FIA-F1

It may not be. When driving, my instrument voltage reading is about 11 volts. I think it should be around 13 - 14. I had purchased a new red top Optima about 6 months ago. It was my second. The first went into another 944 a few years back. After 6 months, that previous one didn't seem to hold a charge even with a trickle charger. I was bummed. So when I switched 944's, I thought I would give Optima another chance and bought my second red top (you know how the negativity goes away, no pun intended). Same dang thing happened. So coincidently, the voltage reading on my 911 was down and the forum suggested it was a bad voltage regulator on the back of the alternator. So not wanting to shell out for a new alternator (Porsche dealer wanted $600 for a inseparable bundled alternator and regulator), I shopped eBay and could get the alternator for around $150. Or, I could buy just the regulator for a fraction of that which is what I ended up doing. I never have luck with short cuts but unbelievably, that worked. My 911 is now pushing 14 volts and running perfectly. I can tell you it was no picnic installing it though. So, back to the 944. I read where you don't have to remove the alternator to install the regulator. By the way, I bought the cheapest new regulator I could find...$8 from China. I got it and it looked like the OEM Mercedes one that comes with the 944. One minor observation, the brushes seemed to extend about an eighth inch to far out as compared with OEM. I didn't think anything of it. This insignificant extra length is causing a great deal of frustration though. You know every mechanics ultimate pain is that little incongruency that causes hours of tedious, championship level frustration because some manufacturing lapse of an imperfect fit. For the life of me, I could not get the regulator to compress the spring loaded brushes and seat on the armature so that I could attach the unit flush to the alternator. Of course, at the end of fiddling with it, the brush broke off into the alternator and fell into a hard to reach nook. Note, all of this May have been avoided if I had taken the alternator completely off but no, I won't just on principle now. I'm waiting for the new Beck Arnley regulator to get here so i can have another go at it.
I think there a couple of messages in here...Optima may not be bad, don't buy super cheap Chinese knock off electrical parts, and go ahead and take the long, proper way on replacing parts.

Arominus

Have you checked the voltage at the battery when the car is running with a multimeter? The dash gauge isn''t always so accurate in my experience.

944Ross

I've heard multiple times that unless the armature rings are smoothed up, new brushes won't seat properly and will ultimately cause more damage. I believe Van did a good write-up on it.

konakat

I used a small strip of electrical tape to hold the brushes in while I got the regulator in place, then pulled it out before tightening everything down. It was definitely a pain in the ***. My voltage gauge is off by about 1.5V, so I would definitely verify with a multimeter before driving yourself nuts.

944Ross

Excellent advice, I found my dash gauge reads about .5v low. I do suspect my alternator is getting weak but it's doing much better than the gauge indicates.

william_b_noble

250K miles on original alternator - I don't think they get "weak", though they can fail.

FIA-F1

Ok, follow up report - I got the Beck Arnley part and immediately noticed the contact surface was already concave as it should be. I struggled a few minutes trying to compress the spring and seat the regulator (while installing the screws) but it wasn't working. So I tried the tape idea and used painting tape but that didn't hold. I switched to clear scotch tape and that did the trick. I left a 5" tail so I could pull it out after the screws held. Everything went back together and the voltage needle is now showing 12.5 volts. Perfect. Definitely will endorse Beck Arnley. I acknowledge the guys point about using a voltmeter but curious...what is the voltmeter reading for all your 944's? Just wondering.

944Ross

"Real" (DVM) readings at the battery terminals:
after a cold start 10 seconds cranking) - 13.8 v
after an hours running, with A/C on - 13.3 v

Dash gauge readings
cold start - about 13 v
after hour run - 12.5
turn on A/C - drops a needle width

All readings are in 80-deg. weather, battery voltage is uniformly 12.75 - 12.8v before starting. Battery is only two months old.

When I said my alt was getting "weak", I'm assuming that the brushes are close to worn out mostly because of the drop in voltage when the A/C is turned on. Could be one diode is blown too.

FIA-F1

Good info. Next time, can you us something more precise?