Season of electrical problems - PeachParts Mercedes-Benz Forum

Carpenterman · #1 04-17-2010, 07:37 PM

The other day, in a rain, the wipers wouldn't work. I pulled over and replaced the red fuse on circuit #6 (wipers, washer). The original fuse looked good---in fact I had just cleaned all the contacts and fuses. And replaced some that looked questionable with new ones. But what the hey. You know how electricity is. Anyway, the wipers now worked.

It wasn't five minutes until I smelled hot plastic and the combination switch was hot on the back, about halfway along the plastic end. I turned off the wipers, and of course, once it cooled the stalk would no longer turn. The plastic was welded in the switch.

The fuse was not blown. Obviously the combination switch was toast. Why didn't the fuse blow before the switch burnt, I wonder? At any rate, I swapped in a new switch from the parts car and so far so good. I checked the motor, the linkages, and the plug on the firewall and they were OK, so if there was a short other than in the switch, I couldn't find it. Never could find the delay relay, even though the melting occurred at the low wiper speed.

While in the fuse box double checking, I noticed that sometime after the wiper incident, fuse #8 (ACC, blower motor, aux. pump) had gotten so hot that it had melted and compressed the end of the plastic fuse, but had not blown the metal fuse strip. Again, why are things heating up but not blowing fuses?

I replaced the fuse, ran the heater/blower/aux pump, and felt to see if the fuse or holder was heating up. It did get warm to the point of being uncomfortable to hold a finger against. The heat was at the lower side of the holder. Is that the feed or the outflow side? The aux pump is running OK (with a supplemental 5amp fuse on the line to save the ACC board in case of seizure) and so is the blower. Is some heat normal at these terminals? And where can I get some ceramic fuses rather than the lame (apparently) plastic bodied ones?

Thanks.

bustedbenz · #2 04-17-2010, 07:59 PM

It's a very long shot but check in the fuse box for metal trash, perhaps even burned fuse strip ends or pieces, that might be bridging two circuits. One way a circuit can melt instead of blowing its fuse is if it's getting power from a different circuit through a mechanical short that shouldn't be in place.

barry123400 · #3 04-17-2010, 08:18 PM

Go looking in the archives for blower motor fuse. Your issue with that fuse is not uncommon. The cure basically is to refuse the blower outside the fuse panel I think and a slightly higher fuse value is allowed.

In otherwords part of that blower wiring was a designed in problem to a certain extent. Usually an electrical engineer will work a fuse at a lower percentage of its trip value and design holders for it that are up to the job. I suspect there was a bit of a misteak and not enough allowance for reasonable aging in that one circuit on these cars.

Pooka · #4 04-17-2010, 08:27 PM

I had a problem like this on a Chevy Astro van. The fuse block itself was cracked and letting voltage run all over the place. That may not be a technically correct way of saying it, but I think you know what I mean.

I just put an inline fuse into the circuts that were giving me trouble.

Carpenterman · #5 04-17-2010, 08:39 PM

Excellent suggestions, all! I will explore these fixes. I thought I had removed all the old metal flakes when I cleaned the box, but I'll double check. I will also look for a crack in the base.

I remember discussions on the blower fuse. I'll search it out. At any rate, are these plastic fuses just crappy in general?

barry123400 · #6 04-17-2010, 09:43 PM

Quote:

Originally Posted by Carpenterman

Excellent suggestions, all! I will explore these fixes. I thought I had removed all the old metal flakes when I cleaned the box, but I'll double check. I will also look for a crack in the base.

I remember discussions on the blower fuse. I'll search it out. At any rate, are these plastic fuses just crappy in general?

On any circuit that draws enough current to heat at the fuse to contact points plastic will deform as most plastics go questionable at about 180 degrees. I always treat contacts with some form of anti corrosion paste when cleaning and renewing grounds etc.

I kind of know then in all likelyhood I will never have to do it again. Eventually car manufactures started doing the same on many plug connections. I really see it as good maintenance of these older cars anyways. I do not think the manufactures really expected these cars to still be running this many years later when originally built.

When the blower motors get a little older. From bearing drag etc they draw a little more current. Unfortunatly our digital meters have a current maximum of ten ampres. So you cannot read that circuits actual draw with out constructing a shunt for the meter.

A shunt is so easy to build to increase the amperage reading ability. You should look up meter shunts on the web. Basically just a piece of household wiring a foot long. Follow the simple instructions posted on using it.