Volt meter mystery
 

I have a strange problem with the volt meter in my 85, it's reading 17 volts and the red warning light next to the voltage reading is on so I figured that I had a bad alternator. But I just checked it with two seperate volt meters and they're both reading 14.5 volts at the battery and at the alternator. What could be causing the volt meter in the car to read 17 volts and turn the warning light on when the actual voltage at the alternator and battery is only 14.5?

Could be that the dash itself is messed up, although I would suspect that it would read zero if that was the case. With the car off, does the battery read 14.5 volts? That seems high for just sitting. Without turning the car on (just ignition power to light up the dash) what does it say? If still 17, then I doubt the alternator is the issue. It could be a bad ground in the dash, or maybe something burned out in there.

Edit: It could be a fuse is blown. I don't have my shop manual with me, but I can check it late tonight if I remember. This seems like something that would be mentioned in there. If it isn't, I'd assume the dash itself is failing.

I didn't check it with the car not running but I will when I get back to where the car is at (I do a/c work and I'm on a call right now, the car is in the parking lot at the office). I'll post what the meter on the dash reads with the key on and the car not running and what the battery voltage is with the car not running when I get back to the office later on.

Ok I just got back and checked it again with the car not running. The volt meter on the dash says 13.5 battery volts and the volt meter I checked it with says 12.0 battery volts. There is definitely something going on with the volt meter in the dash but what would cause it?

My dash does not read the same as a conventional voltmeter. My dash will usually read lower than what a real voltmeter does. However, that could just be due to load on the battery at the time, since I believe that the dash does not have its own parallel run for measuring voltage.

Perhaps a fuse is blown? Those are easy to check. I can check the shop manual's diagram/troubleshooting some time soon.

Since the dash reads 17 when the car is running, but a more normal value when not running, I suspect that the issue may not be fun to find.

Try starting the car with the serpentine belt off. What does the dash read? If it is 17 again, then the issue is definitely not the alternator.

Per the manual, test voltage at Pin D5 for the dash connector. This should be battery voltage. Pin D16 and C16 should be 0. Key on, all three should be battery voltage. If one of them aren't, the issue is in that circuit and not the dash. If they are all good, GM says replace the dash. The LCD fuse, cluster fuse, and Tail fuse, and INST fuse block all part of the dash circuit, in some capacity.

Check the voltage at those pins (will require pulling dash, isn't too hard) to ground, and report back what you find. Don't test resistance for anything, the dash/telltale center have solid state components and could be damaged if you test resistance. Just measure VDC.

I suspect either your dash or alternator are goofed up.

Ok I tried it with the alternator disconnected and the voltmeter on the dash reads 14.6 volts which is pretty much what it reads when the engine isn't running. The entire dash was real dim until I reconnected the alternator, I didn't think it would dim that much with it disconnected if that means anything. I haven't got around to pulling the dash and checking the voltage at the pins yet though, when I do that (maybe tomorrow if I can get the time to do it) I'll post the results.

Sounds good. I beleive the dash dims intentionally when the alternator is not making any power, for the event that it goes out and you need to limp as far as possible. But with the alternator not pumping, you saw a normal voltage? I would begin to suspect the alternator is bad.

The voltage I saw with the engine running and the alternator disconnected (14.5 volts) is about 1 volt more than it reads with the key on but the engine off (13.5 volts). It reads between 16.8 and 17 volts when it's running. Those are the readings on the voltmeter on the dash, the readings from my voltmeter that I use to test stuff is 14.5 volts at the battery and alternator when it's running and 12.0 volts at the battery when it's not running. I double checked it with a second voltmeter just to make sure that the readings I got with the first voltmeter were accurate.

I shouldn't speculate so much, sorry. We'll know better once you've pin-tested the dash connector. From there we can know if the issue is the dash, or elsewhere.

Well I finally got around to removing the instrument panel and checked the voltages at the pins you mentioned, it all checks out good. Also, my cruise control doesn't work and while the panel was out I did all the checks that the factory service manual said to do and it checks out good too. According to the service manual if everything checks out good and the cruise still doesn't work then the instrument panel needs to be replaced. Damn, I was hoping that it wouldn't need that. With that being said, who has the best deal on a rebuilt instrument panel? Who do you recommend that does a good job for a good price?

I go with Batee for anything digidash related. Dude knows his stuff. Got bulb and photoconductor kit from him. I think he does rebuilds, if you send him yours.