Alternator/Regulator strange behavior
 

I recently changed out my alternator with a rebuild. Now when I start the car the alt light is lit until I rev it past ~1300 and then the light goes out and it continues to charge fine at all rpms even back at 900 idle. It's like the regulator isn't getting woke up without giving it a little throttle, but, again, once the light goes out it doesn't come back on until I restart the car.

I have always been able to reach through the window and start the car and let it warm up a minute before I leave. Now when I do that, the battery is draining as the alt is not supplying voltage until the motor is reved up slightly.

The light is working properly too as I checked at the battery. This isn't a big deal, but hoping for a fix short of replacing the alternator/regulator again.

Thanks for any suggestions!

Take it for a drive. You have to wake it back up, above 3k. I recently had the same problem.....

Thanks for the quick reply. It's been about a month since the alt was changed out. The motor has seen some spirited driving since, quite often above 5k.

Belt is tight, pulley is the same. I'm thinking resistor inside regulator, but only guessing and hope it's not. Something is keeping the electrons from flowing to the battery.

'81 911 huh. Did it have an internal reg upgrade? If so was the dash light changed? Is it a Bosch or Valero?

Hi Ronin,
Old one was a Paris-Rhone with internal regulator. Replaced with Valeo with int reg. I think Valeo bought out Paris-Rhone.

Idiot light remains the same. Problem didn't exist before swap.

ok.. you shouldn't have a prob w/Valeo install.

how about start the engine, turn headlights on, and check battery voltage at 2.5-3k rpm? That 75amp alt you have is a good one as it probably should be putting out 90amps max. Your test should read 13.9 - 13.9+

SmileWavy

Idling with alt light on battery reads 12.2v. Idling with light off is 13.9v. The light if functioning correctly. I didn't try the headlight - 3000 rpm mode yet.

My original question is why does it take 1300 rpms to excite the voltage regulator to send the charge to the battery. Once the charge is sent, the light never comes back on even if it idles at 600. It's as thought there is more resistance out of this alternator, than the prior one, but once that resistance is overcome, it no longer is an issue. All grounds were brushed and secured when I installed the rebuilt alt.

Perhaps I will call the alt rebuilder and see if he has any thoughts on this.

search here for "zero-ampere rpm" and "early_s_man" and ALL will be revealed.

I missed that it wouldn't light again at idle.

cramer is right that Early_S_Man / Warren did a good right up on this. I think it was using a resistor in parallel to your dash light in order to lower the excitation field signal on so the alternator sees a bigger voltage demanded to replenish back up. It's common to use a 50-150 ohm resistor to solve this prob.

Sometimes a rebuilt alt will have a voltage drop in the diodes, and a resistance in the dash light wiring which creates a voltage drop in that circuit can cause these problems. Dirty battery connections are another consideration.

I'd still do the headlight and mid rpm test anyway while you're in there.

Thanks a lot, guys. I will grab a 90 ohm 5w resistor and solder it in parallel when I get a chance.