Hmmm… blew an unusual fuse. - PeachParts Mercedes-Benz Forum

Shern · #1 10-08-2021, 11:52 PM

Barreling down the interstate after many electrical gremlin free years, and noticed my instrument cluster dead. Quickly isolated the issue to fuse number 12, swapped at a rest stop and continued along my way without incident.

A few years ago I installed all new ceramic fuses. I think the only thing I’ve ever blown was the classic number eight fuse before cleaning and lubing my blower motor.

This one’s a little odd as it’s such a low draw fuse. The only thing I can think of is my cruise control causing a surge…?

Do these fuses have a service interval that might explain this? Or does current just happen sometimes? I generally tend toward issue in need of resolution.

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vwnate1 · #2 10-09-2021, 01:30 AM

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250 Coupe · #3 10-09-2021, 10:47 PM

While I have seen fuses fail without an overload, their age is usually measured in decades. We lose a fuse in a PLC rack every year or 2 without cause. The racks have been running 24/7 since 1995.

I lean toward an over current event. Do all of the lamps work? It’s unlikely but possible that one could have failed in a way that drew just enough current to pop the fuse before going open itself.

Michael

Rogviler · #4 10-10-2021, 04:04 AM

I also hate when there's no apparent cause for a blown fuse, but it seems like it does happen. I got my wagon over 10 years ago and a few times it's done that thing where the turn signals stop working but they come back if you toggle the hazard switch. But one time it turned out to be that the fuse blew. That was not long after I got it and it hasn't done it since. It's very difficult to find a problem if it doesn't repeat itself consistently.

Shern · #5 10-10-2021, 01:59 PM

Yeah, no kidding... Most I can do is clean the terminals and hope for the best.

Though I do suspect it was the cruise amp. Or rather, me using the stalk continuously to adjust my cruising speed.

jay_bob · #6 10-10-2021, 05:02 PM

Well there are 3 ways these ceramic/plastic fuses fail
Short circuit (Fuse link blown to bits, often accompanied with collateral damage on the fuse holder)
Overload (fuse link melted, can also have melting damage to the plastic fuse carrier if sustained)
Fatigue failure (the link gets metal fatigue from thermal cycling and vibration and just lets go)

You may have just had a fatigue failure and it was just time for it to go.

pawoSD · #7 10-10-2021, 08:45 PM

I had the same thing happen long ago in my W126.....replaced the fuse, carried on, no issues for years and years after that....

Shern · #8 10-11-2021, 01:27 PM

Fingers crossed. Thanks Gents.

Shern · #9 10-11-2021, 03:02 PM

Well this is weird, I just blew the fuse again this morning by turning on my AC.
From the fuse diagram I can see the AC is on a completely different circuit. What on earth would be causing this??

Graham · #10 10-11-2021, 03:12 PM

Suggest locating the detailed wiring diagram for your car and finding out just exactly what else is on that fuse.

I recall having a short in the wiring going to the lights in the rear window switches. The short would only occur when the driver side rear door was opened. This would blow the fuse that also provided lighting to the cluster.

At first, not understanding the problem, I replaced the fuse. Then one time, that did not help. A trace on the cluster circuit board had blown disabling cluster lighting. This is documented here somewhere. I ended up hard wiring the cluster board (and rewiring the back door!)

Shern · #11 10-11-2021, 05:35 PM

Well, the number 12 fuse contains:

Stop lamp, turn signal lamp, instrument cluster, (relay auxiliary fan term. 86, relay power wind. term. 86, cruise control tach, control unit/heat. water valve for climate control system.

The AC compressor and blower motor are located on fuse 8.

The only thing I can think of is that perhaps the "relay auxiliary fan term. 86" refers to the Auxillary AC fan. I suppose that switches on whenever the AC is running. Possibly I have a short to investigate there?

tangofox007 · #12 10-11-2021, 07:56 PM

Quote:

Originally Posted by Shern

The only thing I can think of is that perhaps the "relay auxiliary fan term. 86" refers to the Auxillary AC fan. I suppose that switches on whenever the AC is running. Possibly I have a short to investigate there?

Removing the relay would prevent any short downstream from the relay from being a factor.

jay_bob · #13 10-11-2021, 08:20 PM

Relay terminal 86 is the + side coil of the relay, not the load.

Relay terminals
86 coil +
85 coil ground

30 contact common
87 normally open contact
87a normally closed contact

pawoSD · #14 10-11-2021, 08:52 PM

I'd be suspicious of the monovalve (water control valve) for the climate control, they've been known to degrade internally and start fires in W123s....

Shern · #15 10-11-2021, 10:06 PM

Quote:

Originally Posted by jay_bob

Relay terminal 86 is the + side coil of the relay, not the load.

Relay terminals
86 coil +
85 coil ground

30 contact common
87 normally open contact
87a normally closed contact

I’m not sure what to do with this exactly.

I only have so many fuses left, so I’m reluctant to re-create the problem without a strategy. At the moment, engaging the AC compressor is what blows the fuse.
Unfortunately I don’t know enough about that system to understand where it all starts. I think I read the AC dryer has a pressure switch that activates the fan?
No clue.

Which relay would you suggest I remove? If that is indeed the way forward…