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

[Mike Bellis]

Most alternators do not charge much at all at idle. That means you are running off battery. When you rev above 1200 rpm or so it should start charging and it looks like it does. Your amp gauge indicates the negative discharge at idle. As long as the amp reading shows positive charge after diving a few miles, you have nothing to worry about.

[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.

[cary]

Seeing the same issue for years. I circumvented the harness and ran a fused wire directly from the battery to the gauge. Instead of 10 volts with the headlights on. It now reads 12.5 volts.

[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.

[Spoke]

Short bursts of 16V on the battery should cause no issues. Long exposure to 16V is not good as it would overheat the battery.

To fully understand what is going on the electrical system and layout must be known. That ammeter should be wired from alternator to cabin to battery as Mike mentioned. If not, it would only measure the draw of the accessories in the car.

Do you have a schematic of how the ammeter is connected?

Also, when you saw the 16V, was it stable or bouncing around?

[worn]

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

With multiple voltage meters, alternators, batteries and voltage regulators my lights on always causes a significant voltage drop - say at least a half a volt. The stock alternator is not the biggest in the world. Say about 10 amps to the headlights alone along with the marker lights along with the gauges and running the motor. It all adds up.

[porschetub]

Short bursts of 16V on the battery should cause no issues. Long exposure to 16V is not good as it would overheat the battery.

To fully understand what is going on the electrical system and layout must be known. That ammeter should be wired from alternator to cabin to battery as Mike mentioned. If not, it would only measure the draw of the accessories in the car.

Do you have a schematic of how the ammeter is connected?

Also, when you saw the 16V, was it stable or bouncing around?

Interesting reading and thanks to the OP for starting it,I'am connected to the wiring for the centre console for my gauge, I get some odd readings that don't line up with battery output,seems under load it lines up closer but the gauge is always lower than the battery (IMG:style_emoticons/default/confused24.gif) (IMG:style_emoticons/default/confused24.gif) .
Is the wiring that go's to the centre console not up to it to run a gauge or is it just old wire ? is a direct reading from the battery the only solution.....I can do that if needs be.
Thanks.

[98101]

Do you have a schematic of how the ammeter is connected?

Also, when you saw the 16V, was it stable or bouncing around?

I don’t think I have that schematic, but there’s a lot of records I’m still sifting through. I wasn’t going to worry about the ammeter before because I’d assumed it would be like the test equipment I’ve seen that uses inductive couplers over the insulated wires without any direct electrical connection. If it’s not being done this way ... I’m not sure it’s worth even having an ammeter.

When I saw 16V, it was steady for the 10-20 seconds it took me to shut the engine off. I’m not experiencing any problem starting the car or turning on the headlights so maybe this is something I should leave alone?

Previous owner(s) may not have been electrical wizards, or it may not have been their highest priority. There’s no brake lights or reverse lights, and about half the gauges are illuminated. The steering wheel horn button doesn’t work, but there’s a working horn button under the dash. The back pad light doesn’t work, but there’s a new light added to the dash with its own switch. The parking brake / brake warning light doesn’t work. There’s a tiny black button on the dash; neither I nor the previous owner know what it’s for (windshield washer?). There’s a hidden switch for the fuel pump which I kind of like, not that any thief without a tow truck would figure out how to move it anyway.

Surely due to some sort of oversight, the headlights (H4), driving lights, running lights, and turn signals / hazard lights DO work. But the turn signal automatic cancel doesn’t. As a motorcyclist, I kind of prefer to cancel them myself though.

[worn]

Previous owner(s) may not have been electrical wizards
(IMG:style_emoticons/default/agree.gif)
There’s no brake lights or reverse lights, and about half the gauges are illuminated. The steering wheel horn button doesn’t work, but there’s a working horn button under the dash. The back pad light doesn’t work, but there’s a new light added to the dash with its own switch.

(IMG:style_emoticons/default/agree.gif)

[porschetub]

Do you have a schematic of how the ammeter is connected?

Also, when you saw the 16V, was it stable or bouncing around?

Previous owner(s) may not have been electrical wizards, or it may not have been their highest priority. There’s no brake lights or reverse lights, and about half the gauges are illuminated. The steering wheel horn button doesn’t work, but there’s a working horn button under the dash. The back pad light doesn’t work, but there’s a new light added to the dash with its own switch. The parking brake / brake warning light doesn’t work. There’s a tiny black button on the dash; neither I nor the previous owner know what it’s for (windshield washer?). There’s a hidden switch for the fuel pump which I kind of like, not that any thief without a tow truck would figure out how to move it anyway.

Surely due to some sort of oversight, the headlights (H4), driving lights, running lights, and turn signals / hazard lights DO work. But the turn signal automatic cancel doesn’t. As a motorcyclist, I kind of prefer to cancel them myself though.

Sorry to hear your electrical issues,PO wasn't too forth right about stuff on my car,no wipers,tailights,and worst of all gauges disconnected...every wire (IMG:style_emoticons/default/headbang.gif) (IMG:style_emoticons/default/headbang.gif) .
Got there and you will ok (IMG:style_emoticons/default/beerchug.gif)