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Thu Jul 10, 2025 4:24 pm

I have a 1979 vw bus, and I have a new 70 amp alternator from bus depot, new harness and new regulator. my battery light comes on randomly and I drop back to 12.5 volts, then ill drive a bit and it will start working again. I took the alternator out and got it tested, they said it was good. I just don't know what's going on.

Thu Jul 10, 2025 4:32 pm

why did you install the new parts?

Thu Jul 10, 2025 4:36 pm

lil-jinx wrote: why did you install the new parts?
New engine didn’t know how the alternator was. I also wanted a 70 amp so I put that in and it ran good for a bit then went. The light came on so that I tried a new regulator and it worked for a bit. Then the light came on again.

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Thu Jul 10, 2025 5:04 pm

I would go back to the old regulator first as the first step. Much easier to do than any other troubleshooting step. Quick n' Dirty.

Fri Jul 11, 2025 4:43 am

If the alternator uses a modern regulator, these use the 12 volt feed via the warning light as an enable control. After 12 volts appears on the wire for a while, the alternator starts up. If the 12 volts vanishes momentarily, it shuts down and the warning light comes on as it reconnects for about 0.5 to 1 second depending on the regulator. If during that time the 12 volts varnishes, the timer restarts..
A worn ignition switch or a loose warning bulb or a loose spade connector can make this happen, vibration keeps it mostly connected with occasional disconnections, alternator light stays on, maybe flickers a bit, alternator never starts up.

Its different to older simpler regulators , they would work with a loose wire, as once the alternator has started producing voltage, it does not need the presence of the warning light until it stops.

Fri Jul 11, 2025 7:48 am

timvw7476 wrote: I would go back to the old regulator first as the first step. Much easier to do than any other troubleshooting step. Quick n' Dirty.

I didn't have an original one in there so I ordered another one, it should be here tomorrow.

Fri Jul 11, 2025 10:48 am

mikedjames wrote: If the alternator uses a modern regulator, these use the 12 volt feed via the warning light as an enable control. After 12 volts appears on the wire for a while, the alternator starts up. If the 12 volts vanishes momentarily, it shuts down and the warning light comes on as it reconnects for about 0.5 to 1 second depending on the regulator. If during that time the 12 volts varnishes, the timer restarts..
A worn ignition switch or a loose warning bulb or a loose spade connector can make this happen, vibration keeps it mostly connected with occasional disconnections, alternator light stays on, maybe flickers a bit, alternator never starts up.

Its different to older simpler regulators , they would work with a loose wire, as once the alternator has started producing voltage, it does not need the presence of the warning light until it stops.

so would I not have any voltage at D+ (red/blue wire)? I got someone else's regulator and tried it, same thing went off for a while while driving then the light came on. drove for a bit it went off then came back on

Fri Jul 11, 2025 11:04 am

mikedjames wrote: If the alternator uses a modern regulator, these use the 12 volt feed via the warning light as an enable control. After 12 volts appears on the wire for a while, the alternator starts up. If the 12 volts vanishes momentarily, it shuts down and the warning light comes on as it reconnects for about 0.5 to 1 second depending on the regulator. If during that time the 12 volts varnishes, the timer restarts..
A worn ignition switch or a loose warning bulb or a loose spade connector can make this happen, vibration keeps it mostly connected with occasional disconnections, alternator light stays on, maybe flickers a bit, alternator never starts up.

Its different to older simpler regulators , they would work with a loose wire, as once the alternator has started producing voltage, it does not need the presence of the warning light until it stops.

so I don't have my fan hooked up and have the wires for the fan just hanging out I did cap off the red power one. but there is a blue one that splits from the alternator harness, if that grounds could that cause my problem?

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