Fried Wiring - help needed

Fritz · 12-29-2001, 12:15 PM

Hi all,

I've got a 73 E that is all torn apart for resto/painting/rust repair. When I got it back from the media blaster (easier to see all that rust damage now, sigh), I was vacuming out as much stuff as I could and was spending some extra effort up under the dash (yes, the dash pad is out) and guess what I found...

2 wires (so far) that have the insulation totally burned off of them. They don't appear to be the ground wires so I am assuming they are the hot side. First one I found was to the glove box light, second was to the clock. The burned insulation goes back several inches to the harness bundle. BTW, the clock was working when I tore the car apart.

2 questions:
1st. Any guesses what happened? Too hot a fuse used or fried during a jump start? Regulator problem? Tough to diagnose with the car all apart but I don't relish the thought of fixing it, putting it together and then frying them again.

2nd. Is there a source for a replacement wiring harness (Wayne, if you sell them just let me know and I will place the order, I did not see them in the online catalog)? No, I haven't been to the dealer yet to price it out. I'd prefer to replace the whole harness (oh heck, what's another several hours of work) than make a couple repairs not knowing how the rest of the harness is.

RoninLB · 12-29-2001, 03:42 PM

Looks like you know difference between fried and dried out insulation. High current draw should blow a fuse. I want to know answer to this dilemma. I know that Stoddard can order a harness from Germany. Don't know about other dealers. I use marine grade wireing in my 911 for my projects.

Jim Sims · 12-29-2001, 07:28 PM

Guess what Fritz! In a '73 the clock and glove box light are on the same circuit along with the interior dome lights (their wires may be burnt also) which are protected by fuse position 1 (up in the front luggage compartment). This fuse position should have a five amp fuse in it. What may have happened is the hot side on the circuit became grounded somewhere-probably one of the dome light terminals above the doors and the circuit fuse was too big. The 5 amp fuses are sometimes difficult to find so a higher rated fuse is put in and the result is burned wires when a short occurs. Check your interior lights (dome lights). There are some 20 or sections to a '73 911E wiring harness. Some OEM wiring harness replacement sections are available or can be made up; unfortunately your burned circuit is in one of the major ones. It may be several hundred dollars. I believe your burnt wires are in harness section number 911.612.002.10 (cables for luggage compartment from fuse to instruments); a lot of items behind the dash connect to this section of harness. You really should check it as the overheated circuit could have melted and shorted adjacent wires in the harness bundle. You may also need to replace harness section 901.612.035.20 (cables for interior lamp) which goes to the dome lights; this will likely require removal of the headliner. Not a lot of good news. Sorry.

Jim

APKhaos · 12-29-2001, 07:41 PM

Fear not, Fritz!!
Jim has identified a likely cause, which sounds reasonable.

If the damaged wires have not melted/damaged insulation on other conductors in the harness, then splicing in a new section is a reasonable approach. Use marine or aviation grade wire or the appropriate gauge, solder the splice points, and insulate with two layers of heatshrink tubing [one to insulate the joint, one for extra mechanical support]

If the failure has fried the harness, then try Parts Heaven - I'm sure they have several cadavers lying around that have perfectly OK harness sections you could use. Certainly try this before shelling our the bones for an OEM replacement.

Good news is that you have perfect access for this job - should be a snap.

RoninLB · 12-29-2001, 09:27 PM

Great action happening here. JimSimshas followed his profile and given a researched valuable answer. APKhaos seems to have used it and excited me more. IMHO, I don' t understand how 2 branches of 1 circuit could turn to toast on a 20-25 amp fuse with a short in 1 branch. A slow burn I can understand some what. Not sure of stock wireing in discusson. Would guess about 18 ga. A "fast" burn should blow 25 amp breaker and may have toasted 1 branch not 2, IMO. The "usual suspect", IMO, is cabin lighting circut.
Solder, definately IMO. I use non-lead, 4% silver solder.
Wires, compare aircraft to marine grade. Do what you think is best. Both available mail order.
Used parts, great answer, IMO. Many suppliers, IMO, have pails filled with wires and switches, etc. May be un-labeled. Give them time to find your wires.
Any way, the cabin lite circuit sounds likely, but why 2 branches of 1 circut, unless it's a "slow" burn. I don't want to research this when surrounded by talent.

Jim Sims · 12-29-2001, 10:12 PM

Here's a mechanical engineer's explanation: all the items on this circuit are in parallel. This circuit is the hot side; it goes from the fuse (position 1) to the first item and then to the next and so on. Each item has its own ground and the resistance of that item (clock motor, light bulb filament, etc.) limits the current going to ground at that point. Imagine a pipe flowing water (current) and small holes (resistances) at each item leaking water (current). Now imagine a short as as big hole (very low resistance) leaking a lot of water (current). All of the pipe upstream of this leak (short) will be flowing a lot of water (current). A lot of current generates heat quickly; the power or heat deposited in the wire goes as the square of the current: double the current and the heat quadruples. Also electrical resistance increases as the wire gets hotter making even more heat. All the wire "upstream of the short" in this circuit will be overloaded and excessively heated. I also noticed a branch off this circuit to the front trunk light (on the passenger side of the hood, underneath near the hinge); this is an item also known to cause shorts. If this is the cause the wiring to the dome lights will likely be okay. This bit of harness to the front trunk light is section number 911.612.044.20 (cables for luggage compartment light).

Jim

Doug Zielke · 12-30-2001, 07:54 AM

Jim,
Your explanation reminds me of an excellent Elec. eng. instructor I had years many ago. He could take any scenario and use a lucid example to get it into the heads of his students.
Good post!

Fritz · 12-30-2001, 08:25 AM

Thanks Jim, Tony and Ronin.

Very good explanation Jim, and thanks for tracing through the diagrams. Now I understand why everyone wants the color-coded diagrams. The black and white ones in the Haynes are tough to follow for an amateur like me.

My first suspect is the trunk light. There is a patched section, my guess is it was not put together right after one of the previous paint jobs and was grounded out. I will also look at the cabin lights but I don't recall seeing anything unusual there. No problem with the headliner. When I said the car was all apart I mean it is all apart except for the wiring harness.

I feel much better

Early_S_Man · 12-30-2001, 08:35 AM

Fritz,

Most of the wiring in the '71 diagram on Pelican is identical to your car ... the color codes are consistent (99% of the time) from year to year on the early cars ...

http://www.pelicanparts.com/911/911_Parts/911_electrical_diagrams.htm

john walker's workshop · 12-30-2001, 11:01 AM

the trunk light wiring is usually the cause of this. the light gets disconnected for some reason and the hot wire is left to dangle. the wire end is not completely insulated with a rubber sleeve, and eventually grounds on the body. the wiring burns from there to the fuse. dash lights on that circuit too. most of the time you can just make up a fresh loom for the lights and lighter and tape it to the original.