Factory Tach hookup solved!!?? - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 01-03-2001, 08:30 PM Thread Starter
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Factory Tach hookup solved!!??

This is a question I had and searching through the archives evidently a bunch of others did too. I'm fairly certain I've got a handle on it as far as 6's and 8's go. Right now I have 3 factory tachs of unknown model years except for one. My 8cyl tach comes from a 1981. The other two are 6's and I have no idea. The wiring coming from all is the same. One red w/white stripe and one plain red. Pouring through my schematics and using my considerable Mensa caliber intellect(grin!) I believe I've figured it out. My schematic shows 2 wires coming off the tach, labeled 13A and 13B. 13A can be traced back to a plug coming off the ignition switch that starts off as a 13B but comes to a splice and splits off as 13D and 13A. 13D goes to the firewall connector and 13A is clearly labeled as "tach". I surmise that this plug connects to 13A coming off the tach itself. Now it starts to get a tad funky. This first part comes from a schematic of 80-83 model years. This years schematic shows a 13C wire terminating right at the 13A plug coming off the ig. switch. There is no clearly stated use for 13C. Model year 79 shows a 13B terminating at 13A but neither is labeled as to use. Model year 78 shows 13B terminating at a 13A clearly labeled as "tach". No matter how you trace 13B(78&79) or 13C(80-83) it ends up at the positive side of the coil. On 80-83 my guess is it goes like this...13A from the tach to 13A off the ignition switch and 13B from the tach to 13C which goes to the bulkhead connector. On 78&79 it's 13A to 13A and 13B to 13B(to the bulkhead). On 79-83 13B(or 13C) turns into 13F on the engine side. 13F ties into 13A coming off the 2 prong ignition module plug. This splice contiues as 13B(resistance wire) to another splice. It breaks here as 13D to the side of the coil and 13E on to the starter solenoid. The wiring on the engine side is the same for 79-83. On 78 it gets a little strange. 13B splits off on the engine side as 13B resistance wire to 13D to coil and as wire 78 straight to alternator. On 76&77 it gets stranger still. 13B splits on the engine side as 13B and 78 again. 78 goes straight to alternator. But this time 13B splits as 13 to the coil and as 82 which goes to a 4 prong plug coming off of that model year's electronic ignition pack. And THAT is as far as I'm going. My brain hurts and my eyes are fried from tracing all those lines on my schematics. I wish I knew how to break this post up a bit so it was not just one long paragraph. I realize it's probably going to be a little hard to digest in this form. Maybe someone else can tidy it up a bit and repost it. I'm pretty sure this is the good scoop on this issue but if someone spots something that just doesn't fly by all means point it out. Other than that, if you try this hookup and fry everything from headlamps to tail lights wellll, you didn't hear this from me.

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post #2 of (permalink) Old 01-05-2001, 09:32 PM
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Re: Factory Tach hookup solved!!??

Why won't a 6 cylinder Tach work in a V8 and visa versa. The wire's are the same and externally they look the same. I am having a hard time believing AMC used 2 different Tach's. I am only trying to figure out why. I have not done any research on the internal components of the 2 yet and will as soon as I can take my V8 tach out.

post #3 of (permalink) Old 01-05-2001, 09:44 PM Thread Starter
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Re: Factory Tach hookup solved!!??

I have no idea why they are not interchangable. I do not have a strong enough understanding of electronics to determine that. But I DO know that I have factory tachs labled "6 cylinder" and "8 cylinder" which are otherwise entirely identical externally.

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post #4 of (permalink) Old 01-05-2001, 10:00 PM
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Re: Factory Tach hookup solved!!??

I will try and get the V8 tach out of the Jeep this weekend and take both of them to work next week and find out what the difference is. I will post the findings.

Thanks,
Tom

post #5 of (permalink) Old 01-05-2001, 10:02 PM
 
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Re: Factory Tach hookup solved!!??

where is the 6cyl or 8 cyl label, I couldnt find anytihgn on mine, but I did find a 79 marked on the back, and also mine has 2 red wires, no white tracer, My memory isnt so good, I shouldnt use it as a reference!

im still looking for a V8 to test mine on, so I can for sure see if they are 6/8 cyl specific.


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post #6 of (permalink) Old 01-05-2001, 10:46 PM Thread Starter
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Re: Factory Tach hookup solved!!??

All of mine are marked on the back. Not the back of that cream colored cover that holds it in the dash, but the back of the blue casing. Mine are all stenciled in black, faint but there, which cylinder engine they are for.

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post #7 of (permalink) Old 01-08-2001, 10:13 AM Thread Starter
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Re: Factory Tach hookup solved!!??

Back to the top.

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post #8 of (permalink) Old 01-08-2001, 04:10 PM
 
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Re: Factory Tach hookup solved!!??

I found the marking on my buddys tach, mine is worn off..
Great, so im guessing, Ill hook it up to my inline 6 again, and verify its accuracy with me tach/dwell meter to see if tis a 6 or 8 version.

in the process Ill remeber/re figure out the simple way to hook it up to the col and let you know, if you havent already figured it out.


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post #9 of (permalink) Old 01-09-2001, 07:33 AM Thread Starter
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Re: Factory Tach hookup solved!!??

Something I "did" figure out is that the 6 and 8 versions are definitely not interchangable. I hooked up one and then the other and got readings at idle that were different by a few hundred rpm. I thought I had it all figured out from the schematics I looked at but when I went to hook things up initially something was off. It looked fine at idle but the rpm didn't come up on the tach right when engine rpm was increased. Looking at the factory schematic it "looks" like the tach wire ties into the the junction where the resistor wire and the ignition module wire are tied together. In other words "before" the resistor wire to the coil. I have a centech harness that uses a ballast resistor instead of the resistor wire. I assumed from the factory schematic I would want the tach hooked to the "before" side of the resistor. When it was hooked up this way the indicated rpm wouldn't come up properly when engine rpm was increased. I hooked the tach to the "after" side of the resistor and everything works just fine. Go figure.

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post #10 of (permalink) Old 01-09-2001, 10:26 AM
 
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Re: Factory Tach hookup solved!!??

Hey guys!
I got it!

Clear as MUD! [img]/wwwthreads_images/icons/tongue.gif[/img]

Guess as long as my factory tach works, I got no problem.
If it ever goes south though....

RocknCJ [img]/wwwthreads_images/icons/cool.gif[/img]
A bad day wheelin'
will always beat a good day workin' [img]/wwwthreads_images/icons/smile.gif[/img]
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