No Tach after car heats up

[cary]

My tach died this afternoon at the ax. Once the car was over 190. The tach stopped.
After the car cooled down it came back to life. 3.2 Motronic.
Where should I start looking?

[ischmitz]

The engine temperature should not affect the tachometer signal at all. As long as the engine runs it indicates that the DME receives proper signals from the flywheel sensors and is able to generate spark and fuel signals. The tachometer signal is derived from these signals and put out on a separate pin on the DME.

Not sure what to tell you. Check the connector to the DME and the connections at the back of the tachometer. Maybe the tachometer is bad. A scope would help to tell whether the signal is missing when the tach stops working.

Ingo

[cary]

Don't I have a transducer on the side of the gear box?

[ischmitz]

There is no transducer for RPM. The DME receives the VR sensor signals from the flywheel sensors and puts out fuel, spark and tach signals.

The speedometer is driven off the transmission though because it senses wheel rotation speed.

Ingo

[cary]

I've owned this car for over ten years. Have never taken the connector off the DME. Might be time to look at it. Plus I've had the seats out to put it 5PT harnesses. Might have bumped something.

But why heat related? It's done it twice.

Lets say it's stuck at 1500 rpms. I pull the tach. Pull the signal wire. If it stays at 1500, its the tach. If it drops, its the wiring. Safe guess?

[ischmitz]

Cary,

maybe you can better describe what exactly is happening to the tach when it stops working. I was assuming the needle would drop to zero. If it gets stuck somewhere this sounds like a mechanical issue inside the gauge.

I'd bet the needle stays where it is when you pull the signal wire.

Ingo

[cary]

Sorry, the needle stays at 1200 to 1500 RPM's. Key on, key off. No change. Rev the engine the needle doesn't move.

After the car cools down it drops back to zero and revs like normal.

[MysticLlama]

Mine did that for a long time in the summer, I ended up just buying a used one that doesn't have the issue.

On some of them, based on where exactly the needle sits and the clearance of the glass, sometime when they get hot something expands and the needle gets stuck on the glass, usuallly near the same point, mine was in the 3500-4000 range.

Grabbing onto the bezel and pinching it from both sides will sometimes make it pop loose, but it's a temp fix at best.

[adam911]

Similar thing happened to me once, but with the low tension wire from the coil. When the engine was hot the car wouldn't run and you didn't get a spark. Turned out there was a break in the wire, but not the plastic sheath. When it got hot the plastic sheath expanded and opened a gap in the wire. Took quite a while to work out what was going on.

Could be a similar hting with your tach wire.

Just a thought.

[cary]

I think I'll run it by Portland Speedometer and have them give it a once over.

[ischmitz]

Good idea Cary. BTW, I have used North Hollywood Speedometer in the past. Real nice work and super-fast turn-around.

Your issue is most definetely mechanic and in the tachometer. I can only guess that the heat from the back of the instrument cluster gets to the tach and makes something bind.

Ingo