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4.0L Gauge Cluster to 2.5L (tach question)

D

dakotaguy1999

NAXJA Forum User
Location
Fairbanks, AK; Kodiak, AK
Hello there,

My friend has an '86 2.5 XJ and we have installed a full-gauge cluster from an '89 4.0. Once we got the speedometer cables swapped out - everything worked great, except the tachometer was reading lower than it should. I suspect this is because the cluster came off of a rig w/ the 6cyl.

I have done the swap on a 91-newer XJ, and I know the tach signal came directly from the PCM. I've swapped in a tach from a 4.0 into the 2.5 on one of the 91-later models, and the tach was more or less accurate. I am not sure about the 84-90 clusters though - it appears the tach signal comes directly from the ignition module.

I was looking at the back of the tach cluster, and there is some sort of resistor mounted in the printed circuit board behind the tachometer. Does anyone know if this resistor has anything to do w/ the tach calibration? If so what will I need to replace it with in order to get the tach to read correctly for the 2.5L? Or is there some alternative way to calibrate the tach?

Any help would be greatly appreciated! Thanks in advance!
 
Several times over the years I have seen posts suggesting that changing a resistor will recalibrate a tach for a different cylinder count, but none of the posts I've come acorss know what resistor to change or what values to use. The resistor you found is probably the culprit, but who knows whether to increase or decrease the resistance? Not me.

If I could find the old universal steering column mount tach I had from many years ago, I could pop the rear cover and see if the 4-6-8 switch led to three different resistors, and what their values are. But I haven't seen that thing for ages. Does anyone have an old universal tach gather dust on a shelf?
 
Hmm. I did a pretty extensive search on the web, and I found that there are other vehicles out there that utilize this resistor as well. Unfortunately I was unable to find any information as to which resistor to use, nor was I able to find any info to figure out what resistor I'll need.

Anyone else have suggestions?
 
A friend of mine put a 6cyl tach in a 4cyl (88 IIRC.) He recal'ed it with a pulse generator, I think. Then again, he's an electronics weenie.
 
dakotaguy1999 said:
Hello there,

My friend has an '86 2.5 XJ and we have installed a full-gauge cluster from an '89 4.0. Once we got the speedometer cables swapped out - everything worked great, except the tachometer was reading lower than it should. I suspect this is because the cluster came off of a rig w/ the 6cyl.

I have done the swap on a 91-newer XJ, and I know the tach signal came directly from the PCM. I've swapped in a tach from a 4.0 into the 2.5 on one of the 91-later models, and the tach was more or less accurate. I am not sure about the 84-90 clusters though - it appears the tach signal comes directly from the ignition module.

I was looking at the back of the tach cluster, and there is some sort of resistor mounted in the printed circuit board behind the tachometer. Does anyone know if this resistor has anything to do w/ the tach calibration? If so what will I need to replace it with in order to get the tach to read correctly for the 2.5L? Or is there some alternative way to calibrate the tach?

Any help would be greatly appreciated! Thanks in advance!
Click to expand...
 
THE 2.5 TACH GETS POWER FROM THE IGNITION CONTROL MODULE. AT THE CONTROL CONNECTOR THE FIRST 3 TERMINAL CONNECTOR SHOULD MARKED A-B-C A TERMINAL IS COIL + THE B TERMINAL IS COIL - THE ONE MARKED C IS FOR THE TACHOMETER
[email protected] badge 714 DALE
 
badge714 said:
THE 2.5 TACH GETS POWER FROM THE IGNITION CONTROL MODULE. AT THE CONTROL CONNECTOR THE FIRST 3 TERMINAL CONNECTOR SHOULD MARKED A-B-C A TERMINAL IS COIL + THE B TERMINAL IS COIL - THE ONE MARKED C IS FOR THE TACHOMETER
[email protected] badge 714 DALE
Click to expand...

??????

Okay. And how does this relate to recalibrating a 6-cylinder tachometer to register correctly when used with a 4-cylinder engine?
 
Sorry to bring up an old post - but I finally found a solution for this problem.

I took apart the gauge cluster last night, and it turns out the tachometer is adjustable! There is an adjustment screw on the back side of the tachometer. I fiddled with it for a while, turns out when the screw is all the way clockwise to its stop - the tach is correct for a 4cyl. When the screw is midway through its travel - 6cyl. When the screw is all the way to the other stop - 8cyl.

You can make small adjustments to this screw to fine-tune the adjustment. Unfortunately, the tach must be removed from the cluster in order to access the adjustment screw. The tach is held on to the cluster by a total of 5 screws - 3 smaller ones, and 2 larger ones. It was not very hard, but took a lot of time considering I had to remove/install the cluster, start the vehicle, and compare the tach reading with a handheld calibrated tach...

BTW - this was on a 1989 tachometer (mechanical speedo, w/ RPM X 1000 instead of the older RPM X 100 scale).

Hope this info is useful to others out on the board...
 
Last edited:
XJ,
There is another thread on this same subject somewhere in this forum. It has a lot of info about the 6 cyl to 4 cyl tach swap. I spent two or three nights reading it cause I was trying to adjust the tach in my 87 Dakota. I believe it is in the Modified OEM Forum.
 
sorry to hijack,
but does anyone have notes on how to calibrate when switching from a 4cyl to the 4.0 6cyl? The cluster that swapped in for my idiot lights is out of an '86 4cyl and is now in my '89 4.0. I have pulled the tach out of the cluster but do not see an adjustment screw. Is there a difference between the 86' and 89' tach?
 
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