Intermittant tach on 911SC

[anthony]

In the last 600 miles my tach has stopped twice for brief periods of time. One time it was for a few miles. It was dead at zero. This last time it stopped at 3K rpms and stayed there for 10 miles.

Is the tach itself dying or should I look for a bad connection? Does the fact that it stopped at 3K mean anything?

[anthony]

Anyone have any ideas on this?

[Hetmann]

Mine does this too. Has done it for the whole time I've owned the car, 9 years. I've never figured it out. Seems to be worse when the weather is hot. Every time I'm about ready to pop it out for an inspection and cleaning it starts to work again.

[Westy]

Anthony;

Yep. Went out for a cruise Friday and Saturday, and both times the tach froze. Also, both days were about a hundred degrees. It only happens when it's hot, so obviously it's the expansion-heat issue. I looked at the clear cover, and it appeared to be a little convex. If that's the case, then I might be able to reverse it, however I'm not even sure the cover touches the needle. The expansion could very well be from some other part.

I've had my car for around six months, so I've pretty much learned what the RPM range sounds like, but I agree that it would be nice if the tach always worked. This is just something I haven't gotten around to fixing. There was a thread 4 or 5 months ago about this (I forget, I might have started it) and others said they had the exact same issues. Germany don't get this hot, so it was probably never addressed over there.

B.

[anthony]

That's interesting. It was a really hot day the other day. Next time it happens I'll double check and see if it looks like it's hitting the glass.

The thing is that it just started randomly working when it was still hot. It was only stuck for 10-20 miles out of the 200 I drove.

[Westy]

Yeah, mine freezes, then unfreezes (is that a word?). It doesn't always happen. I also had a thought that because of body flex, that might be a contributing factor. However I'm not a metalurgist expert, so that opinion is free of charge!!!!

[RarlyL8]

Mine has done this for years. It is progressively getting worse.
I have asked several mechanics about this. All have seen it but only a few dealt with the issue directly.
The most reasonalble explaination is that the diodes and other electricals become heat sensitive as they age. Electrical conduction is compromized. When the unit cools off it works again.
The fix is too chuck the tach in the garbage and get a new one.

[justinbaldwin]

i have a similar problem but everytime it happens my battery shorts and loses all power very quickly once i've turned it off.

the tach sometimes drops to zero other times it just goes wild even though the engine is consistent.

i was told that it is caused by a relay sticking in the alternator, and that the solution is to drive over a pot hole, or drop the clutch and give hte gas pedal a good poke. this seems to free up the relay, (if this is indeed the cause) and the instruments all work properly again.

the one time i left the problem to continue when i was on a long drive the battery eventually ran down and the car stopped.

hope this is of some help

regards

justin

[Jeff Higgins]

This started happening in my '72 911T a while back. It turned out to be the points. Although the car ran fine, the point gap, and therefor the dwell angle, were way off. I re-set the dwell and the tach worked fine (and the car ran better). From then on when the tach started going crazy I knew the points needed attention. I have since switched to the Pertronix Ignitor and eliminated the problem altogether.

[justinbaldwin]

what you say makes more sense than my relay story, which, by the way, i was told by the porsche dealer who sold me the car!

i'm sure teh car isn't running completely at it's best so i 'll look into this.

many thanks

justin

[konish]

Justin,
You car does not have points...not sure if you were responding to Jeff's reply...I'm sure you already know that.

Anthony
My tach went crazy when the POS permatune started to go south. It was heat related, but it was also followed by a bad running engine not too long after the first symptoms showed up. Probably not the cause...just more food for thought.

R/
Dustin

[anthony]

My car is running perfectly and I don't think I have a Permatune. I'm just glad to know that I'm not the only one and that it's the tach and not some other weird electrical glitch. If it gets a lot worse it seems like a new tach isn't prohibitively expensive.

[Jeff Higgins]

I wasn't sure whether SC's still had points or not; thanks for clearing that up, Dustin. Do they still send a signal from the distibutor to the tach, even if they don't have points? I guess that was more what I was trying to get at, that the problem may not lie in the tach itself, but rather wherever it gets its signal.

[Westy]

Jeff, that's actually an interesting concept, because whenever my tach stops, it's always at the exact (almost) same spot. If the lens was bowing, I would imagine the point of stoppage would would vary as the expansion/warpage would seldom be the same. How about the rest of you? Does your tach always stop in the same place, or is it random?

[Hetmann]

When my tach conks it is pegged at zero. Just like it's unplugged. If it is working it works. Its like that either on or off.

[konish]

Jeff,
The SC dist sends a signal to the CDI box via a star shaped rotor and a series of magnets inside the dist housing. The CDI then pimps the coil and it sends the juice to the plugs via the cap and rotor.

R/
Dustin

[slider]

yep...mine has done this also, usually after sitting in the hot sun for awhile. Pegged at zero while driving, so I deploy the time-honored tradition of tapping a gauge when you don't like the reading....and it actually works! A few taps on the plexiglass and eventually it wakes up. Like the constant smell of burned oil from dripping on the heat exchangers, its part of the personality of my car....

[RarlyL8]

The key to all of this is heat.
The tach stops working when the interior gets hot. It begins working again after cooling. Mine will be dead after sitting in the hot sun for a few hours. Driving the car for a few miles cools the interior down and it jumps back to life.
I beleive this is the symptom that the original poster is seeing. SCs seem particularly vulnerable.
Ignition and engine problems manifest themselves in other ways.

[Randy Webb]

clean or replace the points

also start thinking about how much a new CDI box will cost you...

[Paulporsche]

Randy,

No points on the SCs.