Tachometer problem

[adam518]

Hey guys. I posted this also at the cherokeeforum, but am still looking for answers. Hopefully, someone has had a similar issue and can point me in the right direction.

 

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.

 

I pulled the gauges & checked all of the connections. I torqued down all of the screws on the back of the cluster that go through the printed circuit into the back of the gauges. I removed, cleaned & reattached the connector at the pcm. I removed, cleaned and reattached the grounds at the block, rear tail lamp, etc.

 

The tachometer does function. The needle raises and falls with rpm, it just reads off by approx. 750 rpm. It sits on or just above zero at idle, but rises immediately if I give it some gas. When I start the Jeep, it shoots up to about 1000-1200 for a second or two and then settles back down to zero. I am stumped.

 

Also, I have no other gauge issues aside from my fuel gauge reading 3/4 when full. All other lights, gauges, switches function as normal.

 

Any ideas?

[HOrnbrod]

Do you have an auto AW4? If so check the speed sensor connector and wiring for corrosion and/or obvious wiring problems up on the left side of the transmission.

[adam518]

Nope...it's an AX15.

[JeffN]

Should wire to the negative side of the coil, speed sensor is for speedometer

[carnuck]

Sounds like maybe dash ground is going bad?

[Zenobian_84]

I had this problem in my MJ as well. The tach would do the same thing you're describing, and then it finally quit working all-together. I cleaned all the grounds, checked all the connections, but couldn't figure out the problem.

 

Decided to then swap over another tachometer, and the problem went away. I'm thinking your tach's starting to experience premature failure? :dunno: Could be wrong, but you could always try swapping over another tach to make sure.

 

Not sure if the fact that my 2.5 AX4 makes a difference in it though, as I'm not sure if it's wired differently for automatics than manuals.

[adam518]

I had this problem in my MJ as well. The tach would do the same thing you're describing, and then it finally quit working all-together. I cleaned all the grounds, checked all the connections, but couldn't figure out the problem.

 

Decided to then swap over another tachometer, and the problem went away. I'm thinking your tach's starting to experience premature failure? :dunno: Could be wrong, but you could always try swapping over another tach to make sure.

 

Not sure if the fact that my 2.5 AX4 makes a difference in it though, as I'm not sure if it's wired differently for automatics than manuals.

 

Thanks. I went to the pull-a-part on Saturday to pick up another tach, but they were closed. I'll try again this weekend.

 

Auto & manual shouldn't make a difference. I do assume 4 cyl & 6 cyl tachs are calibrated differently. It shouldn't be an issue for me, mine is a 4.0 and it seems that 90% of the XJ's I come across are 4.0's also.

[adam518]

It was the tach.

 

I went to pull-a-part this morning & picked up a tach out of a '94 XJ. It now works as it should.

 

On a side note, the Knoxville P-A-P has a 1987 MJ on the lot since 12-27. It is a 2wd. 6 cyl Sport Truck. The rear axle is gone. I took the headliner, tailgate latch, visor hardware and a few other bits.