Goofy Tach Behavior
 

'79 SC. Yesterday, drove 20 miles, stopped for an hour, drove back. Halfway back, the tach became frozen, unmoving, at 3000 rpm. Stayed that way even after the ignition was shut off. Overnight, the tach apparently returned to Zero.

This morning, the tach operated normally for ten-fifteen minutes, then froze around 2000 rpm. I just went out to check, and it has returned to Zero.

I wonder if this problem is just a sticky mechanism. In other words, maybe not electrical but rather just a sticky needle mechanism.

My '79 SC does the same thing. I finally concluded the center of the needle is interfering with the plastic face. Next time it happens, give the face of the gauge a couple of flicks with your finger, and see if that "unfreezes" it. Not really sure what the long-term solution is.

My 87 was doing the same thing and I tightened the battery ground strap and lo and behold..., fixed. It needed tightening too. Good luck.

87 blk coupe

I cleaned the battery clamps and put a fairly fresh, fully charged battery in this car, and the symptoms were still there this morning. Like Jonbot, I suspect there is a mechanical interference with the needle.

Soup - I had a flakey, twitchy (electronic) tach in my '76. I traced it to an intermittent ground in the engine compartment wiring harness. Fixed that and tach is now dead nuts solid. I agree w/the above though, sounds like you have a gauge problem.

What's all this about a '79?

I found a car. Crazy, how important a 911 is to my psyche. I feel like Samson with his hair back.

'79 Euro SC. Polar Silver. Respray. Stunning ten-footer, but the prep work left something to be desired. Couple of rust bubbles.

Black interior, in decent shape. Engine seems pretty happy. Tranny is slightly balky. 164K miles. SSI's. H4's. Strut brace. New shocks.

Fairly well maintained, but in need of my style of TLC. Everything needs adjusting. With the parts I have from my old car, this car will be drinking from the Fountain of Youth over the next few months. And then I will have a bonanza of parts to sell.

Congrats on finding a new car! Where are the pics?

My tach didn't stick, but when I had an intermitant dieing alterator, the tach would become eratic and bounce around when the battery voltage got down to 9.6 - 9.8 volts.........check your voltage?

Superman, my tach just did it again today after sitting in the sun for an hour. I'm now convinced it has to do with heat. I suspect the heat causes the plastic gauge face to expand just enough to rub against the center of the needle. I really hope there is a way to fix it without having to take the tach apart.

Surely check voltage, as it can be a culprit. I've seen a flaky electrolytic capacitor (internal) cause this. Otherwise make sure voltage is good (no overcharge/undercharge condition).

All of this, assuming it's not mechanical (which it certainly could be),...although not as often as electrical (my experience).

Best of luck,

why dont you switch the tachs?

i seem to remember a buddies car doing the same thing. it was a connection on the back of the tach. congrats on the new car. you know better than not posting a pic..

Not to hijack the thread, but my tach only works when the throttle is open. Foot off the gas, and it goes to zero. Does this sound like a ground problem or something more involved?

jonbot......Superman is safe. He lives in the land of "NO SUN" and lots of rain. Maybe that is it the tach is full of water and freezes up solid. Thats it, yup! Or wait maybe its rust, either one, rust or ice. Superdude.............back when Iris was a brown car and all stock she did the same thing. After the rebuild and having the guages changed to white and all it has not happened again.

Randy Jones
1971 911 "Iris the Slut"

+1 on the voltage!