JP007

iTrader: (3)

My Tach has intermittent bouncing needle. I already changed the distributor cap and rotor. Also tried 2 other known working gauge clusters. Could it be the ECM or the control module in the distributor. All stock none modified daily driver car. No cuts in wiring harness. Any ideas? It did run kind of doggy a couple of times but usually runs real good. Clean car under the hood.

Thoughts? I did search forum for ideas.

BrianChevy

These cars had bad tachs when they left the factory. Mine was bouncing so bad and was so unfixable that I just put in an aftermarket one.

Here's a link to my "howto" article.
https://www.thirdgen.org/forums/elec...roc-305-a.html

Good Luck!

JP007

iTrader: (3)

I have 5 clusters of gauges and they all work in my other Firebird just fine but in this one they all do the same thing the needle bounces intermittently. I have heard of that issue but so far none of my 6 Firebird owned never had that problem. Wierd. Thanks

BrianChevy

I also would like to point out the following thread that might be useful to you.

https://www.thirdgen.org/forums/elec...meter-fix.html

JP007

iTrader: (3)

Already read that one. Note I took one gauge cluster from other Bird that works fine and tried it and it does the same thing so the tach is not the issue. I have tried other gauge clusters and they work fine in one car but not in the other.

RBob

iTrader: (1)

It is something in the ignition system. Likely bad spark plugs and/or bad plug wires.

RBob.

JP007

iTrader: (3)

All new wires, cap rotor, plugs. New TPS, still no good. All the clusters work just fine in my other bird so they are good. I am thinking coil or the control module in the distributor or last but not least the ECM. No codes either.

JP007

iTrader: (3)

Replaced the coil and tachometer now works great.

hellz_wings

iTrader: (2)

Old thread, but having a similar issue.. The tachometer is jumping up and down but the engine idles steadily at 750rpm.. So it's just not getting a right reading of the actual true engine RPM.. It bounces slightly around 750RPM.. Any ideas if this is the coil, or perhaps the wire that attaches to the back of the gauge? Thanks!

JP007

iTrader: (3)

Actually after all the bull it was the spark plug wires and we replaced all of them and has been rock solid for years. We replaced the dist and coil and it was good for a bit but the true culprit was the spark plug wires.

hellz_wings

iTrader: (2)

Thanks JP. I'll be sure to check those out too.

naf

iTrader: (7)

bad pickup coil in the dist will also cause a bouncy tach. If yours has never been swapped...

RedLeader289

iTrader: (2)

The connector that hooks the tach to the distributor is pretty shoddy on these cars too, replacing that might help.

hellz_wings

iTrader: (2)

Hmm I'll check all of these too.. The wire that goes from the tach to the distributor is the white one? I've changed the cap & rotor on my distributor but the distributor hasn't been changed since I bought the car in 2006.. I think mine has a coil built in to the distributor since it's an 86.. Have to check.

And my bouncy tach is just what I'm seeing, not what the engine is doing.. So wouldn't a bad pickup coil cause the actual engine RPM to bounce?

RedLeader289

iTrader: (2)

Yup, the white one. Yeah, I would think a bad pick-up would make the engine run rough/miss.

naf

iTrader: (7)

An ignition miss can go unnoticed unless it happens at idle. Even a slight miss from a bad pickup coil will cause the tach to bounce.

of course it could just be your tach, but how long has that distributor been in there?

hellz_wings

iTrader: (2)

Just went through this old thread and forgot to answer naf's question: it's been there since I bought the car in 2006 so I imagine it must be stock or atleast very old. I just ordered distributor cap and coil and will change those. IF that doesn't work, I'll do plugs, and if that doesn't work i'll do wires and if that doesn't work then I'll change out the distributor.

KITT1983

iTrader: (3)

Any updates?

hellz_wings

iTrader: (2)

Going to get around to it this weekend.

FormulaEngland

Mine has something similar by the sounds of it.

When sitting in either Park or Neutral with the engine running, the tach needle starts bouncing up and down. Say that the engine's idle speed is 1000RPM from cold, it will then start to bounce between 1000-1500RPM.
When you put it into Reverse or Drive the needle returns to normal and reads the correct RPM.

Pupkin

Hellz_wings, what was the reason?