I can't find my shorts (fuse problem)

[Ben300SD]

Hello everybody. The interior of my 300SD has been dark for several weeks now, due to a short somewhere in my electrical system. The fuse which connects with my interior lights and my power antenna blew about three weeks ago. I replaced the fuse a couple days later, and the lights worked for about a day. Then fuse then blew again. When I went to replace it the second time, it blew as I installed it.

Today I finally got around to trying to fix the problem. I thought that the problem might be a mechanically stuck power antenna, so I disabled it. The fuse still blew. I checked all of the interior lights for bare wires and found none. It would be quite nice to have my interior lights work again. Does anybody have any ideas as to how I might find where the short(s) in my system is/are?

[Jim H]

Congratulations, you've been selected to receive one of the toughest troubles to shoot!

Basically, you have to find and isolate the wire or device where the short is.

You need a wiring schematic, a VOM (volt-ohmmeter) and some ingenuity. At the fuse block, remove the fuse and measure the resistance to ground from LOAD side of the fuse. A short will give you a very low reading to ground. CAUTION: Be sure which is battery side and which is load side. Measuring the battery side can "smoke" the meter!

How low is low? Normal resistance for 10A is 1.4 ohms or so, a 20A is 0.7 ohms, etc. The formula is Resistance = Volts/Amps where volts can be up to 14 in a normal car. These numbers will vary a little, as a 20A fuse doesn't necessarily blow at 21A!

Good, you say you found a low resistance at the fuse block? OK, check the schematic, find the first connector, open it and check each wire that leaves it. Remember, you're still measuring resistance to ground. Again, one wire should read really low, the rest should be normal. That's the path with the short.

Check the schematic again to find where the next wire goes, find the connector, etc. Keep tracing until there are no more paths for the short. That path, or device, has the problem. Let's say the wire to the door lamp tests "short." The last check is to test from the lamp socket end. Still "shorted?" There's a pinched wire between the socket and the connector. No "short" if you remove the socket? Maybe it's a bad socket.

I hope this makes sense. It's easy to trace on a schematic, and its more exciting on the car, where most of the fun is to find the connector!

Good luck on the detective work!

BCingU, Jim

[be459]

The intermittent short is very hard to find because you do not know when the short is going to come. When wire is shorted permanently, I use a device called Short Finder. It works well for me.

The device has two parts. One part is a flasher type box which is to replace the burn-out fuse and the other part is a very sensitive magnetic field strength meter. The short causes the flasher box on and off repeatedly and you use the meter to trace the strongest magnetic path. You dig in the place where the strongest magnetic path drops sharply and there is the short. The meter is very sensitive. You can detect the magetic field along all wires (thru walls) sharing the same fuse but the strongest path is the one with the short.

I got the device 20 years ago from J.C. Whitney & Co. (www.jcwhitney.com) for about $20. J.C. Whitney is still selling it for about the same price. Harbor Freight Tools (www.harborfreight.com) has it too.

I use it on older cars. I am not sure if it is good for new cars with all the electronics (I am worrying about the flashing causing power surge).

David

[Ben300SD]

Well, I kind of found the short. It is in the left sun visor somewhere. I currently have it removed from my car and all of the lights work fine. I was not able to locate the short in the visor, so I am getting a used one from a junked W126 for about $50. It is hard to find the black visors in good condition, though. Thanks for the help everybody.