Blowing Fuse - PeachParts Mercedes-Benz Forum

mark82 · #1 01-13-2018, 01:29 PM

I fear this may be an unresolvable problem. The number 1 fuse blowing, which is the instrument lights, right parking lights. It will work fine for a day or two, even several hours of constant use, then seemingly without cause it will blow. Been through four already. Recent electrical work has been replacing the headlight switch, but that was months ago and this problem is only a couple weeks old. Ideas?

Also, perhaps related or not, a device I power through the cigarette lighter is immediately blowing its power supply fuse when i plug it in. There is nothing wrong with the device, it works fine in another vehicle. The lighter power is not on the number 1 fuse circuit, so it is powered when that fuse blows. Idea? Bad connection? Corrosion on contacts?

jake12tech · #2 01-13-2018, 01:37 PM

Check your grounds with a multimeter. There shouldn't be any voltage at the ground. Only at the power end. What you're describing indicative of a short.

Corrosion can add unwanted resistance as well. Make sure everything is clean.

also go on w124zone and find the ETM for some diagrams.

Diseasel300 · #3 01-13-2018, 01:51 PM

Fuses blow because of excessive current draw. If it blows once, the problem should be found before replacing it. As you've found, the fuse will continue to blow. There's probably a chaffed wire somewhere that's touching something it shouldn't or rubbed through to the chassis and only pops the fuse when it touches.

sixto · #4 01-13-2018, 02:22 PM

To the extent it’s safe to do, run without with front right parking light bulb, then without the rear right parking light bulb, then without gauge bulbs (remove the dimmer) until you find what blows the fuse. Check a wiring diagram for unlisted devices in that circuit (e.g., license plate lights).

Sixto
98 E320s sedan and wagon

Mxfrank · #5 01-13-2018, 03:12 PM

Removing the bulb doesn’t help if the problem is a chafed wire. Sometimes with this sort of problem, there’s nothing for it but to trace the physical cable run, and eyeball it for damage.

Diesel911 · #6 01-13-2018, 03:15 PM

Don't know what vehicle you have. If the 82 in your member name indicates the vehicle year if the fuses you are using have plastic bodies they bodies melt or distort.

If that is the problem ceramic bodied fuses or the pointed buss type fuses.

What is the amperage of that device you were speaking of?

Perhaps you could run separate wires, inline fuse and separate socket for you
device.

Sugar Bear · #7 01-13-2018, 06:21 PM

The appearance of a blown fuse can give a clue as to whether or it failed due to a direct short to ground or due to a slow build up of heat from a failing component like a blower fan or high resistance connection.

Look at the ends of the severed part of the fuse. Are the ends melted with a glob shape? If yes, there was a slow build up of heat from a component failing and drawing excess current or a bad connection in the circuit like a plug/connector that is loose.

If the severed ends are sharp/crisp it was a sudden high current draw probably due to a chafed wire as stated by previous members. Sometimes they are even sent in opposite directions.

It is common that the last repair is the problem, the headlight switch was replaced recently and is on that circuit. Can you swap it out with another one?

Good luck!!!

BodhiBenz1987 · #8 01-13-2018, 09:05 PM

If my memory serves me, when I had a similar issue it turned out to be wear in the wires that run from the cabin into the driver's side of the trunk and end up at the license plate lights ... they pass through the hinge and over time wear in that area. I'd have to double check a wiring diagram and look through some of my old photos but I know I had to do that repair and I'm pretty sure that was the issue it was resolving.