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1987 300d turbodiesel
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Discussion Starter · #1 ·
I'm having an intermittent but growing-in-frequency failure of the instrument cluster in my 87 300d. When it occurs, I lose everything on the left side, keep the odo and speedo, and lose everything on the right side. The most uncomfortable part, however, is that all vents to the engine compartment open up and blow hot engine air (I'm assuming) into the cabin at a rate which seems tethered to my current speed. Some dash lights remain functional, but afaik I lose the washer fluid, e-brake, oil level, and brake wear lights. I also hear a hum from what seems like the left side of the cluster that stops if I activate the turn signal.

A note of interest: my speedo cable came loose from the bottom of the car recently and pulled itself slack. The speedo has never failed, however.

I suspect a bad ground but I have remained horrible at understanding electrical systems despite becoming fairly car savvy over my 40 years of life. I think a good next step would be to add a new ground wire, but I'm not 100% confident in how to do that. I did pull the cluster yesterday, inspected it and tightened everything back up. It worked for a 2 hour car ride but then failed again. Searching forums fairly thoroughly I've never found anything like this combination of intrument failure.
 

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1995 E320 Cabriolet, 1994 E320 Wagon, plus a brace of 124 diesels
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330 Posts
The speedometer and odometer are entirely mechanical, no electrical power there at all. You are loosing all electrical power to all of the instrument cluster. Yes, a bad ground could explain that. I think that is the most likely explanation and I would check that out first.

The hot air from your HVAC is the default mode when it looses power, so that is related some how. Perhaps they have the same ground point? When you pull the instrument cluster, to the bottom left side is a major ground point or two or three (my '95 E300 has three bolts holding down ground wires there). There is also a collection of grounds underneath and behind the radio, you may need to remove the center console to get a clear look at those.
 
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W124, 230E 5 speeds Manual, 1987
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546 Posts
Before checking for a bad ground. Check your fuses. I once had a such intermittent event and when the cluster failed i could hear relays buzzing inside the fusebox and at the end it was simply a bad corroded fuse contact.
 

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1986/1990 W126
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Yes you wouldn't believe the odd things that can happen from sometimes invisible corrosion on fuse contacts. Even if they look fine. First port of call should always be cleaning all the contacts in the fuse box and replacing them all with the better copper/brass tipped ones if possible.
 

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1987 300d turbodiesel
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Discussion Starter · #5 · (Edited)
Thank you all! I should have mentioned that I did indeed clean contacts, replace fuses, and check for continuity. Then despaired as that’s about the limit of my electrical ability ;-)

I’ll start hunting for ground points today and hope something becomes apparent....

Editing to add: the real culprit here is quite possibly my old engine mounts making a very rattley ride. I’ve needed to replace them for quite a while; the new ones are sitting in the trunk doing me no favors
 

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1995 E320 Cabriolet, 1994 E320 Wagon, plus a brace of 124 diesels
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Think back to see if there was some event that occurred prior to this problem emerging. Was there some repair completed or attempted, or some other event? That may give you a better idea of where to start looking first.
 

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1987 300d turbodiesel
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Discussion Starter · #7 · (Edited)
A couple updates: the buzzing sound is coming from the center relay. Because it’s buzzing, I’m led to believe the relay is okay, but only connecting on one side — which still points to ground.

I finally found a broken ground wire in the trunk, right at the hinge where the power goes to taillights, etc. Could this possibly be related? I’m asking here before attempting to connect because it’ll be a little difficult as space is tight in there lol.

Cluster being out isn’t ideal, but it’s the constant hot air in the cabin that is really driving me insane

EDIT: it was not the ground wire in the trunk (of course, in retrospect)
 

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W163 and General M Gremlin
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Max,
Just following this thread on the side line (for now)...
I have an 87 om603 a fella wants me to look at, after our long weekend up here. Some electrical gremlins.

Have you reviewed the electrical schematics for yours yet?

EDIT:
It's a monovalve and not a duovalve? on the 300Ds?
 
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W124, 230E 5 speeds Manual, 1987
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Since you know what relay is buzing (take a photo of that relay and post it here) you can unplug the relay grab your voltmeter and start testing the base pins to see what s really disconnected the ground or the positive.
 

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1987 300d turbodiesel
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Discussion Starter · #10 · (Edited)
Since you know what relay is buzing (take a photo of that relay and post it here) you can unplug the relay grab your voltmeter and start testing the base pins to see what s really disconnected the ground or the positive.
Thank you. I’m going to have to watch a couple videos on testing the base.

Edited for an update: I procured a circuit tester and found no current to many of the fuses, on either side of the holder, including fuse #5 which seems to be the important one here. (Most forum posts that I've found regarding bad instruments usually end with someone saying "aha! fuse #5 was bad" but I think I can fully rule that out in this case.) Regarding most of the other fuses, I've long known that many of the electronic controls are bad (seat, steering wheel, etc) but it hasn't bothered me.

I also reversed the tester to check a few obvious grounds -- near the batter and below the stereo -- and they were okay. I have not yet tested the grounds where they go in to the cluster.

It’s the turn signal and wiper relay, here are two photos:

Electrical wiring Electricity Electrician Engineering Hood


Hand Passive circuit component Circuit component Jaw Finger
 

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W124, 230E 5 speeds Manual, 1987
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Did you tested fuse 5 with ingition on or off? As if you tested it with ignition off it wont have power.
Get a piece of wire put it on the battery positive and give that fuse 5 some power and see if it solves the problem. Having 0v at the fuse isnt a ground problem it may be a bad disconnected positive wire.
Product Font Schematic Parallel Diagram
 
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