Dave (mysocal911),

Frankly, I can't explain the difference, however, I can state my observations.

1) There is a label on the underside of the deck lid on an original 914-6 that states that you must match the manufacturer of the VF and alternator. Originally, my -6 had a Bosch alternator and a Bosch VR. When I installed the newly rebuilt 2.7 (which also had ITBs, Megasquirt, and distributorless ignition), I replaced the SEV Marchal alternator with a rebuilt one to avoid a problem down the road. I bought the SEV Marchal unit because I knew it would fit in the fan housing. This is when my charging issue began. I could never get the new alternator to charge the new battery, despite chasing all of the connections and voltages. I could never find the problem.

2) I then started my search and when looking at the Pelican Parts website for a '74 911S (the car my new engine came from), PP had a Bosch VR listed but also a separate call-out for an SEV Marchal unit. If they list two different VRs for the same car, then maybe there is some difference that I don't fully understand. I believe the diagrams above point to that difference but I am not an electrical engineer and I haven't taken the time to study this difference until I understood it, but only to the point to recognize that there is a difference.
3) Last night after installing the SEV Marchal VR, I was able to get 13.5 - 13.6 volts indicated at the battery for the first time since discovering the problem. I know that this is not the 13.8-14.6 that is typically expected with other cars but it is in line with others who have posted about the charging voltages they have measured at the battery on early 911s.

So, while I can't state an explanation in specific terms, I have stepped through the problem in a logical manner (I believe), noted my observations along the way, and come to a satisfactory solution that I'm happy with.

Actually, the Bosch regulator would work in that application.

Dave,

Actually, I tried it with two different Bosch VRs and neither made a difference. Not theory, parts in hand and swapped into the car. The only other change I made was to pull the Bosch VR and install the SEV Marchal unit.

When you've done the same on an original 914-6 and have proof, let me know.

I've done it MANY times in the last 20+ years working at many Porsche shops here in SoCal.
Remember, all regulars have the same basic connections; D+, D-, DF & grd. If one has a problem
wiring a simple regulator, then it's time to take it to a good Porsche shop and just consider
DIY tasks like washing and polishing. It's a totally simple wiring effort!

Peace.......you say it works and have done it many times but it didn't work for me. Changing the VR to match the manufacturer of the alternator is all I did and its now charging, where it wasn't before with the Bosch.

Thanks for the info you provided above.

Glad you got it to work. It's likely that the Bosch reg was bad, or if SS it was incompatible.
All mechanical regulators whether Bosch or SEV should function with either alternator.
Furthermore, the original Bosch/SEV mechanical regulators will last forever if not damaged
by an incorrect hookup/severely burned contacts.

Bumping this up as I was thinking perhaps AGM batteries may be an issue with over charging?

Thoughts?

The construction of the battery is different but that would not have had any impact on the issues I was facing.

My original Bosch VR was still good but the SEV Marchal VR fixed the problem. There are some differences in the SEV alt and VR vs. the Bosch units. I actually discussed this with an electrical engineer who is a long time Porsche guy last Saturday and he confirmed as much.

Good info. Thanks for the follow up.

However in my recent post I was referring to the alternator possibly exceeding its rated output. Perhaps, from my research, the new AGM batteries may be a contributing factor?

One more data point. I installed a solid state regulator last year. 911603901m262.
I finally got a chance to measure the output. This is after a half hour of driving and this shot was taken with the motor running at 3000 rpm.
http://forums.pelicanparts.com/uploa...1539317459.jpg