The old bouncing tack, 82 SC
 

We've all heard about the bouncing VDO tach, and I've had it myself occasionally. It usually only lasted minutes and went away for months at a time.

Today it lasted for about 5-10 minutes on the way to work, and I decided to stop the car, kill the engine for about a minute, then restart, to see if anything changed. The tach was still bouncing when I killed the engine.

When I tried to restart, I got a week grunt from the starter as if the battery was very weak, and that was it. I checked the battery for loose connections, then poked around the engine for anything out of norm. Nothing.

After a few calls to arrange for a lift the rest of the way to work, and several attempts to restart, it finally fired up as if nothing ever happened. Elapsed time less than ten minutes. It started bouncing a few minutes later, then stopped, and the remainder of ride was uneventful. It started just fine and was perfect all the way home from work at the end of the day.

Those are the clues. What happened?

I'd look into the voltage regulator and alternator.

Try searching these components and "tach".

Dead voltage regulator or dying battery.

Bouncing tach = high voltage = defective voltage regulator or VR at max output trying to charge dying battery

Turn off, slow crank = low battery

Turn off, wait, crank OK = battery charge has recovered through chemical action.

Check the voltage with the engine off and with it running AT the battery and report that number here. My guess is you're like 12.25V at the battery which is only 25% charge, and something way weird like 15V with it running.

When you say 'bouncing tach', do you mean it jumps around without respect to throttle position, or that it jumps when either accelerating and shifting or lifting off? My tach does this too, and I checked the voltage at the battery while running - seemed OK, about 13-14 VDC.
I guess I'm not sure if I have a problem or not.

Classic symptoms of impending alternator and/or regulator failure.

When my car had the symptoms you described, I pulled the alternator and had a local foreign starter and alternator shop rebuild it for $85. Three years later it's still going strong.

I experienced the same thing with my 83sc. Bouncing tach seems to be the "idiot light" for impending VR/alt failure. I had good readings at the battery, but 2 weeks later it was toast. Installed rebuilt alt and haven't bounced since.

OK, I've got a rebuilt Bosch alternator that was installed a few years back by the PO - maybe it's the regulator? How is that tested - just for voltage at the battery? I guess I should search some threads on this...

Without getting too sophisticated, yes, measuring voltage at the battery will tell you if the regulator is going. But also measure with the system off, it will tell you if the battery is on its last legs, which could cause the alternator to do weird things to try to bring it back to 100% charge.

Remember that the alternator was designed to bring the battery back to 100% charge from a single start, which is like a 3% drop. Asking it to bring a battery back from 25%, or around 12.25 volts, will cause it to overheat from doing a job it was never designed for.

So, battery voltage first, then start engine and measure alternator output voltage.

I had a simular problem, it went away when I changed the breaker point.

Your car doesn't have one so it cant be that....

Good morning.Souds like the regulator,but its my guess. You should test it. If its the regulator the problem, I have a new regulator in the box. Good luck.
Daniel

Hey Paul,
I believe the posters are correct, it sounds to me like the alt/regulator is on the way out. Before you condemn it just make sure all the grounds are good including the ones under the car. Give me a shout if you want to look at it at your house. It might also be a good idea to remove the panel in the trunk area and look for some mouse dammage on the trunk side of the firewall.
I'm glad to see you kept the car!

Hi Rick,
Yeah we kept it! Jan's idea, even! What a wife! She loves the cab (always enjoyed open cars) but thinks the SC is too cute to give up. I have to agree.

Funny that this problem has happened on rare occasions before but would only last a few minutes. I thought I was being smart to shut it down while still in bounce mode, but you see what happened. Since it restarted easily only minutes after the first failed attempts, I have trouble believing that battery had been run down that far to be a problem. And it's only a couple years old anyway. Sounds like a connection somewhere...

Paul

Possibly, but given your symptoms (including starting problems) my money is on the alternator or regulator going bad.

Listen, the VDO tachs, all of them, are a monostable multivibrator or one-shot. When the tach receives an 11V square wave pulse from the TD drive terminal on the CDi, that triggers another transistor to turn on and send a pulse of voltage to the meter movement. The more pulses, the higher the needle bounces, like a volleyball.

When the system voltage is too high, such as when the VR is failing, the tach bounces all over the place. It doesn't help matters that on an 82SC the VR is internal to the alternator, located about 2 inches from the hot engine, inside the airguide, where temperatures are way over a hundred degrees. HEAT is the enemy of electrical components.

The best way to monitor this is with a voltmeter