Another Electrical Quandry--seems to be a running theme today!

[jamesiii]

Hi all,

Been working on my 78 SC all weekend, running down an electrical conundrum that began 3 weeks ago. To make a long story short, a dead battery has lead me to the discovery that the PO had put a 25 Amp fuse in #4 (Front & Rear Parking Lights w/ engine compartment light--left), which should have been a 5 Amp fuse. Result...toasted wiring. The first 4 inches of the wire bubbled up and were burning through ALL of the other wires touching it.

When I was done with the immediate repair, I put an 8 Amp fuse in and the running lights came on for a split second before the fuse blew. When I put the 25 Amp fuse back in (just to test the light for a split second!!!!) the lights no longer worked.

I checked the voltage across all of the fuses in the trunk compartment with the running lamps turned on, and found #3 (Front & rear parking lights--right) pulling .07 volts, #10 (front flasher lights) drawing .02 volts, and #18 (interior clock, glove compartment, etc) drawing .03 volts. Nothing too alarming here; but when I checked the draw from #4 using the volt meter, 12.40 volts were being sucked from the short(s) created by the 25 Amp fuse.

Is there any quick fix to this? Can I get my running lights back by bypassing the existing circuit (isolating it and insulating the cut ends), running the front running lights off of new wiring, and the back lights off of the right rear circuit without any further serious injury to the electrical system? OR...Should I just admit to myself that I have to chase that entire wire down, check every wire it crosses/touches and look for EVERY possible breach in the wiring insulation?!?

I'm hoping there is an alternative to the latter scenario...doubt there is but I'm hopeful. Any suggestions would be much appreciated.

Thanks for your help and Happy New Year!!!

Jim

[jamesiii]

Don't everyone speak up at once!

[Early_S_Man]

Well, it seems like if you can isolate the problem to a particular fuse, you should be able to isoate the problem to a particular lamp assembly. Have you pulled all of the bulkhead connectors loose?

Unless you have four-legged, beady-eyed varmints chewing on your wiring somewhere, the problem is likely to be corrosion in one of the lamp assemblies. if you take all of them apart and clean them, you will likely find the problem!

[APKhaos]

Jim,
Sorry to hear you have toasties in the harness. Not good.

You could isolate the entire circuit, but that would mean running new lines to all three points - not easy to do, and will scream 'shade tree' to anyone who sees it. Hang tough - there is light in your life soon.

Now that you appear to have a solid short to ground somewhere in this circuit, you can do some tracing with your DVM on Ohms.
The key to this is to isolate which section [or sections] of your failed circuit are going to ground.

The first step is to check that you are reading very low resistance from the circuit side of the fuse to ground - the short. Suggest you pull the bulbs that this circuit is serving for this test.

Nest step is to disconnect the wires at each of the lamps on thsi circuit in turn - testing for the short as you disconnect each lamp. What you are looking for is a failure in the lamp assembly itself - corrosion, mechanical damage, etc. Chech each wire for signs of burning - if you find a crispy you may well have found your fault.
BTW, the front trunk lapm is notorious for causing this kind of short - look there first!

Next step - as Warren suggests, a good first step is to pull the bulhead connectors - in order to prove that the harness from the trunk fuse box to the front running light is OK. With the bulkhead connectors isolated, test for the short again.

If it is gone, then the problem is rear of the bulkhead. If it is still there, the problem is between the fuse box and the front running lamp, or bulkhead connector, or both.

If the problem is forward of the bulkhead, then its time to strip the harness some more to locate the other burnt areas, or to take a deep breath and run new lines taped up to the harness. It's possible that the burn has carbonised the insulation on adjacent wires in the harness, so its best to go find it now - before the other shoe falls.

Hope this helps - you should have it nailed in no time!

[jamesiii]

Warren,

I was hoping you would chime in with your .02! I have pulled the light assemblies and did find that one of the rear lamp assemblies was corroded (cracked rear lens) but I didn't think anything about it at the time. I'll pull the assembly & bulkhead connectors and check for APKhaos' tell tale signs of low resistance at each bulb.

Thanks to both of you for shedding some light on this issue of mine. I'll be working on this at night and hope to have some completion by the weekend!

Again, thanks for the help...can't tell you how thankful I am to have this BBS!

Jim

[Eagle Keeper]

I had a similar problem in my 79SC. It turned out that when I pulled the back off the tail light housing, I bent one of the wire plugs just enough to come in contact with the housing when I put the back on. So every time the light came on there went the fuse!! Might be worth a peek!!

[911pcars]

Good suggestions for tracing those elusive electrical problems.

Here's another. Connect a general purpose thermal breaker box in place of your fuse (available from your local auto parts store - about 5-10 amp capacity). When the circuit heats up again (it will because there's a short circuit somewhere), the breaker will open, then cool down again and close once the circuit is cool. While the breaker is opening and closing, wiggle the wire loom and see if you this makes a difference to isolate the short. There are also "tick" tracers, pen-shaped sensors that you pass along the length of a wire. The electromagnetic field the current produces causes the tick tracer to audibly or visually signal. If there's no current, there's no signal.

You can route new wire to bypass the bad stuff, but unless you follow the original loom and use the same color coded wire, this is bad news for troubleshooting years from now or for the next owner/technician. Have you ever tried to troubleshoot a PO's alarm system or audio system wiring? Usually so bad it's easier to start over.

Sherwood Lee
http://members.rennlist.org/911pcars

[APKhaos]

Hey Jim - let us know what you find as you work through this.

Sherwood's breaker suggestion is tricky - the breaker box can cool faster than the copper in the harness. A few cycles of this and you could be doing more damage. Best to keep the juice off this circuit and work with the ohmmeter until you think you have the short cleared.

BTW, if you need to open up the harness - don't worry! There is a really neat heatshrink tape available which does a beautiful job of refinishing repaired or rebuilt harnesses. If you think you will need it let me know and I'll see if I can track down a source.

Good hunting,

[911pcars]

"the breaker box can cool faster than the copper in the harness. A few cycles of this and you could be doing more damage. Best to keep the juice off this circuit and work with the ohmmeter until you think you have the short cleared."

Yes, this is true. I should have added a caveat and suggested using a low amperage breaker so the conductor doesn't overheat. I also suggest not doing this on any circuit that contains sensitve electronics (e.g. audio, alarm, ECU, etc.).

Sherwood Lee

[jamesiii]

Tony & Sherwood,

Thanks again for the suggestions. I'll keep you posted as I muddle my way through.

I've got the determination to make this car the beauty that's within, but without your direction this effort would be difficult to sustain!

Jim