts are much cheaper to replace.

Wilhelm

I will start using Metric only when the duodecimal system is adopted!!

Last edited by bandhmo2; 06-10-2007 at 05:08 PM.
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post #3 of (permalink) Old 06-10-2007, 11:45 PM
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A quick way to isolate fuel vs ignition.....hook up a timing light to any one of the plugs.....if it flashes while your are cranking, then the CPS and other related devices are ok. Start thinking fuel. As correctly noted above, the proper pressure should be at least 40 PSI and when you push in the valve stem on the fuel rail it should blast out.

Yes on the DVM...I never leave home without one....and you should consider investing in an OBDII scanner. I paid $90 for mine off Ebay....I use it all the time....mainly to confirm certain operating parameters on my SBC.

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post #4 of (permalink) Old 06-11-2007, 05:08 PM
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Let's drop back 20 and punt before you start throwing parts at the Jeep. That's likely going to cost money, and you'll end up introducing problems in the Jeep as you throw the parts in. You think the problem's bad now... just wait, it will be three times as bad.

OK, start simple. If you don't understand how the ignition and fuel system works, here goes....

In Start mode, the fuel pump turns on for 2 seconds and pressurizes the fuel line, then turns off. The ECU turns it on and off.

The ignition and fuel pump are both powered by the AutoShutDown relay. This relay sends voltage to the positive side of the coil.

So, if you've got Spark, and you don't have fuel, you know the ECM is likely working... because you've got spark. The voltage to feed both systems come from the same place.

Now, if you don't have fuel and you have spark, the CPS is working too, as it pulses the ECM, and the ECM then takes over and enables the ASD realay.

See how easy this is?

Now you know that the problem's likely fuel related. If the Jeep's got spark, and the spark is in time, and it's not got fuel.... it's a pump/regulator/fuel line related issue.

A pump can be damaged by just sitting. They varnish, and freeze up.

Going on the symptoms you described I'd try the following.

Spray some starter fluid down past the throttle plate.

Does the engine start, and run, on the starter fluid?

If yes, then you've got a fuel delivery problem.

If no, then all bets are off and you've got to recheck spark and timing.

If it does run, then look at the fuel pump relay. Is it passing voltage on to run the fuel pump? It can drop out and die on the vine. This is NOT the same as the ASD relay.

If the fuel pump relay is good, then are you getting voltage to the pump? It's time to drain and drop the tank, and remove the sender/pump assembly.

Now that you've got it out, try hooking the pump up to 12 volts and SEEING if it runs. If it does, that's good, but not a guarantee everything will work. It may not be providing the pressure needed. Try running a jar full of Seafoam and gas through the pump, and then back to the jar again. It will help remove varnish.

After you've cleaned up the pump, and know it is working, then put it back in the tank and see what happens. It may even be the pump has lost it's ground. Things like this happen.

If you don't have a wiring diagram, email me, and I'll email you one back.
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post #5 of (permalink) Old 06-11-2007, 05:59 PM
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Quote:
Originally Posted by LEVE View Post
Let's drop back 20 and punt before you start throwing parts at the Jeep.
Letís drop back 30 and throw a fresh set of plugs in it. Hasnít grandpa been starting this thing and letting it idle over the last several months? It had problems before that too. See what the old plugs look like. Are they black?

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post #6 of (permalink) Old 06-12-2007, 04:54 PM
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Make sure you have a fresh or fully charged battery,
Then call me, I will let you borrow a CPU,
Ricky

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AND, in my avatar, the girl on the right..she wanted me...
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post #7 of (permalink) Old 06-18-2007, 06:03 PM Thread Starter
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Thanks Ricky, I should be back in Charleston somtime around the first of july. I appreciate your help. RYAN

97 TJ SPORT...2" SKYJACKER LIFT KIT, 30X9.50X15 BFG ALL TERRAINS, BLACK STEELIES, SAIL CLOTH SOFT TOP, SONY MARINE GRADE BOAT STEREO.

97 CHEROKEE SPORT
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