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-   -   Strange Voltmeter Issue (http://www.hotrodders.com/forum/strange-voltmeter-issue-222953.html)

Leoman 08-16-2012 10:05 AM

Strange Voltmeter Issue
 
My '72 Camaro runs great, but in the past year I've had an intermittent problem that is driving me nuts. At times, the voltmeter pegs itself to the left, past 0 volts. When this happens the dome light fails to operate. Everything else seems to work. The battery terminals read over 13 volts (meaning it's charging), the exterior lights, dash lighting, heater, other gauges etc. all seem fine.

I had replaced the alternator and jumped out the old voltage regulator months ago, but it has failed to address the problem. I obviously have a faulty connection someplace. But what circuit do I home in on? Everything under the dash seems ok visually. What common circuit would my voltmeter and dome light occupy when everything else works?

It's hard to trace because even if I disassemble the dash (as it is now) the problem will disappear and then reappear. No wire tugging (so far) seems to have an effect.

vicrod 08-16-2012 08:17 PM

amps
 
That model car/year had an ammeter not a voltmeter.
I believe you are seeing an amp draw that is causing the ammeter to go below 0 amps.

Check the dome light for shorting to ground. I would expect that a fuse would blow on the dome light circuit or you could remove the fuse and the problem will go away.

vicrod

Leoman 08-17-2012 09:55 AM

Quote:

Originally Posted by vicrod (Post 1583518)
That model car/year had an ammeter not a voltmeter.
I believe you are seeing an amp draw that is causing the ammeter to go below 0 amps.

Check the dome light for shorting to ground. I would expect that a fuse would blow on the dome light circuit or you could remove the fuse and the problem will go away.

vicrod

Makes sense, but my meter doesn't read -40/+40, rather it goes 0/18, and pretty accurately reads terminal voltage according to my DVM. Sure that's an ammeter?

On the other hand the deflection is about the same as an ammeter, I'd guess. Straight up with a charged battery and the ignition off, right-deflect with the alternator working, left deflection with the car shut off and the headlights on. So maybe just a faceplate repaint? Still, if it is just a repaint and it's an ammeter showing a short, that short would be drawing over 40 amps! Plus if I disconnect the battery under those conditions it's still pegged left. I'd think ammeter springs would pull it to the center. How confusing. I'll pull the lead off the meter and see if anything still works.

In any event it's sure worth a try checking the dome wiring!

vicrod 08-17-2012 01:30 PM

no amps
 
The shop manual I have shows an ammeter but what you are describing sounds like a voltmeter. In that era when a meter was used instead of a indicator light ammeters were more common.
Try connecting another voltmeter to an ignition ON source and monitor both meters over time. It may be that the old meter is bad. Try to find a replacement. Good luck.
You may find a bad connection if you carefully disassemble the dash.

vicrod

Leoman 08-17-2012 03:55 PM

Good advice. BTW Classic Industries lists an ammeter as 1970-75, a voltmeter as 76-78 that looks just like what I've got. Oddly though, I have a replacement dash panel stashed away, and I looked at that today. It has the same voltmeter! Gotta be from the 76-up cars. The car had this stuff when I got it 14 years ago, but I never had this problem until recently so it can't be a catastrophic mismatch. BTW the meter reads 8-18, not 0-18, so the left pin is a true 0. My bad.

I unplugged the hot lead from the meter and everything (even the dome light) still worked. So definitely not an ammeter on the same line as the dome light or anything else. Only God knows what my real wiring diagram is after 40 years :sweat:

I'll keep pluggin'! Of course, everything is working fine now, even the dome light (I took it off the roof and it seemed to have good looking wiring... yanking on them had no effect on anything, good to bad or bad to good).

At least I'm pretty sure I have an open circuit somewhere and not a short. Bad ground always a possibility I suppose.

PS nice Vicky!

dogwater 08-18-2012 07:29 PM

Dome light-Replace the switch that the door pushs on when closing the door.Some where I read that when you change out an amp gauge an put in a voltage gauge it a big no no to connect the volt gauge the same way as the amp gauge was.What I would do is disconnect the gauge connect your volt meter to it an see if it acts like a voltage gauge,such as turn the key on,read voltage then turn on the head lights,should be a volt drop,then start it up should be a volt rise.If it doesnt do that then sump tins f-up.

Leoman 02-27-2013 09:38 AM

Many months later, I seem to have the answer. It was indeed the dome light circuitry (duh), and it was at the door switch, driver side. How did I find out? When it acted up again, I tried adding another ground connection from the battery to various places with a test lead. Except that I clipped my test lead to the positive terminal! When I made contact with the valve cover, large sparks ensued along with a nice 'pop' from somewhere. After realizing my mistake, I wondered what I had blown. To my amazement everything seemed to work, but when I opened my door the dome light didn't go on, par for the course when the voltmeter was pegged low. I was convinced I was back to square one again, this time with a blown wire/fusible someplace, but in the middle of a streak of bad language I noticed that the voltmeter was reading 12v! Finally 1+1 reached 2 in my fat head and I realized I'd had a partial short in the door switch wiring. When I mistakenly shorted hot to ground through the valve cover, it blew the short (or the switch) sky-high! Nothing else seems to have been affected.

Someone was watching over me I think. I suppose all the other stuff I fixed getting to the real issue was a good thing to do anyway :embarrass

PatM 02-27-2013 01:11 PM

Thanks very much for "closing this loop", so to speak. All to often, we (I) don't see this kind of follow through on the problem resolution.

Many more miles . . . .

PatM


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