Generator Light On While Engine Running

73ess · 05-09-2007, 09:21 AM

The generator light in my '71 Targa stays lit while the motor is idling and goes slightly dim at higher rpms, but is still lit. Thought it was the voltage regulator, but I replaced it and the light is still staying lit when engine is running. Must be the alternator? Should I just replace the alternator or is there something easy to check on the alternator and possibly an easy repair?

Thanks,
Steve

vash · vash's Garage

have you tested voltage at the battery yet (car running)?

73ess · 05-09-2007, 09:32 AM

Yes, it was 16.5 to 17 volts. I think that's too high. Actually, I know it is because the battery is also leaking a bit after the charging system has charged it for a while. So after a 30 minute driver or so.

Jim Richards · 05-09-2007, 09:39 AM

Yes, way too high. You should see something roughly in the 13-14V range. I'm wondering if your high reading is due to a bad voltage regulator or a bad diode(s) on the alternator. Many shops can test and repair alternators.

vash · vash's Garage

riteway south of market. they do great work. i paid $125.

lateapex911 · 05-09-2007, 01:09 PM

Way too high indeed. Should be right around 14.6 IIRC. Odd that the regulator didn't clear that up. Warren, or John Cramer (as well as Steve or Grady or some others) will have the answer I bet.

welshdragon · 10-28-2021, 02:22 PM

Hi there
I have the same issue with my 77 911
I’ve tried 2 new voltage regulators and both give a reading at the battery of up to 17v
Did you get to the bottom of your problem
If so can you pass that on cheers

Trackrash · Trackrash's Garage

A bad ground or bad connection with the blue wire to the regulator can cause that problem.

Walt Fricke · 10-28-2021, 06:28 PM

You sure you have the VR hooked up right? The '77 (mine, anyway) came with the external VR, which is easy to replace. But I recall the connection from the alternator to the VR was a three female pin molded plug, so hard to miswire.

Next step would be to pull the alternator and take it to a shop which tests such things. If the VR replacement won't fix it, it pretty much has to be in the alternator itself - a bad diode or so somewhere inside, or some kind of internal short. 17V is way too high. At 15+V I started smelling sulfuric acid smells from my battery.

spoke · 10-29-2021, 10:14 AM

There are 3 spades on the VR, DF, D+, and D- just like the 914. Here's the 914 charging system schematic. The 914 system is similar to the 911 although I believe the 911 alternator has more diodes than the 914 alternator.

Check all 3 voltages at the VR with respect to chassis ground, not the engine case.

D- should be zero volts.
DF will bounce around 5-10V depending on level of charging.
D+ should be equal to the battery voltage.

Measure battery voltage on the POS terminal to chassis ground.

D+ and the battery POS should be equal in voltage. If unequal, then it points to an issue in the alternator.