An other 930 bouncing tachometer

[Plangis]

I am new to this forum and I read a lot of comments about what causes a tachometer to bounce. On my car, the problem is clearly related to engine heat. Each time the engine heat needle passes the first white line, the tachometer starts bouncing and stops. It will come back alive as soon as the engine temperature falls below that critical level.

To solve the issue, I first sent the tach to North Hollywood and they sent it back mentionning that it was working well. Next, we changed the CD box. No change. Then, we changed the voltage regulator and the wiring behind the alternator and the fan. Same result.

I really dont know what to do next.
Any suggestion ?
Thanks a lot for your help.

[dap930]

Remove the 14 pin connector at the rear of the three fuse panel, clean any corrosion from the pins and reinstall. See if that helps.

[Plangis]

Super !
Thanks

[Groovie Ghoulie]

Mine did this when the CDI box was about to die! as the car got hot the CDI box would start acting up. When it goes, it goes. car will be dead on the side of the road!

You can get it rebuilt by ecudoctors with a 5 year warranty for about 500 bucks.

[Groovie Ghoulie]

If you have big money you can get a more modern unit from classic retrofit

https://www.classicretrofit.com/collections/cdi-ignition/products/911-cdi-6-pin?fbclid=IwAR0K_UxTmHqVC-9Nvely6t2_Np9zS3_UHTdR21R8JQmoBvW-hKQaTy3i0bo

[flightlead404]

Or you can buy the stock 930 cdi I have on the shelf lol. I also have two coils, a distributor and most of a harness

[timc]

Mine (tach fluctuating) was due to faulty Alt years ago....

[pkabush]

Originally Posted by timc

Mine (tach fluctuating) was due to faulty Alt years ago....

Same here. A dying battery or a bad alternator will cause your tach needle to bounce. If you put a volt meter on your battery and have someone rev the motor you should see the voltage go up to around 14V. If the volt meter doesn’t move when the engine is revved than your alternator is faulty.

[911TerryT]

Originally Posted by timc

Mine (tach fluctuating) was due to faulty Alt years ago....

Same for me. I had to pull a remaned Bosh alt and have it gone thru. From some research I don't think I am alone with a reman Bosh going bad early....

[JacobS911]

had a bouncing tach recently as well.. turned out to be lose connection on the ground terminal of my battery.. hopefully something as simple as that on your end!

[timc]

Originally Posted by timc

Mine (tach fluctuating) was due to faulty Alt years ago....

So I guess I jinxed myself, as mine started fluctuating again, as well as just hanging at 5K. Turns out my battery was going bad, reading 10 volts when charged, and finally just completely died. Replaced it and tach is now normal, with good battery voltage.

[timc]

Originally Posted by timc

So I guess I jinxed myself, as mine started fluctuating again, as well as just hanging at 5K. Turns out my battery was going bad, reading 10 volts when charged, and finally just completely died. Replaced it and tach is now normal, with good battery voltage.

Bouncing returned , although in a much more limited form. Never a problem at idle, and only after driving car for 30-45min, then it would just hang at around 2500-3500 or head straight up to 5-6 and remain. My guess is the Alt was sending spikes when hot. It’s been 15years and over 100k so I just went ahead and replaced, this time with new Valeo. Last time was new Bosch. Also gave me a chance to clean up engine bay.

[RSstop]

There is a remote chance that If your 930 is running the emissions from 86-89 with Speed relay under the drivers seat, when this malfunctions it can destroy the tach signal at the CDI box. Most 911's run directly between the CDI box and tach. The emissions 930 also runs a signal wire to the speed relay.