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post #1 of (permalink) Old 01-02-2001, 06:19 PM Thread Starter
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How to wire factory CJ tach.

I've got a factory tach for my 304 I want to wire up but I'm a little unclear on what goes where. Not counting the lighting there are only 2 wires coming off the tach. One red and one red with a white tracer. On the centech harness I used there is only one wire behind the dash that is labled for the tach. This wire terminates at the green wire side(negative?) of the coil. Should this wire attach to the red on the tach or the red/white? And where does the remaining wire go? I have a schematic but the tach portion is a tad hard to read. I "thought" that the red w/tracer went to the neg side of coil and the plain red I figured went to power. If I try to do this the tach looks like it wants to read but as soon as I make the connection the motor dies. Ditto if I reverse the connection. Seems I am shorting something. Same thing happens if I attach one of the wires to ground. Any ideas?

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post #2 of (permalink) Old 01-02-2001, 08:09 PM
 
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Re: How to wire factory CJ tach.

I posted this same question aseveral months ago.

I have the same tach as you 2 wires red and red/white
i was told its from like 82-84 something like that a really narrow year range for that particular hook up, and a few folks said it was difficult to wire up, and its for a a 6 or a 8 cylinder..

well, I tested it on my WOrk truck, a Ford ( duraspark ignition) 300 staight 6.

I just probed around until I got a reading that looked right, if I remember correctly it was the negative side of the coil to one wire, and ground the other wire,or possibly it was the 12 on the coil to the battery, I remeber thinking it made no sense to me, its been a while since I did this, but i rememebr it was very simple once you goofed around with it a bit.

maybe a search for that old post will turn up the specifics if its posible to search back that far.

Ive never understood the difference between a 6 cyl tachmn ad an 8 cyl tach either if they both use the same duraspark ignition system?, if mine doesnt read properly with the V8 Ill ad a potentiometer and try to tune it.
can anyone explain that to me?

OzarkJeep

"If thats the way that you want it, well thats the way I want it more..."
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post #3 of (permalink) Old 01-02-2001, 10:57 PM
 
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Re: How to wire factory CJ tach.

The two red wire tach is a 6 cyl tach and won't work on a V-8.
I've been involved with and trying to find answers for this old question for over 2 years now. I't just something that has bugged me so I keep trying to understand it ....
My last attempt at this involved trying to trace where that danged red # 96 red wire COMES OUT on the engine side of the fuse block.
Finally, using a 10X jewelers loop to spot on the diagram the part of the exterior module it was in and PhotoShop to blow up and enhance the diagram, I found it! I touched it up pixel by pixel so it can now be clearly seen.
# 96 red (interior) becomes # 13F red (exterior)
See pic ...


CJ-/5.2Mag, our resident certified Jeep master mechanic explained this problem best recently .....
"....tach on CJs 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."





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post #4 of (permalink) Old 01-03-2001, 09:25 AM Thread Starter
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Re: How to wire factory CJ tach.

I don't know what year the tach came from but it is printed on the rear very plain 8 Cylinder. I have two others that are printed 6 cylinder. The one marked 8 has 2 red wires, 1 with a white stripe. Where does 13F terminate in the engine bay? Have you been able to determine that?

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post #5 of (permalink) Old 01-03-2001, 11:03 AM
 
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Re: How to wire factory CJ tach.

Spook-
That's the jist of the major confusion and why it's so hard (at least for me) to try and understand.
I don't have the diagram for an 8-cly so my findings so far are based on a 6cly.
It doesn't 'terminate', per say. At least on a 6-cly.
The engine bay 13F red w/trailer seems to go into a loop.
13A spurs off the loop and connects to the plus side of the coil where it joins with 13C red w/trailer which I believe goes to the capacitor.
This loop goes into and out of the ignition controll module which seems to be the key as to how the proper signal is created.
Part of the post coil leg of this loop is the light blue 14A and 14B wires which incidently splice(?) at the slot immediately next the #50 slot and the #1 slot in the above illustration.


Pic of the extertor fuse block - 1983 CJ-7. Middle right shows the location of # 13F Red w/white trailer Tach wire.

Welcome to the CJ Tach Club!
Be sure to post any success or any insite gained on this subject ...
one of the deepest mysteries and thorn in the side of CJ Jeepdom.



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post #6 of (permalink) Old 01-03-2001, 07:24 PM Thread Starter
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Re: How to wire factory CJ tach.

Looking at your initial post here I'm not sure where you came up with the #96 Red wire. The only 96 wire that I can find that has anything to do with the tach is a #96 violet and thats for the 4 cylinder. As far as I can tell that wire comes out on the engine side as nothing. Using your photo as reference it would be 2 boxes to the LEFT of 13F which makes it nothing. At least according to my schematic. But my schematic doesn't go into much detail as far as the 4's go. Anyhow, see my new post on this subject because I "THINK" I've got a handle on it. At least as far as the 6 and 8 cylinder.

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