alternator light glows above 2000 RPM's

[rrcrawford70s]

I own a 1970 911 S, with the original 2.2 motor. Tonight while driving, I noticed the alternator light was glowing. The light would go dark at idle. This was only occuring while the lights were on.
The batteries were replaced last week, after it refused to start.
Is it the diodes, V.R. , or the alternator?

TIA

[ischmitz]

I can be all of the above. Worn brushes or a bad diode would be my guess. Also the old style regulators do go bad.

I had one bad diode in my 74 resulting in a glowing alt light at all the time (only noticable in the dark). Also my light would sometimes come on at low idle and I had to blip the throttle. Didn't feel like fixing it since I changed to a 3.6 engine soon after.

[3.2 CAB]

Best case, it could be as simple as belt slippage at the higher rpm. If the belt is too loose, it will try to slip around the pulley, at higher rpms the faster rotation trys to sling it out of the groove. Worse case, regulator, diodes.

[304065]

What failure mode would give you voltage at low RPM but not at high rpm, that would change state rapidly? If you had a diode that was overheating at high RPM it probably wouldn't cool down and start working again that quickly.

I agree with the belt slippage analysis. There should be about 1/4" of play in the belt when you push on it perpendicular to the belt. The tension is set with the shims on the pulley- see tech article on the main board or do a search here.

[Joe Bob]

Check play....then put a multi meter on the battery posts....after you fully charge the battery.

Should be 12.5 volts at rest. 13+ at idle and 14.5 max at 2500 rpms +.

[Lorenfb]

You need to check the voltage at the alternator besides the battery. A dimly
glowing light can indicate a another problem other than an alternator problem.
It can be as simple as a voltage drop from the rear to front. The light senses
the voltage at the ignition key AND the voltage at the regulator (D+). If D+
is greater than the battery voltage, the light glows. If it's bright, then it could
be an alternator problem.

[juanbenae]

as 3.2 said start with the belt. try to rev it out of gear and listen for some belt slippage. then if thats not it do what the others said.

[Zeke]

While proper belt tension and a good belt are very important, I don't think a belt slippage problem could cause a light to come on after it had been off at idle and low RPM. That would be saying that the fan was actually turning slower in slippage than it was when at idle. I could be wrong, but it doesn't add up for me.

I guess your oil temp guage would be the litmus test.

[targa80]

Since you said you see this at night with the lights on then it seems to me that your alternator/voltage regulator can no longer handle the load applied. During the day is the light on with driving lights off or on ?( or is it to hard to see). At night try shutting off the lights to see if the lamp goes out. Or hook up a volt meter to the cigarette lighter socket. You can get an adapter for the lighter pretty cheap. You can monitor the voltage as mentioned earlier.