Electrical Gremlin! - PeachParts Mercedes-Benz Forum

PanzerSD · #1 06-20-2006, 10:07 PM

My Car ('80 SD) keep blowing the "dash lights and indicators" fuse, I'm not an electrical dummy, just don't have the time to hunt for the fault.
It blew thru 2 8 amp (proper fuse) fuses, and so far a 16 amp fuse.
What's the most likely problem here? I have upgraded tail lamps but they've been running on that first 8amp fuse for a year. There is a little bit of corrosion on the terminal that the fuse snaps into, could this be causeing the higher current draw??

Thanks in advance,

Ryan

dkveuro · #2 06-20-2006, 10:25 PM

Check the illumination reostat for short to ground.
Pull the instument panel ....it's on the back side.

I was in Saskatu....Sashc....Canada, in 2002...how'd you get on with those rough pot holed roads up there ?
And what's with the old biddy at the border just south of Regina ?

.

P.E.Haiges · #3 06-21-2006, 01:04 AM

Got,

No, the corrosion on the fuse terminals can't cause more current to flow, but it can cause less current to flow.

I have seen corrosion on the fuses and terminals make a circuit go completely open.

P E H

PanzerSD · #4 06-21-2006, 01:23 AM

Okay, I decided to get off my lazy ass and check things out.

The tail light upgrade is only a simple change from the little round single filament bulbs to the better 1156 style bulbs. I removed all those bulbs...Pop...there goes the fuse. Ok, not the bulbs. So I scratch my head a bit and take a closer look and notice that my Dash lights, rear indicator lights, my Passenger side indicator lamp, and only one (drivers) rear inboard marker lamp is not functioning. whih I'd expect because they are all on the same circuit. but why leave the two indicator lamps lit when the entire circuit is down??

So I played some more, the Fuse 8a. Doesn't pop with the car off.
if I turn the lights on they come on. if they're off while I start the car the fuse doesn't pop. If they are on when I start the car, the plugs glow, when I turn the engine over, the lights in the dash dimm then pop goes the fuse.

So My determination was that something is overloading that circuit when the starter engages.

On further investigation, and trial and error I tested the battery. 12.25V no load except clock and radio.
Car running, no load again just the radio and clock. 12.60V UH OH I said.
Alternator or battery. an obvious and servere problem when I think of the glow plugs.

But is the blowing fuse and the faulty battery and/or alt. connected some how? could the voltage regulator malfunction in such a way that it sends a surge thru only one circuit?
I will consult my ETM and see what I see....

anyone have any ideas or theorys?

P.E.Haiges · #5 06-22-2006, 01:37 AM

Got,

U are not getting an over voltage problemm, U are getting a short circuit.

This is demonstrated by the lights dimming beforethe fuse blows out. The short circuit causes a high current in the wires and the resistance of the wires causes a voltage drop just before the fuse blows out.

Nothing wrong with your battery but your voltage regulator is causing a higher voltage than usual. It should be a nominal 14 volts. However 14.6 volts isn't enough to cause a fuse to blow, that's only 0.2 volts higher than normal.

P E H

PanzerSD · #6 06-23-2006, 12:27 AM

Quote:

Originally Posted by P.E.Haiges

Got,

This is demonstrated by the lights dimming beforethe fuse blows out. The short circuit causes a high current in the wires and the resistance of the wires causes a voltage drop just before the fuse blows out.

Ok, if it's a short, then when it happens during starting there must be a break somewhere that is making ground from some vibration from the starting....

My alternator is going bye-bye soon anyway I think, from the 12.60V when running.

Jeremy5848 · #7 06-23-2006, 12:45 AM

Quote:

Originally Posted by Got_The_Benz

Ok, if it's a short, then when it happens during starting there must be a break somewhere that is making ground from some vibration from the starting....

In many vehicles, when the "ignition" switch (even the factory calls it that in the 300D wiring diagram) is turned all the way to "start," many accessory circuits are cut out, to minimize battery drain. The circuit that is blowing fuses may be one of them. Once the engine starts and the key is released, the circuits are re-energized and the fuse blows.

In any case, P E H is right, you have a short somewhere. It must not be a major short or the fuse would blow instantly. The lights dimming and then the fuse blowing suggests that the short is drawing just more than the fuse can support, say about 20 Amps.

I would suggest removing every bulb on that fuse and check out the connections at each socket while removing the bulbs. Start with the easiest and proceed. If you can get a (say) 10 Amp mini circuit breaker at an electronics shop, wire two clip leads to it and clip them across the fuse terminals. That will save you from constantly replacing the fuse as you gradually eliminate one item after another.

Each time you remove something from the circuit (and examine its wiring), try the circuit to see if the breaker or fuse pops. Keep going until you find something, perhaps a wire that is touching something that it shouldn't, a wire with frayed insulation, two crossed wires, a wire on the wrong terminal, etc. It may be a slow procedure but hang in there.

PanzerSD · #8 06-23-2006, 10:25 PM

Yeah, I have a game plan, gonna reserve my Dad's garage and start seeking...

I'll let ya all know what I find.

Ryan

PanzerSD · #9 06-24-2006, 01:02 AM

I'm reviewing the Electrical diagram for my SD, and I happened to notice the glove box diagram. if the ground is broken or shorted out.........your brake lights won't work...

pure German silliness. LOL

Furious D · #10 06-24-2006, 01:10 AM

Also, don't forget to check the grounds. If things are not grounded properly, and voltage is really low, strange stuff can happen.

PanzerSD · #11 06-24-2006, 10:34 PM

I wasn't under the hood for 30 seconds when it made itself obvious...there was a wire pinched between the battery and battery tray. it jiggled when I started the car and would short out.

Thanks for the help guys.