Crazy RPM Guage

The RPM guage on my '82 SC intermittently goes wild, jumping to redline and back or staying at 5000rpm. Obvioulsy the engine is running normally. It also seems that I can sometimes induce this effect by switching on certain switches such as my heater fan, or wipers. I believe this to be a ground issue, but that is only a guess. I would appreciate any advise or similar experiences and fixes.
Thanks
Bob Bosak

Bob:

Your rev counter is an electrical device which is somehow connected to the breakerless points of your ignition. When my 1983 SC did this, I changed the "points" and all was well again. It's a cheap and easy fix....Andras

Hey Andras, how can you change the points on an SC?!?

Many years ago, the tach on my SC once went wild like yours, Bob. The battery failed shortly thereafter. I replaced the battery and that solved the problem.

I don't know if that's your problem, just thought I'd pass along my one experience with a wild tach.

Brian

I see where Brian is going with this one. I
have a 930 now, but my last car (911sc) would
have an excited tach needle sometimes. For
me it would happen , for example, if my car
sat for a while and when I started the car
, due to a lower battery charge I found this
would occur. The battery light would be on
and this is how I would know this. A blip
of the throttle, the light went out and the
tach levelled out. After this it would not
happen again. I feel that a low battery
charge or voltage will cause this problem..

Just had this happen over the weekend. All gauges went berzerk, including the tach . . . all while dealing with a dead battery.

AAA jump started my 911SC, pulled the cables from my battery, the engine ran for about one minute, gauges freaked out, and the engine died.

Replaced the battery and all is well (except that the system isn't charging and will replace the alternator this week).

I'd bet you've got a low battery, if my experience is any indication.

------------------
S. Chris Edmonds
1983 911SC Targa

Thanks for the responses. The battery was the problem, seems like during my last service, the plate that secures the battery in position was not secured. The battery slipped all the way forward (great brakes) putting enough strain on the positive terminal to loosen it. This morning I awoke to a dead battery. Anyway, secured, tightned, jumped; all is well. Once again, thanks.

Brian:

Sorry for the confusion. When I had this problem, I just followed the advice of our dealer's techie at a tech session, and replaced the innards of the distributor (you do have a distributor, don't you?), and the problem went away.

This was some years ago, and I don't remember which actual components I changed, but I could research it for you, since I keep all records of things I have done.

But, as everyone else has mentioned, this tach problem is electrical related, not to the tach, but to the battery or the "points" or something in the distributor. Thus a check of those components will help solve the problem.

Mea Culpa for not being more specific...Andras

Bob,

I had the exact same problem today ('81 911SC). The RPM guage began acting up on the way to work. When I parked, I could not restart the car. Battery was very weak. The cause I think is a corroded ground strap at the battery. I replaced it and everything seems fine.

911SC

Hi,
The same thing has been happened for the past two days. The battery is a few months old, so it couldn't be it...although I haven't checked the straps to it. I did find out that after I cleaned the distributor points free of carbon, the tach no longer jumped around.

Jeff

Jeff:

But Brian just said that you don't have points!

Could it be that you did something else in the distributor? It has happened to me. The tachometer is connected to the distributor, so if something goes dirty or loose in the distributor, you will get a tach doing the St.Vitus Dance.

Anyway, I'm glad all these points (no pun intended) lead everyone to understand that an errant tach is often linked to the ignition systm, and is not automatically an indication of a bad tach.

Keep the shiny side up......Andras

[techguy]

Bob,

I have your exact problem! However, I have a new battery, and a new set of points. The car runs great and just like you the tach bounces around then settles down while driving at a sustained rate. I can activate the tach a bit by turning on fog or main lights. The tach is bouncy but settles down right until 4500 rpm then zing right to 7500. The car is a 72 Targa with a 75 2.7 motor.

History,

I remember the tach bouncing a bit when I bought the car then last summer the car died on a fun run. I put in a used coil and Permatune while troubleshoting my ignition problem. I tracked the problem down to a set of points that went bad and a coil that worked one min then dead the other. With the Permatune and matching Bosch coil that came from a working car and a new set of points the car runs really well...except for the tach. This is sounding electrical to me combined with I am sure is a dried tach dampening mechanism.

[klatinn]

Hi,

From my own experience:
The Min. Voltage limit of operation of the 911 tachs is kind of high. They require a minimum of close to 12V to operate. If the supply voltage of the tach falls below, it goes erratic.
Battery voltage is the first thing to check. Then check the voltage at the + terminal of the tach. Sometimes corrosion in the ignition switch or somewhere in the 12V connections to the tach can cause enough voltage drop to let it's supply voltage to fall below min.

Regards,
Klaus

[scottb]

A bouncing tach is often the first sign that your alternator is going (or has gone) bad.

[techguy]

Thanks guys, I know the wiring is in decent shape since I have been thru almost the entire system. Maybe I will start by having the alternator bench tested along with the voltage regulator. I have been told the voltage regular is on the inside of the engine compartment near the CD Unit...L Shaped Config????

[caliber60]

Always go to the simplest route first without touching your points or alternator. When your computer does not work, it might just be your power plug is unplugged.

[techguy]

You must also work in IT, I deliver enterprise systems training...tell my students the same thing