Tachometer goes crazy at random

[Trellabor]

Ok, so let me first start by saying that this issue has been apparent for about 2 months and happened out of nowhere on my 1991 SHO MTX with 69k miles.

I originally thought that it was the crank or camshaft sensors, so I performed the 60k service replacing both sensors, water pump and the DIS module along with all new belts, hoses, etc etc. The problem persists after all this has been changed with new parts.

Now, this does not in any way affect the way it runs and or drives. The Tach will be reading RPM's perfectly fine, then all of a sudden it 'freaks out' and will randomly jump to about 4k rpm. If i happen to be at a stop/idle when it occurs then the tach drops to 0 rpm but i can tell the car is still idling perfectly. There is no pattern to it at all, totally random.

Anybody else seen this? I know about the car dying, bucking, etc that is caused by the CPS sensor when the water pump fails, but all of that was replaced on my car and I have NO drivability issue at all. Would the gauge itself fail? It's driving me nuts. Thanks for your time.

 

[Kens1992mtxSHO]

My tach did this but it was accompanied by the engine bucking. Try bouncing the tach of the rev limiter

 

[CademiaX]

I have has similar problems with the speedometer on other cars, it would jump around all crazy. The cause was the last place I would have looked, the battery.

Not sure if that can have the same effect on the tach. I have read about this problem in here before, not sure what the fix was, but pretty sure it was an easy one.

 

[Trellabor]

The battery eh? Thats interesting, didn't think of that. Hopefully that is the problem as it would be a cheap fix, and I really have no clue what else it could be.

 

[sdpatt]

You addressed the sources and assembler for the tach signal: CPS, CID and DIS. The condition you describe sounds more like an intermittent/short circuit on the signal feed to the tachometer than a sensor fault. Is there a local ground in the instrument cluster or is the tach input a two-wire feed? If the CPS signal were intermittent, your engine would be cutting out.Have somebody observe the tach while you perform the "wiggle test" on the CID and DIS wiring and connectors and possibly the instrument cluster ground.

 

[oneqwicksho]

My 91 did the same thing in high school after I did a tune up it was a grounding problem on the coil pack, and I had a short at the battery.

after I fixed those two the problem stoped