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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1981 240D 4sp manual. Ivory White.

Subscribed .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

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

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.

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.

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1981 240D 4sp manual. Ivory White.

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.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

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

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

Fingers crossed. Thanks Gents.

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1981 240D 4sp manual. Ivory White.

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??

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1981 240D 4sp manual. Ivory White.

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!)

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

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?

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1981 240D 4sp manual. Ivory White.

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

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Terminal 86 is the 12V side of any relay and is connected to 12V. In this case both the aux fan and the power window relay terminal 86s are powered from fuse 12.

Don't know if you have this, but section 24 has the wiring diagrams and shows the wiring for the various components powered by fuse 12.
Model 123 - OM616 Maintenance Manuals

If everything is working with a new fuse, it wil be hard to troubleshoot until the fuse blows again!

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

I hope you get to the bottom of this soon, electrical problems can cause serious damage .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

Certainly nothing I've done, though in the past, I have noticed the fan seems to activate almost instantly.

Thank you for this. Pardon my inexperience, but as I have manual windows, you are referring simply to the Aux fan relay? Would unplugging the Aux fan be the same as removing the relay or is that too far downstream?

A five-pack of fuses just arrived from Amazon so I suppose I have about four shots at getting this right.

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1981 240D 4sp manual. Ivory White.