Wild and crazy tach

[Stephen T]

I could use a little help trying to resolve an issue I am having with my 72 S. The tachometer will occasionally swing wildly for a couple of seconds and then be fine for a few minutes then go nuts again. It is most noticeable at lower RPM’s, (the needle will swing the wildest) but will also happen at higher RPM,s (but the swing is less violent). The car runs great and this phenomenon seems to have no effect on the performance in any way (but it bugs me).

A little history that may shed some light on this subject; when I first bought the car, about three months ago, I had an problem where the car would not idle. If I let the RPM’s drop below about 2k the car would stall. I had a guy who is very 911 knowledgeable look at the car. Before working on the car he asked me one question… “does the tach needle jump around?” I told him it did and he said it was most likely the points not opening enough. So he adjusted the points set the timing and the car has run fine since. But the tach still jumps and the idle is still low but the car does not stall.

If anyone has a theory on this please share it/them with me.

Thanks,
Steve

[Steve@Rennsport]

Steve:

Erratic tach readings are usually indicative of dirty points.

Cut some business cards into 1/4" wide strips and spray them with some electrical contact cleaner before you drag them through the points while they are closed.

That usually does the trick.

(FYI..Installing a Pertronix Ignitor eliminates the points and takes care of this once and for all)

[Mr9146]

FYI..Installing a Pertronix Ignitor eliminates the points and takes care of this once and for all

Unless the adapter plate the Pertronix pickup is mounted to comes loose ... then the "Disney Clock" tach symptom will return.

Ask me how I know ...

[Stephen T]

Well, I pulled the points gave them a good cleaning and reinstalled them with no change in the tach performance. Is there another place I should be checking?

Steve

[37yrold911]

the next thing to do is to inspect/clean up all the grounds near the points/trans/motor/gauge...

[tp67911s]

Everytime I have had your problem it has been the ground(s).

[CamBiscuit]

I've checked my earths and have installed a Pertronix and it still bounces around....

I think the final step will be to send the tach itself off to be reconditioned.

[210bhp]

Steve
A friend of mine had a similar problem recently and did what you have done so far. The problem only resolved itself with a new set of points.
Regards
Mike

[304065]

What is the voltage measured at the battery with the car running? What is the voltage measured at the battery with the car off?

A jumpy tach is a symptom of either a defective ground, or a bad voltage regulator, or bad wiring somewhere. As these cars continue to age, oxidation in the terminals gets worse and worse, resulting in high-resistance connections and voltage drops, resulting in more heat in the wires, increasing oxidation, and so on. . .

Get the voltages and post them here. 13.8 volts at the battery with engine running, and at least 12.6 with engine off are a good start. Also, check the:

Ground strap between engine and trans, both ends;
Ground strap between engine electrical console (the black panel to which the CDI is mounted) and the body sheetmetal
Ground strap between pin "D" of the CDI unit (a 1/4" male faston terminal" and ground
Black wire with purple stripe from the points to the tachometer-- this is the tacho drive wire

The VDO tachs are what EE's call a monostable multivibrator or one-shot. When the tach senses a high-impedance pulse in the "C" circuit from the points it means the points have closed to charge the CDI. Current in that circuit isn't very high, only about 420 mA, which is why the points don't wear out on a CDI car (post-'69 E and S and later T) like they do on an SWB-- there's simply not that much current flowing. Anyway, when the pulse is sensed, it fires off another transistor that sends a pulse of voltage to the tach which bounces the needle. As the revs rise, there are more bounces to the needle, which bounces it higher, like hitting a volleyball, and the tach indicates higher.

When the voltage is off the levels at which the internal transistors fire off are changed, and the amount the tach needle bounces is different than spec. Therefore, wobbly tach. If the voltage regulator is putting out a rock-solid 13.8 or so volts this should not be an issue.

It could be possible that your tach is on its last legs but I really doubt it. Check all those connections and test and report back here.

Good luck!

some fun reading here: http://www.early911sregistry.org/for...ad.php?t=10817

[Stephen T]

John and others,

Thanks for the tips. I have my homework cut out for me. I will play with this over the week end and post back what I find.

Steve