1987 300D fuse #5 ... will a bad rheostat blow it?
 

I was just remarking to my dad earlier tonight how I haven't really had to do much with the 300D lately, and sure enough, on the way home, when I got in it outside the grocery store, all the gauges were kaput (everything but speedo and odo). After recovering from the minor coronary after not seeing the oil gauge rise, I drove it home (it was pouring rain and I had no flashlight) to the garage and found Fuse 5 blown ... didn't look like a break from it being an old fuse, but from too much draw ... had a burn mark on the plastic part of the fuse.
This explains the lack of gauges, and replacing the fuse brought them back. I'm inclined to think I should sort out the reason the fuse blew though, rather than have it blow again at an inconvenient time/place. I was fiddling with the dimmer last night, and it's very squirrely ... lights go out when I turn it all the way up, and I have to turn it up and down to find a spot where the lights come back on. Would a bad rheostat blow the fuse? Most accounts on the forum of bad rheostats involve the lights just dying. Tonight all the lights stayed on the whole time. I did see the lamp control is listed as being on Fuse 5, so I'm thinking this could be the cause of the blown fuse. Just wondering. Going to replace the rheostat anyway, because I want it to work, but I want to know if I still need to look elsewhere for the cause of the fuse blowing (i.e., brake lights, etc.).

Oh boy. Fuse 5 powers a lot of things. Many of the are very important.

Unfortunately the rheostat has nothing to do with that circuit.

Brake lights
Reverse lights
Gauges
Cruise control
Trigger for the warning buzzer relay

were the loads on fuse 5 I found in 5 minutes of searching the diagrams.

Given your past history I would be looking at your rear brake lights. Didn't we have a problem back there before that I helped you with?

Ah, I was going by the list inside the fuse box, and now that I'm looking at the wiring diagram, may have read lamp monitoring wrong. I don't see reverse lights on here, though? They are listed as fuse 7 in the E-Class Owners Bible and I think that's what I'm reading on the diagram. But yeah the brake lights are definitely on there, and they jump out at me since, yes, I had a problem with a short in one of the rear tail lamps, which caused the brake lamp to light in lieu of the tail lamp. I replaced the bulb holder and it was resolved. Then I had an issue with the side markers and one tail lamp blowing the #3 fuse, which I thought was resolved with fixing the wiring at the trunk hinge. Maybe not! :( Will inspect the tail lamp assemblies again.

... ugh, this also seems to be tied to every warning light in the cluster. Going to lose my mind.

It won't blow the fuse again. This drives me crazy because it's basically impossible to figure out the culprit. Yet I don't want to drive it because I could end up miles away from home with no oil pressure gauge, low oil sensor or brake lights, which are kind of essentials.
I checked from amp draw at the fuse box and get about 1 amp without brakes, and 6.5 amps with the brake pedal depressed. Is that second one high? I guess it wouldn't take much of a deviation to blow the fuse from there, but I have no idea what the deviation was or how to recreate it.
Guess I will pull the cluster and see if there are any warning lights wires that look suspect.
Should I just drive it a bit (without going too far) and see if it will blow again? Because aside from visually obvious problems, I don't know what to look for when no fault is present anymore.
Another thing that crossed my mind was the lamp monitoring unit ... since I had issues with other lights and fuses blowing, as mentioned in above post. Again kind of hard to test when it isn't doing anything offensive now.

Could be the brake light switch on the pedal. I had an issue with my old W123 300D blowing the brake light fuse and that switch was the culprit. There is a 87 300E at B & H over in Elkton if you need parts. For now I would just replace the fuse and see if it fails again.

Thanks! I might actually have a brake switch in one of my buckets of bits and pieces I got from hanno a few years back, or elsewhere. I find all sorts of little gems I didn't know I had. What color is the interior of the 300E in Elkton, just out of curiosity? I broke the third brake light housing while inspecting it, plus mine has always had a "melted" spot ... so if it's palomino maybe I'll grab it.

Its red. There is another there with (I think) the Palomino interior. Not 100% sure on the interior color but it is lighter. The grille from that car is now on my 300E though :D If you go they are in the NE corner of the yard. I may go back tomorrow if its not so cold and windy.

Uggggh this one. This is the one with black purple wires? I ended up pulling the fuse panel and putting in butt connectors to isolate the cause. Still have fixed it, just isolated the ones that kept the gauges going.

Stupid question and an offer:

1) If #5 blows on the road, can you just replace it to get home?
2) If you can't find a Palomino W124, I can grab the high-mount surround for you from a local yard if you like...

I spent hours on this. Sometime fuse would hold up for a drive only to blow the next time. I think mine is somewhere in the horn system. Something I def wanna fix with the drivers around here....

1) At this point, yes, I could just carry a few extras and probably be OK (I'm probably being a bit overdramatic, as I tend to be, haha). My concern is if it gets worse, and starts blowing them repeatedly while off in the middle of nowhere. I guess I'll see how far it goes on a new fuse now, though! I have not driven it yet since, just took the Jeep tonight.
2) I'll check the local one Billy mentioned, but if it doesn't work out, I'll let you know, that would be great! :)

Yep, sounds like it ... black/purple/red ... one thing that confuses me (granted I'm easily confused by wiring diagrams) is that the dome light is listed on this fuse, but works without the fuse (and is also listed on a different fuse). The dome light was flickering with the door open before ... so maybe I'll check that too, though I assumed that was a door issue, not the lamp.

Well, it certainly does seem to be the case that little parts like that fail during the worst times possible. So only drive it on nice, sunny days and it'll never blow again :D.

Could it be that the dome light coming on with the doors open or something is controlled by #5 but that its general function is controlled by something else?

Well, this strategy seems to be working so far ... hasn't blow again yet. :P

I did check out all the wiring under the rear seats and into the trunk to see if there was some disaster involving the stop-lamp wires, but visual everything was fine.

I'm waiting and watching.