How to locate a short circuit?

[Tom in seattle]

The #5 fuse blows.  The nomenclature on the fuse cover says it is for turn indicators, stop lights, back up light and fuel gauge. There are two wire sets attached to the #5 fuse panel.  I have detached them and eliminated the fuel gauge as the culprit.  This leaves the lighting system. The wire set goes into a large bundle and I can't trace them past that.   The wire colors are a strange twisted red, blue ,yellow and I do not find that color of wire in the tail light assembly.  With the wires disconnected from the fuse panel there is no power in the offending wires.  Up until now I have had no problem with the lighting, turn signal etc. all were operating just fine.

If you can help me hunt this gremlin with techniques etc, you input will be greatly appreciated.

[Minerva]

I usually trace a difficult to find short circuit with a bulb, get a bulb in a bulb holder and put 2 wires to it, put one wire in either side of your blown fuse and the bulb will light bright. You've power one side and the short will complete the circuit with an excellent earth.
Then start disconnecting things bit by bit, perhaps start with the rear loom connector which would isolate wiring front to back. Once the offending short is disconnected the bulb will either dim or go out depending on what's switched in in the circuit.

[Benz Dr.]

I've found a lot of wiring problems underneath the sill trim where it runs to the back of the car. All it takes is one pinched wire and it can be very hard to find. You should be able to determine that it's the cable going to the back of the car but not where the problem lies. If you find your problem there you will be obliged to remove the sill trim to locate the actual short.

[waltklatt]

Tom,
Try hooking up a light tester to the fuse panel #5 and then leave the key in run position(remove fuse for pump).
Then unplug left taillight assembly, if test light goes out, then you found the culprit.
If, not then do the right side, and so on.  You will find it.
Did you have a short before?  What changed just before blowing the fuse?

Walter

[Tom in seattle]

I haven’t solved this problem yet. From the wiring chart in the owners manual I find that the #5 fuse provides current to the brake light, backup light and the brake fluid level warning system.  I have disconnected the brake fluid sensor and the short  is not revealed, same with the systems of the instrument cluster.  Here’s where it gets interesting:  with the brake light system wires disconnected from the fuse box I get a hot wire from a light tester (key does not have to be turned on) at the disconnected wire. Also at the tail light box (both sides) I get a hot wire from the turn signal bulb socket. Additionally I ran a 12 v wire directly from the battery to the tail light area and found grounded circuits at the tail, stop and turn signal wire ends of the plug socket(disconnected).  This makes me believe the problem is in wiring leading there, though it could be elsewhere I always am suspect of the last thing I was doing: connecting wires to the auto transmission under the car and laying on my back across the door opening while looking up under the dash at the fuse box as well.

I’m encouraged to read that there could be a problem under the metal door trim.  I will investigate there as this could be the location of a short to the tail lights.  Also I will reconsider my wiring at the transmission.  I will completely disconnect it to see if the problem goes away.

Question:  where is the backup light switch?  Could it be inside the auto trans?  I don’t think it’s inside the shift lever box, just a light there I believe.  Thank God for  alcohol!

[teahead]

when you remove the #5 fuse, does one of the "tangs" have continuity to ground?

Maybe remove all the lighting connections (headlights, taillights, sidemarkers if you have them), and start connecting them one by one until it blows the fuse.

[Tom in seattle]

Problem Solved!  Maybe.  On recommendation of Benz Dr I removed the sill chrome piece and inspected the plastic covering around the wire bundle but could find nothing punctured.  I thought maybe if I attached a test light and manipulated the bundle I might see some indication of a flicker to locate an issue and guess what?  The short is no longer there.  I’m sure hoping it doesn’t recur as I reassemble it.  I’m sending my love and doing a preliminary happy dance for Benz Dr who got me this far.  And to all of you my thanks.  I did not know the wiring took that route.

[Benz Dr.]

Well, your welcome. 

 However, I don't think you're out of the woods yet. Putting it back together and expecting no more problems is a bit like putting air in a leaking tire and expecting it to never go flat again. Somewhere under that sheath is a bad wire just waiting to mess you up somewhere in the middle of nowhere.  Don't be that guy...........