4.0 lt tachometer working in 2.5 lt XJ

Gentlemen,

I have found an instrument cluster from a 4.0 lt Commanche (probably 1988-1990 era), which fits great in my 1984 XJ, with one small problem. The tachometer shows only 2/3 of the real reading.

A search in the dealership didn't produce a corresponding part, they have stopped making them it seems.

Has anyone ever tried to modify a 4.0 lt tachometer to work with a 2.5 lt engine? It shouldn't be too difficult, or maybe it is impossible, I do not know.

All help would be appreciated on this subject.

Kind regards

I have a 4 cyl Metric cluster for $25.
swap the tach or use the whole cluster.
e-mail [email protected]

Another option is to recalibrate it. But I am unsure how it is done. I would assume that there is a pent. on it for claibration.

Ghost said:

Another option is to recalibrate it. But I am unsure how it is done. I would assume that there is a pent. on it for claibration.

Click to expand...

Hey Ghost,

You and I have the most ancient XJs in this forum LoL. How does it feel to say 1984? Only yours has a "young heart", I am planning to draw on your expertise, when I get the $$$ to do the transplant myself.

I got that cluster from an abandoned Commanche and it looks brand new, so I though it would be nice to have it in my truck, since it has all metric gauges (Celcius, Km etc), which is what we use here.

This weekend, I plan to remove the dash again and have a look, I'll check if there is some adjustment or anything, but somehow I have the feeling that these gizmos are fixed, at least until someone with some electronics knowledge goes in. I am not the one who can do that.

Tnx and rgds

A friend of mine did exactly that a few years ago, recal'ed a 6cyl tach for a 4banger. IIRC, he used a pulse generator and an oscilloscope to do it.

It can be fixed. I am an electrical engineer and I fixed mine. It was a 2.5 and I made it work with the 4.0L after the stroker swap. Where are you located?

By the way my rig started out life as an 85. The cluster is from an 87, the engine from an 88, the tranny from a 90, the rear D44 from an 88, the drive shafts from a Bronco. Nothing except the skeletal body is original.

old_man said:

It can be fixed. I am an electrical engineer and I fixed mine. It was a 2.5 and I made it work with the 4.0L after the stroker swap. Where are you located?

By the way my rig started out life as an 85. The cluster is from an 87, the engine from an 88, the tranny from a 90, the rear D44 from an 88, the drive shafts from a Bronco. Nothing except the skeletal body is original.

Click to expand...

C'mon, Tom, HOW did you fix it? John isn't the only person who needs this information, ya know. This question comes up every few months, and always there's some vague reference to "Oh, you just change a resistor," but nobody seems to know which resistor to change, or what the old and new values should be.

By the way, John is in Greece -- I don't think it will be cost-effective for him to send his tach to you for modification. I looked at the tach on an '89 cluster, which should be the same as the one he has, and I saw what appeared to be a potentiometer on the little circuit board on the back of the tach. Is it as simple as turning the screw on this potentiometer?

That screw you're looking at is the secret, Eagle. The real trick is figuring out how much to turn it.

ChiXJeff said:

That screw you're looking at is the secret, Eagle. The real trick is figuring out how much to turn it.

Click to expand...

I guessed as much. Not having access to an oscilliscope and pulse generator, I was thinking of removing the tach from the cluster and wiring it to the coil, then hooking up a standard idle tach in parallel and adjusting the XJ tach until it reads the same as the tune-up tach.

Any reason why that shouldn't work?

Gentlemen,

Please pass this info along. As Eagle told you already, I live in Athens, Greece, so it's not easy for me to either get a new tachometer from a junk yard, or to send mine to someone in US for mods.

Even if doing the mode requires access to an osciloscope, I could arrange for that, I have a friend who is repairing ham radios, I am sure he can do the necessary mod for me, so PLEASE (begging), tell me how to do it.

I am so happy with my new cluster, and the only thing which spoils my happiness, is that tachometer.

I am sure that you good Samaritans in will come up with a solution, like you always have, for me.

Thank you!
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John SV1CEC

Careful, there are trimmer pots for adjusting the fine tuning and linearity on some boards. They will not allow enough adjustment for a 4/6cyl switch. On my board, the resistors are laser trimmed and there is no pot. I will try and get the pictures posted today on what my board looked like. If it is the same I can give instructions on how to modify it. You don't want to fubar it before you know what you are doing.

old_man said:

Careful, there are trimmer pots for adjusting the fine tuning and linearity on some boards. They will not allow enough adjustment for a 4/6cyl switch. On my board, the resistors are laser trimmed and there is no pot. I will try and get the pictures posted today on what my board looked like. If it is the same I can give instructions on how to modify it. You don't want to fubar it before you know what you are doing.

Click to expand...

I'll be looking forward to your instructions Tom.

Sincerely appreciated.

Rgds
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John SV1CEC

ChiXJeff said:

A friend of mine did exactly that a few years ago, recal'ed a 6cyl tach for a 4banger. IIRC, he used a pulse generator and an oscilloscope to do it.

Click to expand...

If you can ask him to provide us with some instructions, I could certainly appreciate it.

Kind regards
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John SV1CEC

Here is a picture of the front of my cluster for comparison


Remove the screws shown.

If the PCB is like shown we will be able to modify it easily.



The pictures show the trace that must be cut and the resistor to be added. I will need to work out a proceedure to measure the existing resistor and a formula to allow you to calculate the resistor value.

Tom --

The photo of the front of your panel looks like the old (Type 1) panel used from 1984 through (mid?) 1987. That circuit board doesn't look at all like the one on my 1989 tachometer. What year is yours from?

old_man said:

If the PCB is like shown we will be able to modify it easily.

The pictures show the trace that must be cut and the resistor to be added. I will need to work out a proceedure to measure the existing resistor and a formula to allow you to calculate the resistor value.

Click to expand...

Tom,

I just want to tell you that this is not the tachometer I had in mind, I already have two of these clusters here, the one which was in my truck before the latest swap and another one. These are working fine, they are from a 2.5 lt, 4-cyl. truck.

The tachometer I need to modify comes from a 1988-1990 truck. Its zero indication in at 9:00 o'clock. The instruments in these clusters do not just plug in from the front and get secured by screws from the front. They are secured by screws which are entered from the rear of the cluster. In these clusters, even the film with the circuit is different than the one on the cluster in your picture, and also the gauges are different.

However, since a lot of other people (as Eagle pointed out) have the same question, please do provide the additional details, for others to benefit, when you have them. I am sure they will appreciate your help.

Many thanks for taking the time to take these pictures and post them here, I sincerely appreciate your effort to help me out.

Kind regards
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John SV1CEC

I'm the one ChiXJeff mentioned. (I'm a new member - just joined, although I have been reading the forums off and on for quite a while.) I put the cluster from a late '80s XJ into my old '85 - the only ones I could find at the boneyard were from 6 cylinder XJs. Poking around the tach, I discovered that they are just using a frequency to voltage converter - the tach guage itself is nothing more than a voltmeter. I hooked the new tach up to 12VDC power, and gave it an input with a function generator. I checked the output of the function generator with a frequency counter. I then adjusted the trimpot on the converter to get the correct reading. (IIRC, the tach gets its signal from the ignition module, and that is just a pulse train that comes from the pickup in the distributor. In the 4 cylinder, 1 revolution of the distributor equals 4 pulses, and that equals 2 revolutions of the crankshaft, so 2 pulses equals 1 RPM. In the 6 cylinder, 1 revolution of the distributor equals 6 pulses, which equals 2 revolutions of the crankshaft, so 3 pulses equals 1 RPM. I set the function genearator on 2kHz and adjusted the trimpot so that the tach read 1000 RPM - that seemed to do the trick.) It's been a few years since I did this, so I am a bit fuzzy on the details at the moment. I do have another 6 cylinder tach that is unmodified, and I am working on a writeup (with pictures even!) about how to do this. As far as I know, this will work on the clusters with the cable driven speedometer - I am not sure about the full electronic clusters, since I haven't played with one of those yet! All this leads to a question that I have - once I get this written up, how do I go about posting it in the Tech Articles area?

I suppose it would help if I read the *&%#@ posts. Just went back through the thread and saw that Tom's cluster is from an '87, so it's the older (Type 1) cluster that's like the cluster John removed. I'd still like to know how to convert that style, also, but the immediate problem is how to recalibrate a tachometer in a Type 2 cluster (1988 - 1990).

You're sure that potentiometer won't do it? (Sure would be nice it it would.)