89 TTA tach seems off??

[kwiktsi]

Anyone ever experience this? The tach in my car is off by about 1/2 when it revs. I will be full throttle, motor winding out and the tach shows 2800-3000 when it shifts.. If I rev it in neutral, it is the same way. Also, it seems to be a bit "jumpy" while I am driving. It will bounce 100-200 rpms almost like if it were missing or something. Any ideas on what to look for? I was going to look for a loose wire or something, but figured I would ask in case it was a common problem. Thanks in advance.
Joe

 

[iroc22]

It is a very common thing for the tach to be a little off from the factory. They can be fairly inaccurate. Though I have never fiddled with mine so I couldnt tell you how to fix it

 

[kwiktsi]

I have seen them off before, but not by half, usually only a couple hundred RPM's .. There has to be something wrong somewhere- a loose wire or something.. Thanks.
Joe

 

[turbota]

Check all of the ground wires on the block (at the rear of the eng. on passenger side of the intake) and the ones under / behind the dash. Have you done any engine work lately ? or Had the instrument panel out for something ?? These things (TTA's) have a "thing" about good grounds.
The Tach is grounded through ckt. #150 (black wire .8mm thick, to grd #200. The feed wire is white .5mm, and runs through the tach filter (which is also grounded) to the tach from the CCC Ignition Module. I think the pin location on the ign. module is "E"
Good luck --- TURBOTA

 

[LT1GMC]

Check the back of the tach itself, most GM cars use a jumper back there to make a tach work on 6 cyl or 8 cyl, if a V6 engine was used on a V8 set tach, it would read low by a good bit. I believe its labeled if there is one back there.

 

[kwiktsi]

Thanks!! I was wondering if it was something like that since that is how it appears to be working. I didn't know GM used a jumper, I will look for it. Thanks.
Joe