BlazinBWF

I have a 1979 911sc that I have had a few years now and generally during the warmer months it goes into storage and I take out my landrover defender 90 because its a convertible and it can pull trailers. My land rover is currently in the shop so I have been driving my 911. When the car is parked outside and it gets hot in the sun the tach will get stuck and it usually takes a while for it to break free and function normally.

Has anyone else had this problem and is it something that needs to be fixed or is that just the nature of the tachs?

Thanks
BWF

Chuck Jones

I have a similar problem...but in an '87 slantnose turbo I just bought...the tach will jump around a bit when the car is going slower...when I say jump around...it might jump from 2K all the way over to 6K, bounces around a bit.... then smooths out when the turbo kicks in and runs smoothly from then on until the car stops at a light, then bounces around a bit when the car starts up again. I thought that it acted sort of like the old mechanical speedometer. when the speedo cable was getting worn out..but I guess the tach is electronic(?). Any thoughts or suggestions here?? Chuck

theiceman

From what I have read bouncing tachs ( if not the tach itself) can be traced back to bad grounds or a bad alternator. I would check the grounds on the car as it is least expensive first, see if you can swap in a tach , and lestly look at the alternator.

e911

I agree with iceman check the electrical connects - does cost anything and many times this is the problem. Do forget to check the connections on the tach itself. I had a speedo that was jumping around - pulled the speedo and found one of the wire loose. Its real easy to do.

Peter Zimmermann

The bouncing tach, from my experience, is almost always an alternator failing. Unfortunately the bounce usually reflects high voltage spikes, which can damage electrical equipment on board. Put a volt/ohm meter across those battery terminals and check it out at both idle and 2500 rpm. I don't want to be an alarmist, and the problem could be something else, but check out that alternator!
Pete

Carlos Tornero

Hi, just experienced this exact problem with my ‘81 SC. Did you ever figure out the problem?

theiceman

His last post was over 14 years ago so i seriously doubt you will hear back.

Mark Salvetti

The stuck tach in hot weather is due to the plastic (not glass) window expanding and coming into contact with the needle. To test this, use a small suction cup to pull on the “glass” and the needle should free up.

The fix is to pull the gauge and reverse the “glass”. It’s not flat. The inside is convex, which is causing the problem. Reversing puts the convex side facing out.

Mark