ammonman

My '78 911SC has me stumped. The SEV Marchal externally regulated alternator randomly charges, then doesn't charge. The dash light comes on with key on but usually will not go out until RPM's are >3K. Randomly, the light will stay off when the RPM's fall and the alternator will charge, even at idle, as verified with a volt meter showing 13.8 VDC at the battery. However, more often than not when the RPM's fall below 3K the alternator light will come on and the alternator does not charge at idle, again as verified with a volt meter at the battery showing 12.2-12.5 volts. The alternator is recently rebuilt and tested at a local shop that I've had positive results from for the last 15 years. The voltage regulator is a new Porsche OE unit from our hosts and is Beru branded. I have verified less than 0.5 ohms resistance from all connections on the alternator to their respective connect points:B+ to 14-pin/#2 Fuse/Battery positive cable in frunk; Df to voltage regulator; D+ to voltage regulator and alternator light socket blue wire; D- to voltage regulator and to battery negative cable in frunk; alternator ground strap to battery negative cable in frunk. I'm considering installing the 90 ohm resistor across the charge light to ensure enough load to reliably bootstrap the alternator.

Any suggestions are appreciated.

gtfhercules

Sadly, I do not have any ideas myself, but Daddy's my 1978 911 SC is also experiencing this rather strange issue.

The last time I looked into it, I read that checking the factory grounds would be a good idea, and perhaps replacing the ground by the gearbox.

Following!

ammonman

Thanks for the response. The ground strap from the trans to the chassis is new and connecting pads clean and shiney. Same with the battery ground cable. Alternator ground to engine case is in place and solid with low resistance. The fact that it charges at high RPM but won't sustain once RPM drops has me suspecting the Beru voltage regulator needs addition current on the Df connection to keep the field coik energized similar to the internal regulator. Will install a 100 ohm 5W resistor soon and see if the solves it.

theiceman

im still thinking its going to be the regulator or the Alternator .. I do realize you have rebuilt one and replaced the other , so i am assuming you had this identical issue with previous regulator ( and i hate assuming anything ) , if that is the case i would say its your rebuilt alternator and they didn't catch the issue . You said yourself it is intermittent.

ammonman

The alternator is still a possibility. I have a 5w resistor to put across the alternator light wiring on the way. If that doesn't stabilize the field coil charge I'll be taking the alternator back to the rebuilder.

systemsc

Just put a jumper wire on pins D+ & DF of the female regulator pins. The alternator should fully charge to 14V. Avoid an RPM greater than 1500 or you'll over-voltage the battery.

theiceman

if it does Loren , does that make the regulator the suspect ?

systemsc

Yes, most likely, or wiring/light.

ammonman

Looks like the alternator is going back to the rebuild shop. I put everything back together and started the car up to let it warm and ensure the rpm stayed below 1500. The alternator immediately started charging at 13.8VDC but after a minute or two of running the dash light came on and it stopped charging. I then put a jumper from Df to D+ at the regulator and the best voltage the alternator makes is 11.95VDC.

theiceman

called it lol ....

That thing is such a pain in the *** to get in and out, i feel for you .. Wonder of one on the diodes has a dodgy weld.

ammonman

Easier to swap than a Honda Accord I once owned. Agree it seems load/thermal related like a bad joint or a break in the winding.

ammonman

Circling back to close out the thread. The alternator repair shop had to put a new stator in to address the issue. My own fault for neglecting to send the key and retaining nut for the fan the first time around so they could test after the first bit of work. Now the G light comes on with the key and drops out as soon as the car is running.

Thanks to all for the tips.

systemsc

The least likely part that fails in an alternator!

theiceman

Thanks for following up with us , so many people dont do that , good to hear that you got it resolved ...

I bought a " spare " alternator 15 years ago in case mine dies thinking it was about 40 years old .. its still on my shelf...
But i have to tell you on the audi forum i am on the number one complaint for systems not charging after putting Bosch rebuild alternators in ... is the alternator .... not relevant here but thought it was kind of funny...

Good work !!