TheDon007

I have a pesky problem on my 1981 5-speed, and I am a little stumped. At start up, the voltmeter on the dash pegs up to about 14v, and slowly drops to 12v and sometimes a little under 12v with about twenty minutes of driving. I have the idea in my head that the longer I drive the more I am draining the battery. However, the battery does not drain down.

I measured the battery, 12v reading. Measured the reading at the alternator, steady 12v. I let the car sit for a day or two and took the measurements again, with the same results.

I am about to take the dash apart to get to de-bug my climate control, and I thought I would check the leads, wiring, and/or connectors at the indicator. My first thought would be that if the voltmeter might have a nut loose at the connection point.

How do I check the meter to see if it is good/bad? Also, if the voltmeter is in good shape, where else should I be looking?

I am away from my book so I do not have access to the schematics or diagrams.

As always, thanks for the assistance.

the flyin' scotsman

007..............the 12v measurements you've taken; are they with the engine off?

Starting with a good DVM read the battery engine off; should read 12 ~ 13v.

Start engine, read battery again; should read +14v.

Dennis Wilson

The voltmeter on your 928 uses the power source that feeds the other instruments, lights etc instead of having a direct wire from the battery. On my 78 there are 7 connections between the battery and voltmeter. A lot of resistance can build up during operation. Cleaning of grounds, fuse and relay contacts and the multipin connector to the instrument panel should help if not entirely correct this problem.

Dennis

docmirror

Actually what's happening is quite normal. when the battery is used to start the car, it is drawn down a bit by the motor. Once the motor is done, the alternator comes online, and notices a fairly low battery voltage, so it begins charging at a fairly high rate. This allows the voltage regulator to operate 'wide open' if you will, for a few minutes. The sense for the dash voltmeter comes off the 15 buss through fuse 12, then H6 on the CE, then 10R and 5M on the dash connector. So, you're getting a pretty raw sense of the voltage from the alternator, and well before it reaches the battery + terminal. As long as it doesn't go above 15VDC and it relaxes in a few minutes, don't worry about this.

You may have a battery that's getting weak, and will start the car fine but has little reserve capacity. You may want to use a hydrometer and check you electrolyte condition. It can be done with the battery in the car. The voltmeters are also notoriously innacurate, but you can do a very primitive calibration from ground to 5M on the pod witha DVM. Turn on the key and the volmeter should be very close to the DVM. Then start the car, and watch the two for similar readings.

<edited for puncuation>

Doc

SharkSkin

I was thinking the same thing as Doc as I read the first post. The variation as you run the car sounds normal. So does the inaccuracy. In my particular case, I found the voltmeter itself was so accurate as to not be worth bothering with -- the real problem was connections between the battery and the meter.

I posted some info on refreshing the connections in the cluster here.

TheDon007

thanks for the input. Question on the dash meter. I get what I call some "needle wobble" at times, like the needle is bouncing, although very minutely. That was why I was wondering how I could check to see if the indicator itself was failing.

neilh

Follow sharkskins cleaning procedure.. you will find two small brass nuts on the connections to the voltmeter ( and others) remove the nuts, clean the contact surface on the printed circuit strip, and reassemble.
You will be amazed at how much better the gauges read!
Neil

SharkSkin

Bouncing like a fraction of a needle width? Could be just the regulator, or a circuit that is not a continuous draw, like turn signals. IIRC there are vacuum solenoids in 80+ cars that could cycle on & off, maybe causing slight variations. What is the period of the variation, roughly; 1/2 second, couple seconds, ???

docmirror

Don, this is yet another indication of a weak battery. Every little load causes the needle to jump slightly. It also could be a resistance in any of the circuits that loads the system moderately like the AC, lights, etc. Or, also could be a resistance in the connectors of the back of the voltmeter that need cleaning. It's a 26 YO car, and resistance happens.

Hey, I want a drive the King Air next time you're in DFW airspace. Failing that, I'll take a backseat ride in that thing in your avatar if they make a 2 seat trainer. I have a MB ejection seat cert from about a billion years ago.

Doc

TheDon007

Doc- now that they lifted the Wright Amendment, it's cheaper to go Southwest to Big D.

Not to go off topic, but this will give you an idea of 3,900 rounds per minute. When you watch it early on, you can not only see the aircraft but the pilot shake with the first burst from the 30mm.

http://www.youtube.com/watch?v=zhPwaApe4Rk

docmirror

Anything's cheaper than a King Air. We had the little brother of that gatling gun in the AH-1L. 20mm three barrels. I couldn't hit **** with it. 'Course my gunner had coke-bottle glasses, maybe had something to do with it. The whole bird just shook like a rag when he fired that thing.

Doc
37 Cord Beverly
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63 Stude Avanti
70 Lambo Espada
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83 Ferrari Mondial cab
90 928GT
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