940 RPM needle jumping

1997 945T.

I recently noticed that the needle of the RPM meter sometimes jumps back and forth when keeping steady speeds.. I only noticed it happening between around 2.300 and 3.000 rpm. It jumps up and down by around 300rpm or so.

I replaced my CPS and fuel relay few weeks ago, because my car sometimes only starts when giving it a bit of throttle (didn't solve the problem - thread here: http://forums.turbobricks.com/showthread.php?t=333280)

Could both those problems be related and maybe a sign of a failing power stage? I read on a few threads that this could be causing issues like that.

subbed, i am experiencing the same issue, minus the starting issue; my tach will go to zero once the car is at idle

I suspect failing power stage.

Thanks. I did some reading, but I want to understand the power stage a bit more.

So, if the RPM reading comes from the CPS, what part does the power stage play in influencing the RPM reading and how? The car does not suffer from any misfires and the actual revs are steady while driving.

Try unplugging and cleaning the terminals on the power stage and connector first and plug back in. If that improves things, you probably just had some corrosion on the contacts.
Dave B

Another thing to check is the tightness of the coil primary terminal connections.

Try this, turn the car off, disconnect the battery and wait a couple hours. Reconnect the battery, and turn it on. Does the idle swing like crazy for a few seconds? If it does you have some basic ignition issue, plug gap, wires, clogged injector, etc. and the computer has adjusted your timing to compensate at lower rpm.

^LH 2.4 is going to do that no matter what. The computer has to relearn everything. When I swapped out the dead battery on the '92 when we first got it, same thing happened. Engine ran a bit randomly when first started, but then settled down by the time I got back home. You don't have to disconnect the battery, tho. Just pull the fuse that provides power to the LH-Jetronic system.

With respect to the wiring on the coil, make sure it's tight, but while you're there, take a wire brush (or wire wheel on you Dremel, set at the low speed) and clean the terminals up a bit, both the male fast-on terminals on the coil itself, and the female terminals on the wires. Apply a small amount of Ox-Gard to the terminals once they're reattached.

Another thing to do is to check the grounds. I once had a 242 that would lose tach output while running (would also stop accelerating at the same time, but develop a high idle in neutral). Turns out, the ground screw at the relay bracket had corrosion on it. Ran the wire directly to the battery, since it was long enough and I figured that I might as well prevent it from occuring again. Problem was solved.

Thanks so far guys, very helpful. I will try all of that on the weekend.