Alternator grounding

[phil fleming]

1981 SC I have screwed down my shroud and loaded on all the ancillaries without checking whether I had a ground strap on my alternator connecting under the shroud to the engine block . Peering in and facilitated with with torch light, I cant see a floating ground cable behind the fan and I cant recall as to whether I had to remove one when dismantling the engine? It looks like the harness coming from the back of the alternator is one contained harness and exits out through the shroud into the main harness that comes forward . In other words looks like grounding is via strap on blower plus internal cable in the harness. I didnt want to start removing bits again if the grounding is done via harness and transmission to engine ground.

[HaroldMHedge]

The alternator ground strap is separate from it's wiring harness.

Here's a discussion and photos of the installed alternator.

Where does the ground strap attach to the alternator

On my 87 Carrera, the alternator ground is a wire vice the woven strap in the above thread and connects to a stud and alternator housing.

If you can't see the strap with a torch/flashlight, a mirror or borescope might help in seeing the connections.

[phil fleming]

Thankyou. I cant see a braided strap when peering in. The conductors coming from the alternator seem to pass through the shroud without a ground leaving the harness and being avaialble to ground as the picture shows on your reply. Was there a different arrangement for later cars as I have read that might be the case?

[E Sully]

Do these help?
Picture of brown insulated ground wire on my 1986 alternator.


Technical bulletin from Porsche

[phil fleming]

Thanks - I think it does help as under bulletin even though for a 911T and mine is an 81 SC it states that a ground wire to crankcase was not provided (could cause interference) and so when i took off my shroud with blower housing I dont think i had to undo a ground strap. I didnt disturb the internal wiring to the alternator just lifted the lot as one So I am going to assume that I dont need to investigate any further

[Walt Fricke]

The alternator housing is, basically, already grounded. The alternator body is the alternator ground, it is bolted to the fan housing, and the fan housing is strapped to the engine, which is bolted to the transmission, which has a ground woven wire to the chassis.

Porsche seemed to think a separate ground wire from the alternator housing to the engine was useful, and maybe it is. My engines have it, so I have dutifully connected it. But it always seemed to me it was not necessary. I am dubious that aluminum or magnesium oxidation between the housing and engine would introduce a meaningful resistance. But from that tech bulletin, it looks like Porsche somehow left this ground wire off - by mistake? The bulletin suggests that this might be a problem. It doesn't suggest that the ground wire itself is a problem, which it couldn't be. Superfluous, maybe. Not a problem.

However, if you left it off but aren't having problems, you'd be OK.

[E Sully]

You may have misread it. The bulletin is for early 911T through '83 911 SC's. It states that interference may occur on cars without the strap, and to install one if issues with radio interference and battery discharge occurs on models without the ground wire. It also says to install one on cars that do not have one if the alternator is removed for any reason.
As an electrician, I would highly recommend installing the ground wire. It offers a much better ground than the mounting strap from the housing to the engine. Oxidation does cause significant issues electrically over time. Many issues develop in older cars due to poor ground conditions.