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Looking for a 87-90 Tachometer


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In trying to upgrade my gauge cluster to the full gauges, I pulled a cluster from a Cherokee and everything worked except the Tach which burned out when I put power to it. I'm wondering if anybody has an extra Renix Tach I can pick up.

 

Thanks

Romain Tweedy

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Well, I'm really confused now. I pulled a full cluster to upgrade the original dummy cluster. The original salvage tachometer's board broke when I disassembled it for cleaning. The screw was rusted into the circuit board and the board broke before the screw let go. I tried a repair that was cosmetically ugly but seemed OK. When I plugged up the full cluster, everything worked fine... except... the Tach gave readings for a few seconds and went to zero. A resistor clearly burned out on it. Replacing the oil pressure and temperature sensors with senders went smoothly and everything else is perfect.

 

I got a new cluster from 87MJTIM, good man, and the Tach looks perfect. I put it in the cluster and plugged it up and... nothing. The Tach's board looks perfect. I've cleaned the copper contact sheet on the back side and even tried a different one. All the fuses are good, all the contacts are clean and making contact, but there is no tach activity at all. Any idea where to look next? If all else fails, I'm going to put a 4" round photograph of a nature scene over the Tach and call it at that. LOL

 

My MJ is an automatic, but everything I've read says that doesn't matter. The Renix Engine Monitor shows I have RPM so the Renix computer is reading it OK. It's just not getting to that gauge for some reason I've yet to determine.

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The tach signal comes from the ignition control module, and given that you admittedly had a very dirty repair I would expect you probably burnt out that line.  Not sure how it's constructed internally but if you shorted it to either ground or 12v I'm sure it wouldn't be pretty.  Given you burnt out a resistor, I think that's likely.  I think it's supposed to be a 0-12V PWM signal, and you can test it at pin 1 on diagnostic connector D1, but you probably need an oscilloscope to really check the signal.  I'm pretty sure the computer derives the RPM from the CPS and not the ICM, so if the module wasn't driving the speedometer line I don't think that would be reflected by the REM. 

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The Renix models also have two diagnostic connectors on the passenger side of the engine bay, behind the shock tower. One of the connections in one of those connectors is a tach signal. If you have (or can borrow) an idle tach or an aftermarket tach, you can test with that to see if the tach signal is making it as far as the diagnostic port.

 

If you haven't already done so, download the Renix electrical manual. You'll find full information on those two diagnostic connectors in that.

 

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Thanks for the technical support. The electrical system is certainly my weak spot. ΩhmC213 (found it) to C203 is a longer run than my multi-meter leads. I'll have to improvise a longer cable to test the continuity.  scaleless, I'm not sure where D1 is.

 

I don't see any obvious burnt wires. Had that before on a '77 FIAT Spider after an engine replacement and, yes, that was a real mess. Nothing like that here.

I'll report back after testing.

 

Thanks again.

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This should help.  The illustration is from a 4cyl, but, you know, it's the connectors under the yellow caps on the passenger fender is the important part.  The wiring is the same.

 

tuneup_4_cyl_html_3278fc90.jpg.87df30570157fce2c2c75447e213b348.jpg

 

 

As Eagle said, you could connect a known working tach to test.  Don't forget to ground it to the chassis.  This signal comes from C213_C that Ohm mentioned, so you could test from there, too, if you can access it while the vehicle is running.

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I walked through some tests this morning.

There IS continuity between C213_C and C203_12.

The IS NOT continuity between either of them and ground.

Hooking up to D1_1, I can read voltage while running and using the REM2 to read the RPM, I get:

o   Idle @ ~800RPM gives 1.11 volts

o   ~2,000 RPM reads 1.23 volts

o   ~3,000 RPM reads 1.34 volts

 

Next would be to hardwire the Tach to D1_1?

There are three connectors on the Tach, 1st says Ignition (I'm guessing 12v), 2nd says ground, 3rd is unlabeled.

I don't want to blow this Tach if it's good, but I'm guessing I'd wire 1st to battery +, 2nd to ground, 3rd to D1_1 ???

 

You guys have been great so far. I'm learning a lot.

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So, I direct wired the Tach and ZERO.

I have a tach from a '91-'96(?) cluster and hooked it up. The electronics in the cluster are all different but it has the same 3 inputs.

It hovered slightly above zero at idle and idling around 3-4K RPM, that gauge went up to about 1K RPM. The reading is not right and it from a post-renix MJ.

 

Is the new tach I got bad do we think?

 

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19 minutes ago, Romain said:

I have a tach from a '91-'96(?) cluster and hooked it up. The electronics in the cluster are all different but it has the same 3 inputs.

I think you might need RENIX stuff.

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I think it's more likely there's an internal fault in the ignition control module, but we can't be sure if you're not sure that's a good tach.  Given it's not working on either the original tach or the new one, I would bet on the ICM.  If you know someone with an oscilloscope you can test the output at the diagnostic connector to be sure, but since you tested continuity from the ICM to the tach and it's not shorted to ground, it's definitely either the ICM or the tachs.  Good repair shops usually have an oscilloscope lying around, so you can probably ask them to test it for you.  Should be a 12v peak PWM signal, I think.  Kinda noisy.  Lacking that, I would just throw an ICM at it, since they're only like $40.  

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OK. You guys are geniuses. I replaced the Ignition Control Module (ICM) and the Ignition Coil since I was in there.

IT WORKS. You solved it. I'm so grateful, I'm going to wear my ComancheClub Tee Shirt the rest of the week.

 

Now I have to figure out why the fuel gauge stopped working during all this. I haven't begun researching that yet, but I'm so so so close to putting my dash back together.

 

Thank You scalelessΩhmMinuit, and Eagle for the group effort technical support. Of course thanks to 87MJTIM for providing me a functional Renix cluster.

 

You can't see much on the old parts since it's an internal problem, but I put a pic of them just in case/for fun.

 

Romain

 

IMG_0580.jpeg

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