4.0 lt tachometer working in 2.5 lt XJ

[Eagle]

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?

That's a pretty decent explanation right there. Too bad the older tachs don't have the potentiometer on-board.

I believe to get the write-up added to the tech archive you submit it to Kejtar. He'll circulate it to a couple of members for grammatical review and to be sure it's as clear to readers as it is to the author, and then he posts it. (At least, that's the way it was being done, but I don't think anyone has added any write-ups to the archive for a long time.)

If there's any way you could eventually expand this to cover the Type 1 tachs and then the '91 thru '95 and the '96 - '01 tachs, that would be even better. Thanks for solving a long-standing mystery.

 

[Ghost]

Eagle, for info on the newer tach you might want to check with A-Mechanic. Are you aware of what he does and where he works? I hope he does not shoot me for this! :cheers:

 

[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?

djinn,

Many thanks for jumping in this one, I sincerely appreciate it. If you can post some more info, it would be great, like which trimpot you adjust.

As for your write-up, if you want it hosted in a web server (apart from ), I would be more than happy to put it in mine, with all credit going to you, of course.

Looking forward to some more details, so I can do some tests this weekend.

Kind regards to all.
-----------
John SV1CEC

 

[djinn]

So I rummaged around in the garage, and I found both the old cluster from my '85 (type I tach, I believe), and one of the replacements that I found at the boneyard (type II tach). The type II has a manufacture date of 1/18/89. There are 3 electrical connections along the back of the (small) circuit board - one labelled +IGN, one labelled GND, and the other labelled in some sort of oriental script (I believe that this is +12VDC, IIRC). You shouldn't have any trouble making the adjustment - there is only one trimpot on the circuit board (while posting late last night, I couldn't remember if there were one or two trimpots). The only reason that I used the frequency counter along with the function generator was to make sure that I had the desired frequency going into the circuit - if necessary, I suppose you could trust the markings on the function generator (the one I used was a bit old, and had not been calibrated recently, hence the extra check that I did). I will work on the Tech Article for this (probably this weekend), but I hope in the meantime that this info will work to get you going.

 

[SV1CEC]

So I rummaged around in the garage, and I found both the old cluster from my '85 (type I tach, I believe), and one of the replacements that I found at the boneyard (type II tach). The type II has a manufacture date of 1/18/89. There are 3 electrical connections along the back of the (small) circuit board - one labelled +IGN, one labelled GND, and the other labelled in some sort of oriental script (I believe that this is +12VDC, IIRC). You shouldn't have any trouble making the adjustment - there is only one trimpot on the circuit board (while posting late last night, I couldn't remember if there were one or two trimpots). The only reason that I used the frequency counter along with the function generator was to make sure that I had the desired frequency going into the circuit - if necessary, I suppose you could trust the markings on the function generator (the one I used was a bit old, and had not been calibrated recently, hence the extra check that I did). I will work on the Tech Article for this (probably this weekend), but I hope in the meantime that this info will work to get you going.

djinn,

Am I correct in assuming that the IGN connection goes to the coil and the 12VDC goes to the truck's main engine switch?

Tnx again
---------

John

 

[djinn]

djinn,

Am I correct in assuming that the IGN connection goes to the coil and the 12VDC goes to the truck's main engine switch?

Tnx again
---------

John

Yep, that would be it. The +IGN is the pulsing signal that gets converted to a voltage by the freq/voltage converter chip - this is where you put the signal line from your frequency generator (along with the GND terminal). The GND is ground for both signal and power, and the 12VDC terminal provides power for the circuit. I hope this helps out some!

 

[SV1CEC]

Yep, that would be it. The +IGN is the pulsing signal that gets converted to a voltage by the freq/voltage converter chip - this is where you put the signal line from your frequency generator (along with the GND terminal). The GND is ground for both signal and power, and the 12VDC terminal provides power for the circuit. I hope this helps out some!

It certainly does my friend, it certainly does. All that remains now, is to find a way to have both tachometers connected, so I can compare the old one with the new one, as I calibrate it.

What I plan to do, is to put three small screws with nuts on the back of the cluster, where the tachometer connects to the circuit. Then from those screws I can run two groups of wires, one to each tachometer. In that way I'll have the two of them side by side and do the adjustment.

When you do this trimpot adjustment, I assume that you change the whole scale of the gauge, i.e. if both tachometers are showing 900 rpm at iddle, can I expect they will show the same at say 3000 rpm and 5000 rpm? Of course I'll check at least at two rpm indications, just asking.

Thanks djinn, much appreciated.

Kind regards
----------
John SV1CEC

 

[djinn]

It certainly does my friend, it certainly does. All that remains now, is to find a way to have both tachometers connected, so I can compare the old one with the new one, as I calibrate it.

What I plan to do, is to put three small screws with nuts on the back of the cluster, where the tachometer connects to the circuit. Then from those screws I can run two groups of wires, one to each tachometer. In that way I'll have the two of them side by side and do the adjustment.

When you do this trimpot adjustment, I assume that you change the whole scale of the gauge, i.e. if both tachometers are showing 900 rpm at iddle, can I expect they will show the same at say 3000 rpm and 5000 rpm? Of course I'll check at least at two rpm indications, just asking.

Thanks djinn, much appreciated.

Kind regards
----------
John SV1CEC

Quick update - I hooked up the type II tach that I have here, and the markings on the board are not quite right (at least, on mine here). As you look at the back of the tach (with it out of the cluster), there are 3 terminals. From left to right, they should be hooked up as follows: Leftmost is +12VDC, the middle is GND, and the rightmost is the pulse input. Of course, looking at what you are going to do, you don't really need to know which screw gets hooked up to what - just make sure each tach is hooked up as if it were mounted in the cluster, and you will be fine. Adjusting the trimpot should take care of the whole range (at least, it did on my old one!). Making the new tach read the same as the old one should work fine.

Actually, your method is easier than what I did - but I didn't have a 4 cyl. tach to compare with, so I had to do it the hard way! Let me know how it all works out!

- djinn

 

[djinn]

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?

An update to my own post: my math was wrong! (That's what I get for trying to do this by memory alone, after studying all night for a physics test!) On the 6 cyl., 3 pulses = 1 revolution (that much was correct). Setting the function generator to 100Hz (NOT 1kHz!) will get a reading on the tach of 2000 RPM. Adjusting the trimpot so that the reading is 3000 RPM on the tach will calibrate it for the 4 cyl. (Since, on the 4 cyl., 2 pulses = 1 revolution. 100 Hz on the function generator will give a reading of 3000 RPM.) Sorry about that!

 

[Eagle]

An update to my own post: my math was wrong! (That's what I get for trying to do this by memory alone, after studying all night for a physics test!) On the 6 cyl., 3 pulses = 1 revolution (that much was correct). Setting the function generator to 100Hz (NOT 1kHz!) will get a reading on the tach of 2000 RPM. Adjusting the trimpot so that the reading is 3000 RPM on the tach will calibrate it for the 4 cyl. (Since, on the 4 cyl., 2 pulses = 1 revolution. 100 Hz on the function generator will give a reading of 3000 RPM.) Sorry about that!

????

Lemme see, one Hz is one cycle per second, or 60 cycles per minute. 100 Hz is 60x100=6000 cycles per minute. Divided by 3 pulses per revolution for a 6-cyl engine = 2000 RPM.

Divide 6000 cycles per minute by 2 pulses per evolution for a 4-cyl engine = 3000 RPM.

Got it. It took me awhile to figure out how you got there, so maybe this will help others who are as mathematically- and conceptually-challenged as I am.

But if you hook up a 4-cyl tach and a 6-cyl tach side-by-side and just synchronize, the math doesn't matter. (Just have to be sure to adjust the right unit!)

 

[SV1CEC]

????

Lemme see, one Hz is one cycle per second, or 60 cycles per minute. 100 Hz is 60x100=6000 cycles per minute. Divided by 3 pulses per revolution for a 6-cyl engine = 2000 RPM.

Divide 6000 cycles per minute by 2 pulses per evolution for a 4-cyl engine = 3000 RPM.

Got it. It took me awhile to figure out how you got there, so maybe this will help others who are as mathematically- and conceptually-challenged as I am.

But if you hook up a 4-cyl tach and a 6-cyl tach side-by-side and just synchronize, the math doesn't matter. (Just have to be sure to adjust the right unit!)

Which I did this morning and YES, now my tachometer is close enough to the real numbers. It's maybe 200-300 RPM lower, since my trimpot run out of adjustment. I guess if I wanted to take it off and take it to my electronics guru, he could match up the appropriate variable resistor (or even a fixed value one) for my case, but as it is now, it is close enough.

So folks, yes, on a Type 2 cluster, you can adjust the tachometer for either a 6-cyl. or a 4-cyl. engine. That makes it a universal upgrade for those with older clusters.

I would like to thank everyone who participated in this discussion. You've allowed me to use my new instrument cluster without paying a dime.

Thanks gang!

Rgds
----------
John SV1CEC

 

[rsalemi]

And if you don't want to do all this; I still have a 89 cluster for a 4 cyl with metric guages for sale for $25 . NOS cluster - lower the mileage on your XJ - tell people its the repalcement for the Liberty!

 

[SV1CEC]

And if you don't want to do all this; I still have a 89 cluster for a 4 cyl with metric guages for sale for $25 . NOS cluster - lower the mileage on your XJ - tell people its the repalcement for the Liberty!

Bob,

You will probably hear from Eagle about that cluster, but do I remember correctly that it is a Type 1 cluster or not?

Rgds
--------
John SV1CEC

 

[rsalemi]

It is a type II (so described) and Eagle already has one - I have only 1 left.

 

[Eagle]

It is a type II (so described) and Eagle already has one - I have only 1 left.

Bob --

The one I have is a Type 1. Metal clip on speedo to retain cable. Recessed gauge contacts. Definitely not a Type 2 cluster.

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