billdrah

I am hoping to pick your heads from a moment about a 1978 US market car I have been looking at. The car is not local to me so I am relaying what I hear second hand hopefully it makes sense to you. So the car is having some electrical troubles that seem to involve a large black / white wire that powers the headlight motor. It seems this wire is overheating and has damaged other wires in the same loom and is now allowing power to jump multiple systems from the shorts it has created. I am being told this is a “known” problem has anyone heard of this, should I run away like the wind? Is this a loom that is available? Does this signal to me that more electrical issues will surely follow?

Any advice would be greatly appreciated.

The Forgotten On

It sounds like a bad connection causing either a voltage drop or high resistance. Could be internal to the relay, could be elsewhere in the loom. We wont know without seeing the car.

I wouldn't say that exact one is a well known problem but replacement harnesses are available for ~$400 from Porsche if you need to replace the entire front loom like I am in my 81.

jcorenman

There's a couple of things wrong with this description. But generally anything described (by a seller) as a "known problem", probably isn't. And I've never heard of a melted headlight wires as a "known problem".

Certainly there are some unfused wires that can melt and cause shorts, but those are rarely the ones that cause trouble. And the headlight motor is not unfused. And it is not a large black/white wire. But there are probably some melted wires which have created some shorts, and you won't know which until you dive in.

A snapshot of the WSM electrical diagram for the headlight motor for the '78 is below, the "V8" dashed rectangle near the center. The early wiring diagrams are pretty easy to read: The top is generally the various 12v feeds (exception: bus 31 is battery negative), bottom is chassis ground, current generally flows from top to bottom (except for Bus 31). The headlight motor drive is a 1.5mm red/black (red wire with a black stripe), fed by relay J36 through fuse S19. If the motor jammed or shorted it would blow the fuse. If the fuse were then jumpered by an idiot POP then the red/black wire would melt.

So maybe the PO was color-blind, maybe they were also clueless on electrical matters, none of that matters-- something melted. The real questions are (1) how good are you at reading wiring diagrams and sorting electrical things (or how willing to learn), (2) how much doesn't work on this car, and (3) what is the price, and (4) do the first three equate to a project that you want to take on. It isn't that hard, there are only four relays to operate the headlights.

Understand that 78's are special, they were the first year, were the simplest electrically, and the purest expression of what the designers had in mind. And the first and last years will always be the best investments if that is a factor.

Good hunting!

Landseer

When there is harness burn-through, it is my experience that it isn't localized. Instead, the wires fry intermittently throughout the harness.

Cross feeds then are havoc.

That said, if everything else seems solid mechanically it might be a good project. Don't underestimate the scope of the electrical fix though, it needs to be thorough.

Check with Roger Tyson on availability of loom at 928sRus.

jwillman

A harness from Porsche for $400. Did you buy one of those and from where? That sounds too good to be true.

Ducman82

front engine loom or FRONT car harness? at 400 id buy them all. i had to replace the front car harness on a 91 GT a few months ago due to horribly made aftermarket 14 pin harness that had the wires wrong and toasted the grounds on the front harness. so many wires were melted and cross feeding power to each other. it was a mess.

The Forgotten On

I bought mine for that. Just go to Jim Ellis Porsche, Sunset, Sonnen, etc. They can get the harness for $400.

I bought mine from Jim Ellis for reference. P# 92861200106 It is the front end harness, not the front engine harness.

https://www.porscheatlantaperimeterp...861200106.html

The harness I got was made in 2002 so there is stock that has been sitting a while in the Porsche warehouses. Sometimes you get lucky with Porsche in that they don't change prices for an eternity.

Wisconsin Joe

First off, most of the "known problems" are listed in the "New Visitor" sticky.

While, in general, 'electrical problems' are common, the headlight system (in general) isn't one of the common ones.
Ask the seller to point you towards forum posts or writeups on it if it's that 'well known'.

Second, it sounds like this car has been rewired to at least some extent. That could mean all sorts of things. Only a few of them are "good things."
As was noted, if a wire 'fries' and melts, it usually damages other wires that are close to it.
Generally, that means replacing the whole harness (or a large section of it).

OTOH, the electricals in these cars is pretty basic. With a couple of exceptions, its just wires, fuses, relays and motors.
Nothing is integrated, like it is on modern cars.

It isn't something I would automatically scream "RUN AWAY!!!!" from, but it is something that would be of concern.
The primary concern isn't this particular issue, but more of a "what else has been done like this?" question.

Where is the car? In any long distance purchase, it's always wise to get a good look at the car from someone who knows what to look for.
Depending, you may find someone on here who is close and willing to take a look for you.