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post #1 of (permalink) Old 08-26-2000, 10:07 AM
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factory tach and TBI problems

Hello, I recently posted a question concerning getting my factory tach to work with TBI buildup of my 258. I have removed all of the unnecessary stuff under the hood to clean up the wiring situation and apparantly the little box on the passenger side fender controls the tach. A gentleman replied to me and was going to send a wiring diagram of that box, but I lost my e-mail provider - if you read this please reply to my new e-mail listed below. Otherwise, does anyone else know how the factory six-cyl. tach works and how I can use it with my new setup.

Thanks,
John
[email protected]

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post #2 of (permalink) Old 08-26-2000, 12:18 PM
 
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Re: factory tach and TBI problems

You didn't fill out your bio, or give details on which injection system you are using...
So we are going to have a hard time guessing what it might be!

Post the year of the jeep, along with what fuel injection system you went with, and we'll try to help.

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post #3 of (permalink) Old 08-26-2000, 01:51 PM
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Re: factory tach and TBI problems

I apologize for the lack of info. Here it is:

I have a GM TBI system w/ 747 style computer that I am using to control ignition timing. This is a junk yard buildup. I am still using my stock distributor and coil with the vacuum and centrifugal advance disabled. I have disconnected everything in the original wiring harness that is no longer necessary, that includes the little module (it has a name, but I can't remember what it is) that sits between the radiator and starter solenoid on the passenger side fender and also the Motorcraft ignition module. I am told that the tach signal came from this mystery module.

My questions are:
1. Since I want to use my factory tach, what kind of signal does it get? Note that the tach has only two wires, not including the light bulb. From this I might be able to make a little signal conditioner. I originally assumed that it acted like a SunPro or other inexpensive tach from AutoZone or wherever that takes a signal from a hot wire and the negative side of the coil.

2. Barring that, what wires on the mystery module control the tach and how should they be connected? I can reconnect that module somewhere more convenient.

Thank you,
John

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post #4 of (permalink) Old 08-26-2000, 03:08 PM
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Re: factory tach and TBI problems

I don't want to start a war here or anything, but why don't you use the HEI dist which is disigned for the injection? The hard part would adapting the dist. to fit into the AMC engine, which is already done by some of the earlier HEI swaps. The advantages would be in the ign module. I know that GM's module has a switching chip inside it for the electronic spark timing. This chip uses a 5 volt reference to switch to computed timing. You could use the same 250 chevy dist. that is used in the HEI swap, and pin the advance weights and remove the vacuum advance. The tach would just hook up to the coil ground terminal on the cap that is marked tach. Trying to modify and join the motorcraft ign and the GM computer may not be a good idea. Also some of the GM modules had special built in signal modifiers that converted the analog signal to digital signal for proper computer input.

I like your idea, it should turn out to be a real cool setup. There is also some helpful info on the diyefi site, which offers some custom eprom programs.

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post #5 of (permalink) Old 08-26-2000, 05:49 PM
 
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Re: factory tach and TBI problems

JOHN!!!
I still don't know what year your factory tach is...???....
Same as the Jeep I assume, what might that year be???

--------------------------------

No "WAR" Dorfs, I think you may be correct.
There is no reason why he can't use his Motorcraft ignition because all it does is provide crank timing location anyway, but the GM ignition will provide him all of the plugs and wiring compatability.

He is, however, only asking about the tach not working correctly, which means the he already has the vehicle running.
Now, for that tach wiring diagram.......

Do you have the orignal distributor for the injected system?
If not, we will figure this out. If so, it may save you a lot of time just changing them out.

Us the 250 CID I-6 Chevy distributor, from about '76, and it will have a cap that doesn't have the coil in the cap, and fill it with the electrical guts of the injected distributor.

the only thing you will have to modify is the coil wires to use a remote coil, and then you could update to the new style of E-core coil and leave that old GM HEI coil in the trash where it belongs.
That should solve every problem right down to the connectors!

If you need any of the I-6 distributor housings, I have a couple around here for cheap if you want one.
Let me know.

I'm not starting from the begining on this one, because if you did a 'Junk yard' install of a factory fuel injection system, you already know what I'm talking about. If you need help with specifics, let me know, I'll be glad to help.

Dorfs and Dave will be good sources of help.
Dave in particular is real sharp with what will and won't interchange in the 6 cylinder distributors.

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post #6 of (permalink) Old 08-26-2000, 11:59 PM
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Re: factory tach and TBI problems

"Otherwise, does anyone else know how the factory six-cyl. tach works and how I can use it with my new setup"

If you are talking about a late model CJ I have never heard an answer to this question. It comes up a couple of times each year. It would be nice to finally get it figured out.

What kind of Jeep and what year do you have?

Here is one for you....

My 85 CJ7 has a factory tach. The Jeep had the 258 and I installed GM TBI fuel injection using the 727 computer. With that addition I completely removed the stock computer. I then installed GM HEI and removed the stock ignition module, coil and every wire going to each of those ignition components. The HEI has a terminal for tach source but I did not use this. Much to my confusion, the tach still worked and was as accurate as ever. Im not sure where it gets it signal. It cant be related to the stock ignition because I removed every bit of it.

The Alternator is the only remaining thing I can think of that could be sending a signal but that just doesnt seem right. The only original wiring in my Jeep was that which goes to the starter solenoid and alternator.





Regards,
LarryM
"If I can't go faster I'll go longer."
post #7 of (permalink) Old 08-27-2000, 03:49 AM
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Re: factory tach and TBI problems

OK, here is what little bit I know about tach's.
There are two types that I know of, not counting the new ones connected to the computer modules. The one type has 3 wires connected to 12V, Gnd, and Point's side of the coil. The other type has 2 wires, one is connected to the ignition switch and the other to the coil's 12V terminal.

If were talking about 80's CJ's then the 6 cyl and 8 cyl tachs are the two wire type and are connected in series with the coil. Spliced into the power line from the ignition switch. The 4 cyl tach is the other type.
If you look at the wiring diagrams in your Haynes book you'll see the two different hook ups.

Now, how does the 6/8 cyl tach work? Here are my theories. It works like an AC ampmeter. When the points (or transistor in the module) close then power flows from the ignition switch through the tach, then through the coil and through the points to ground. When the points open power stops flowing through the tach, so every time the points open and close there is a pulse in the power line that the tach picks up. Convert the pulses to a voltage and measure it with a meter. I believe what they use inside the tach is a small transformer a couple of diodes and resistors and the meter movement. I havn't taken mine apart yet but what I could see down the light hole was the small transformer and some diodes.
My tach was made by Thomas G Faria Corp, Patent 3005155.
That Patent is to old (pre 76) to be online, at least I couldn't find it, but I did find a newer one by "Faria". US04059799.
http://patent.womplex.ibm.com/cgi-bi...d/US04059799__
(If you get a not available error on this site try it several times)
That's where I took the attached picture from.
My guess is that if you removed the two wires going back throught the fire wall you would have the stock tach.

Now how to connect it up to the computer or ignition module? Don't.
Connect it between the ignition switch and the coil ( ) terminal. I'm assuming the coil is still powered from the ignition switch.

Be careful connecting these tachs up, if you hook things up wrong you can burn things out. With the "Series" type tach power flows straight through. If this doesn't make sence to you don't hook it up.
This is my limited understanding of tach's. If someone knows anything more speak up before this guy blows up something.


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File Type: tif 9-199611-SeriesTach.tif (11.1 KB, 2 views)
post #8 of (permalink) Old 08-27-2000, 10:43 AM
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Re: factory tach and TBI problems

Dorf, you are absolutely right. I would be going about this the wrong way by not using a GM computer "recongnizable" ignition module. But, TeamRush is also right, I already have the thing running and have used the stock GM HEI control module from an '89 Astro Van w/ 4.3L. This module has four wires going to the computer, two coming from the distributor (these are for the magnetic pickup and gives the computer engine speed and, of course, mechanically set ignition timing) and two wires to the coil (positive and negative). Note that the positive side of the coil is at full voltage (14.4 when running) as this is necessary for the module to operate correctly. There is NOTHING left of the original motorcraft ignition system and engine management system. Further this Jeep is a 1981 so I didn't have a BBD carb computer to deal with. When I first started the Jeep, the tach did work, but unlike someone's post, it was not accurate at all. This tach is, in fact, a two wire setup that has a constant hot on one side (red/red wire) and also has a red/white wire.

What I should have done before I took the Jeep apart, was to put an oscilloscope on the tach to determine what kind of signal it got in terms of frequency (hopefully 6/2 the engine speed, a V8 would hopefully have a freq of 8/2 the engine speed) and amplitude (voltage or amperage, probably amperage since it works at 12V) and was the signal some sort of square wave or analog wave or something else. I should have then gone out to the coil and seen if the signal was the same taken from the positive to negative side. If not, the mystery box is conditioning the signal somehow / someway and then I would be in trouble.

I would be happy to reconnect the mystery module in a better location (from a wiring standpoint) and use only those terminals that I need to get the tach running. Does anyone know what the pinout of that box is? Else, we could investigate the signal going to the tach on someone's Jeep that is working correctly and put this issue to bed once and for all. I haven't looked at the patent that someone forwarded, but your description jives with what I was thinking and have seen on inexpensive SunPro tachs: connect one side to positive and the other side to the coil. I, like you, would be concerned about doing this with my stock tach as a replacement is fairly expensive. I would like to know that this would work before I hook it up!

Thank you all,
John

post #9 of (permalink) Old 08-27-2000, 11:52 AM
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Re: factory tach and TBI problems

MaxW82CJ7 is right about the tach on CJs, it reads the pulses from the + side of the coil, most aftermarket and late model factory tachs read off the negative side of the coil. The hookups for the factory CJ 6 and 8 cylinder tach are under the dash, in series with the ignition feed to the coil. DO NOT hook the factory tach to the coil in the manner of an aftermarket tach, it will fry! If you haven't played with the wiring under the dash, it may still work with your EFI. The 6 cylinder tach is not calibrated correctly for a V8, so this may account for the innaccuracy found by some. The factory 4 cylinder tach works like the aftermarket tachs, getting the pulse from the coil negative, but it also won't read correctly with a 6 or 8. The exception is if you are using Mopar EFI, factory or Mopar Performance, which supplies 2 pulses per revolution (4 cylinder reading) to the tach. There is a place in Riverside, CA. United Speedometer, that can recalibrate the 4 cylinder tach to accomodate a 6 or 8 cylinder. I'm not aware of any aftermarket tach that would fit in the original dash hole perfectly. You either have to get a 2 5/8" tach, and make a spacer ring to cover the larger hole (A diff side bearing spacer works well) , or get a 3 1/8" tach and enlarge the hole.
Hope this helps, good luck!

post #10 of (permalink) Old 08-27-2000, 09:11 PM
 
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Re: factory tach and TBI problems

HEY MAG!! I was wondering where the resident Jeep guy was...???...

I don't have a wiring diagram for an '81, so I had no frame of reference.
I had a sneaking suspicion that it was a positive feed for the signal...
Because of the run on problem some jeeps had when the wiring was screwed with, and the fact that if you unhook the tach is some jeeps they wont run...

He may have to use a tach filter and/or a coil capacitor to get it to work, and he will for sure have to get a V-8 tach for it to be correct.

Good save there MAG!

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