Acceptable voltage discrepancy, Voltage at dash vs. voltage at battery Options

[98101]

I started my car after sitting a few days and noticed the dash voltage meter went up over 15V. In Tech Tips 700 I read that overvoltage could lead to battery acid spills, so I quickly shut it off.

Prior to that, I'd noticed the dash gauge reads lower than I'd like when all the lights are on.

I checked the voltage at the battery under different circumstances and they weren't so extreme in either direction. So now I'm wondering what the acceptable differences would be before you'd start to expect a poor (ground?) connection somewhere?


https://www.youtube.com/watch?v=S6BcQC-cjXo

[Spoke]

There is a lot of resistance in the 40+ year old wiring and connectors in the 914. Measuring the voltage in the cabin with lights on could easily yield 2-3V lower than at the battery.

The voltage at the battery is the only voltage worth measuring WRT the voltage regulator and alternator.

[98101]

Thanks Mike and Spoke.

I was also confused by the 15+V reading I saw on the dash gauge when I first started the car. I didn’t catch that on video unfortunately.

[Mike Bellis]

Thanks Mike and Spoke.

I was also confused by the 15+V reading I saw on the dash gauge when I first started the car. I didn’t catch that on video unfortunately.

I see -15 Amps and +11 Volts when you start it. The Amp gauge is bogus installed in the cabin unless there are two very large wires running all the way back to the alternator. If there is, make sure they are fused or you may burn the car down. Otherwise, throw the amp gauge away. There is no place to correctly connect this inside the cabin due to the way the factory wired everything. At most you will only be measuring one of 4 circuit paths. The low voltage on the gauge is mostly due to age of the wiring. Age of the ground plane (chassis) and the fact that the gauge is analog and your meter is digital. Both units process RMS voltage differently and will never read the same. There is also substantial voltage drop inherent in the factory wiring.

[98101]

I see -15 Amps and +11 Volts when you start it. The Amp gauge is bogus installed in the cabin unless there are two very large wires running all the way back to the alternator. If there is, make sure they are fused or you may burn the car down. Otherwise, throw the amp gauge away. There is no place to correctly connect this inside the cabin due to the way the factory wired everything. At most you will only be measuring one of 4 circuit paths. The low voltage on the gauge is mostly due to age of the wiring. Age of the ground plane (chassis) and the fact that the gauge is analog and your meter is digital. Both units process RMS voltage differently and will never read the same. There is also substantial voltage drop inherent in the factory wiring.

To be more clear, the nearly 16V reading I saw was not part of the video I posted. It was about a half hour before I shot the video, when the car hadn't been started for a few days. I was not expecting this at all, so I didn't have a voltmeter connected to the battery to capture the reading there. I shut down immediately to protect the battery. But it seemed more normal by the time I got the phone cam out and the meter hooked up to the battery. Would an overvoltage reading in this circumstance indicate any important fault, or just the gauge being connected nearer to the alternator than to the battery?

I'm not referring to the ammeter, which I've been ignoring for now. I would have assumed it was using inductive pickups not carrying much current. But I haven't actually discovered how it's connected yet.