ahenderl

The tachometer on my '91 MJ seems to be having issues.

A couple of weeks ago, I was driving on the interstate at a steady 2200 rpm and noticed the needle would drop to 1500 or so for a minute or two and then pop back up to where it was supposed to be. It did that a few times and then seemed to work as it should.

This weekend I was driving and noticed that at highway speed, my tach showed about 750 when it should have been more like 1500. I pushed the clutch in and let the motor settle to idle and the tach dropped all the way to zero. Even though the needle is off, it does go up and down as I rev the motor. It has never completely quit, it just reads wrong.

Any ideas?

I assume that I have a bad ground somewhere. Does anyone know specifically where the tach circuit actually grounds in a '91?

Thanks

cruiser54

Try this:

Improving the Instrument Panel Ground
The ground point for the complete instrument cluster on your XJ or MJ is located up under the driver’s side dash. If you lay on your back and look up under there with a flashlight, without wearing a hat, you will see a black wire attached to a shiny piece of metal almost directly above the hood release ****. The screw will have either a ¼" or 5/16" head on it.
This ground point is responsible for handling the ground circuit for the following items: Dome lamps, Seatbelt and key warning, trans comfort switch, wiper switch, headlamp switch and delay module, fog lamp switch, cargo lamp switch, all instrument panel grounds and illumination, power windows and door locks, cruise control dump valve, and a few more things.
The problem is that where the ground point is located does not have a good contact with the chassis where the ground should be. The solution is simple.
Make up a jumper wire with #10 gauge wire about 10" long. On one end, crimp on a ¼" round wire terminal. On the other end, crimp on a 3/8" round wire terminal.
Remove the screw from the existing ground wire and attach the small terminal of your jumper so that the original wire and your new jumper share the same attaching point, one over the other.
Look above the driver’s side plastic kick panel just forward of the top of the hood release ****. You will see an 8mm stud there. Attach the large terminal end there with a washer and nut over it tightened securely.
**Special note for Comanche owners: Make your jumper wire 12" long and attach it on the driver’s side kick panel close to the fusebox on the 8mm stud.**
　
Revised 11-29-2011

ahenderl

Thanks...I'll give it a shot.

kissthis66

My guess is the screws in the back. I had the same issue with my speedometer not working. Pull the cluster and you will see that there is a plastic strip that carries the current to ur speedometer , tach, oil volt water etc. tighten all those screws back down. What happens is the heat and cold will make the plastic expand and contrast but the screws being metal wont do the same so over time they lose connection. I tightened all mine back down and it all started working like normal. It will take all of 20 min to pull it, tighten them and toss it back in. It's an easy thing to do and only requires a screwdriver.

ahenderl

OK. I pulled the cluster and all of the screws on the back were a little loose. I put it all back together...still doesn't read right.

I then made a jumper wire, ran it to the ground under the dash and grounded it to the door jamb...still doesn't read right.

Any other ideas?

kissthis66

Maybe the module is going out? from what I read its located behind the driver headlight???

http://alarmsellout.com/support/diag...01991-2001.pdf

Just a guess but hope this possibly helps

Norsk62

Also check the signal wire from the ECU. I've seen these wires get damaged causing the same symptoms you are experiencing.

ahenderl

Thanks for the link. I am not sure what they are referring to as a module? They are right that the tach wire is gray/blue.

I did pull the plug from the PCM. The connection appears nice and clean. I cleaned what little dust there was and reconnected it.

I cleaned the engine block grounding stud, steel wooled all of the connectors...and still..no dice.

Basically, the tach has three wires that operate it...an ignition hot, a ground and the signal from the distributor. It does function and moves up and down with rpm, so I assume it is getting a signal.

I don't know...maybe it is just going bad. I think I may pick up another tach at the pull-a-part and see if it works better.

ahenderl

For anyone else out there experiencing this problem...it was the tach.

I picked up another tach at local jy for right at $11 and it works great.