Hello All,

It seems like there have been some issues with bouncing tachs over the years, but a lot of them seem to apply to the 1990s models. In my case, I own a 1991 (with ECMLink) and noticed this happening some time ago, but forgot about it. The drive on the way home revealed the tach bouncing around if I kept the RPM steady at about 4k. I will attach the video to this post. I will check the wiring this weekend, but was wondering if anybody has dealt with this lately and have some knowledge that could be shared with me. As part of troubleshooting a different problem, I have tested and replaced the coils, PTU, and CAS. I've checked wiring and nothing stood out, but clearly I possibly missed something since this is acting up. Any ideas will be greatly appreciated.

Thank you,
Oscar

Tach is driven by the output of the PTU on the 91 and later cars or by the tach gate on the coil assembly on the 90 and earlier cars.
It's a direct connection to the gauge cluster on the 91+ cars and via a noise filter on the earlier.

The ECU RPM input on all generations is from the CAS sensor (I forget if it's the crank or cam that drives the ECU interrupt).
There is a condenser (capacitor) on the power feed to the coil/PTU and it's job is to filter noise and stabilize the voltage since there are large current pulses from the charging and firing the ignition.

So beside noise it's also possible that the gauge cluster at fault. Since you have ECMLink, do you see the same bouncing in it's logs?

Steve,

Thank you for the reply and all the information. I looked at the wiring diagram for the 91+ cars (mine is a 91) and I see the condenser that you are talking about. I have replaced the coils with a new one while I was troubleshooting a different issue. That's why I don't think that is the issue. I can take it out and test it out again to see if it's good.

Unfortunately while I was driving and saw that issue, I wasn't logging with link. I haven't driven it since either. I can tell you that the engine didn't feel like it was bouncing around and it felt pretty steady. However, I was hearing some slight popping coming from the exhaust. that is part of another issue.... Unfortunately

Oscar