The fuse (#3) for the interior lights and right rear tail lamp is blowing and I have been unable to find the short circuit. So far, I have pulled the instrument cluster and tried to find the ground with the fuse removed. I thought it might be the lamp rheostat, so I cleaned it and resoldered the connection. It held for a few minutes, but then it blew again.

I have the M-B electrical troubleshooting manual, so I do have good wiring diagrams. My next step is to just try to isolate and check each of the loads.

Does anyone out there have some insight into likely short candidates or any troubleshooting shortcuts. I have both an analog and a digital meter and am usually pretty good with electrical problems. This one has me stumped.

Thanks.
Brent Smith
Atlanta, GA
1986 300E 274,000 miles

I remember a similar post in another forum months ago. I believe the poster finally found some fault in the rear tail light. He took some care in cleaning up the contacts and wiring back there and it solved his problem.

I hate electrical gremlins! Could be anything!

__________________


1989 300E

2005 Acura TL

2011 Hyundai Elantra Limited

Swing hard! Take chances!

Does that fuse power the licence plate illumination? If so check wires around trunk arms.



.

Both license plate lights are from Fuse 3. I neglected to check them when I had everything (fuse block, bulb check relay) apart. With Fuse 3 out, I removed the license plate bulbs and checked the resistance to ground of the lamp wires back to the bulb check relay and appears that they both are shorted to ground. I will still need to pull the lamp check relay to verify, as I suspected the relay was bad until I read your note.

It looks like finding the short and/or replacing the license plate wiring all the way back to the bulb check relay plug could be a real pain.

After pulling what little hair I had out, I found the short. It was the right rear marker lamp.

I took a blown fuse and used it to solder in wires to a miniature 8 amp circuit breaker so I could test under load without wasting fuses. Anyway, the right rear marker lamp positive contact is clamped in a piece of insulating plastic. It had cracked and allowed the contact to occasionally touch the lamp base (ground) and short out. I used JBWeld to repair the plastic part and all is fine.

Previously, I found out while trying to troubleshoot using only resistance to ground measurements that cold lamp filament resistance goes up by a factor of about 15 in the first 0.1 second or so when power is applied. So, what looks like a possible low resistance, say two or three ohms, will really draw much less current than it appears.

Brent Smith
Atlanta, GA
1986 300E 274,700 miles