Charging question on a 914-6 Options

[db9146]

Well, I have made a temporary conversion of my -6, inserting a rebuilt 2.7S with ITBs, MS3, and distributorless ignition in place of the tired 2.0L. Before putting the engine in, I decided it best to install a new alternator along with a new engine/alternator wiring harness.

The alternator is for a 1965-1977 911 2.0-2.7 that uses an external VR.


and here's the new wiring including a new ground strap to the engine case.


The large red cable goes to the starter, the smaller red wire is D+, the black is DF, and brown is D-.

I thought the problem might be the battery because of its age but when I checked the output of the charging system at the battery, there was none. I replaced some of the dash lights with LEDs but made sure not to replace the GEN warning light, which is not coming on despite the alternator not charging.

Should I next pull the D+, DF, and D- plug and check for alternator output across DF and D+ momentarily?

If there is output, what then?

If not, what's next?

[db9146]

Well, I think I figured it out.

All 914-6s were set up for the Bosch style VRs which work with the Bosch externally regulated alternators in the stock 2.0L engines. The Bosch VR's housing fits nicely onto the stock -6 relay board. However, if you switch engines to a larger -6 that has an externally regulated SEV Marchal alternator like the one in my new engine, you can't use a Bosch VR.....you have to wire in an SEV Marchal regulator. This was my solution to get my charging system to work properly after installing a '74 911S 2.7L in my 914-6.

I actually gutted an old Bosch VR and made up a small harness that could plug into the stock relay board connector under the VR but then it exits through the rectangular hole in the relay and wraps back up to make the connections on the new SEV Marchal VR. Doing it this way keeps any water out of the relay board connector under the Bosch VR.




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/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 ( (IMG:style_emoticons/default/piratenanner.gif) ). 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 other 911s/930s, including our own Spoke above..

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.

There is another similar thread I joined in on over on the PP board here that has some diagrams Alternator Problems which explain the differences between the Bosch system (alt. and VR) and the SEV Marchal system.


LarryM, thanks again for all the help figuring this out, for the posts you added, and for the IMs as well! (IMG:style_emoticons/default/beerchug.gif)