How to troubleshoot instrument light short 74 911

[not-so-fast]

After suffering the dreaded instrument light rheostat melt down of the headlight switch on my 1974 911 Targa and successfully repairing all the wiring and replacing the god-awful-expensive switch, and fusing the instrument circuit off switch post #58a, the fuse blew immediately when I restored power. Thankfully it did its job and confirmed that a short in the instrument light circuit is probably what fried the headlight switch. I had recently had the speedometer overhauled by a shop that specializes in that and shortly after the speedo was reinstalled I experienced smoke coming from behind the dash the first time I turned on the headlights. I immediately went back to the shop that had pulled and reinstalled the speedo and the mechanic checked all the connections. We ran the lights and no smoke was evident. Months later driving at night is when the switch had a nuclear meltdown.
So my question is, when Ii originally researched this forum for information on how to replace the headlight switch I remember reading someone's post that mentioned they experienced an instrument light short after a shop had rebuilt their speedo, but unfortunately didn't bookmark the post and now I can't seem to locate it. In any case I am most interested in how I can troubleshoot to locate the short? My first thought is to pop the speedo and remove ll the light fixtures, then test each one individually. But is it possible the short is occurring within the speedo itself?

[strictly]

The possiblities are

A) wires in speedo not connected in the right place
B) fault with speedo when rebuilt (but it is fairly simple...)
C) other wires in the wrong place.
D) other problem (two wired some how connected together during the "job")

I would troubleshoot it like this

1. Get a multimeter, with an audible sound when you connect both the black and red leads together when in "continuity mode"

2. disconnect your battery earth/ground

3. connect the mutlimeter black wire to ground/earth, and check by touching the red wire to another ground/earth....the multi meter should make a sound

4. now pull one wire off at a time, and connect the red lead multimeter to it, with the black on ground. You should not hear any sound, repeat with the wire connected in the place it was.

5. investigate any that make a sound. Investigate for example by

5A. looking at the wires

5B. using other wires to replicate the orginal design, but without going through the entire harnes..

It will require looking at the wiring diagram, some of the wires are

• "live"
• "switched live"
• earth/ground
• guage

Its been a while, so i cant comment precisely, but looking at the wiring diagrams will help. Post some pictures of the wiring on the back of the gauges.

[fintstone]

Depends on how you hooked up your fuse. Most connect one end of the fuse pigtail to the switch (58a) terminal and the blue/black wires that were on 58a to the other end. There should be two blue/black wires. One to the heater/vent panel control lights, ashtray light and the temp control lever light and the other to the gauge lights.

There is a third wire from 58a, it goes to the Hazard flasher indicator light (glows red).

If you hooked both blue/black wires to the new fuse, then a good start in troubleshooting is to disconnect one and eliminate half the potential places it could be shorted.

As far as the speedo, the only thing that could cause that would be the gauge light connector grounding out against something. That would be pretty easy to check (and obvious to the naked eye). It is pretty simple circuit. Anytime one works on a gauge, there is a potential to knock a wire off that one or another.

[not-so-fast]

Thanks so much, strictly and finstone, for the rapid and insightful replies. I have operated the car for nearly 20 years and have never once had an electrical issue until the speedo was worked on, which is why that is most suspect in my simple mind. I shall perform more troubleshooting per your advice and post results along with more pictures soon.

[fintstone]

I have a '74 Targa (my second) and have fried that switch as well....so let me know if you have any questions or need me to look at something on my car for you. It is usually caused by a wire being pinched behind the heat/ventilation control panel or the frunk light wire...but it certainly could be behind any gauge.

[not-so-fast]

finstone, kind Sir,
You wouldn't perchance happen to have a decent copy of the wiring diagram, would you? The only one I have is in a Hayne's manual and is barely legible.

[redranger]

My headlight switch burned up on my 86, pretty terrifying to see a flame at your knee while driving. In my case, it was a wire from the clock that had shorted. It took out the lights between the driver and passenger seat where the heater controls are. I replaced the switch and added inline fuses. It’s been a few years and it’s been fine been fine since, except for those lights between the seats that don’t work. If I recall, it was a blue wire that caused the issue. I’d start by pulling gauges out one by one and looking for burnt wires.

[timmy2]

https://www.dropbox.com/sh/4guggxlnz7holqv/AAAf8SZvYdIPJnpptPQJisDHa?dl=0

74 factory schematics. Print and paste together.

[fintstone]

That is the one I use too. Let us know if you have any questions...it is challenging at first.

[not-so-fast]

timmy2: can't thank you enough for this diagrams! At last something I can read legibly!
And to all, good news: I found the short rather quickly! The center wire in each of the bulb fixtures is free to move longitudinally within the fixture once the bulb is removed. Evidently when the mechanic was replacing one of the bulbs, the center wire slid outwards slightly such that, during insertion of the new bulb, the wire folded itself sideways and pressed against the inner bore of the bulb housing creating the short condition. I guess it's an easy mistake to make.
I will include a photo of the fuse installation in the instrument lighting circuit, similar to other threads on that topic.
I don't know what I would do without this valuable forum of information. My sincere thanks to all the contributors.