W124 inst. lites STAY ON !!

[MisterLarry]

I've torn the "threads" apart and find no help with my problem. I can't get the instrument lights on the 300E ('93) to turn off.

I'm assuming it's in the light switch and that I might luck out if I remove it, spray and/or heat it, to hopefully remove any dirt and/or condensation.

Am I on the right track??

[mbdoc]

Are they on all of the time the key is ON, or even with the key OFF?

[babymog]

I suspect the instrument panel ground.

[MisterLarry]

Yep, they are on even when the key is off & pulled out & the doors locked for the night.

Re: the ground connection possibility, assuming that is NOT fused, would that not cause other malfunctions I might look for in troubleshooting?

[babymog]

The cluster is grounded, and there are circuits that are energized all of the time that need the ground to be energized/lit. Usually when more than one circuit is energized but not completely it is something common to all, such as a ground, and the electrical path is in through one / out through the other. Since the dash has both; unswitched +12v and grounds, it is possible that the ground is weak. The instrument panel parts are grounded together mechanically also.

Try another thing: Release the bail and unplug the wires on your alternator. I don't know the year of your car, but I believe that all years can be unplugged in this way. A glowing alternator light with the key off could indicate a bad voltage regulator/alternator.

[MisterLarry]

OK - my BAD! The lights that will NOT turn off are the lo-intensity "night" lights as opposed to the Warning Lights (for lack of a better term). So the back-lit icons around the light switch, the seat position buttons in the doors, the dash vent, etc. Now that I think about it, I'm not sure about the heater panel buttons. I can check after the sun goes down.

All the red & yellow dash warning lights turn off. All the exterior lamps turn off.

Anyway, does that sound like a ground connection? While I'm thinking out-loud, are these all connected to the dash rheostat? Could that be the culprit? How would turning OFF the light switch NOT turn power off to the rheostat?

Oh, bother !!

[babymog]

Got it, something entirely different. In the 124, there is a light control "relay" box behind the speedometer that controls backlighting intensity.

I don't know the wiring by memory, so am not sure whether there is an unswitched power to that box.

The headlamp switch however does have separate contacts for the instrument lighting, and it could have failed closed? It is a job to get to the headlamp switch, need to drop the bottom dash panels to do so, but it does unplug. If unplugging the switch kills the lights you need a new switch. If not, the next stop in my opinion, the dimmer control box.

Have you tried turning the switch further to the left into the "park lamp" positions to see if the instrument lights turn off? Do they still dim/brighten with the rheostat in the dash?

[MisterLarry]

Oh - what a good question.

I turned them to both park positions the other night. They stayed on! But I didn't try to dim them. I'll try that in a few hours when the sun goes down. It's just too bright outside right now to see if they're on and/or off.

FYI - I had the rheostat replaced about 5000 miles ago and everything was working fine for quite a while after that. However, the car (and therefore the light switch) has about 200K and a couple decades. That's a lot of use.

Let's see what happens after dark.

[MisterLarry]

Back to the question asking if the rheostat has any impact on the lights that stay on - NO. But I did discover that the rheostat only works on the instrument lights in the dash, not these back-lit illumination bulbs. The intensity on these (around light switch, inside window switches, under gear shift lettering) is only reduced by the activation of the light switch itself. They get dimmer as soon as you turn the headlight switch to parking lights.

I have looked at the Haynes manual and found that their schematics read easier than those in the Owners Workshop Manual (published by Technibooks) that I was referring to. According to Haynes, the Illumination-dimmed as well as the Illumination-not dimmed are routed through the instrument panel from the light switch, via the light control unit. But it doesn't appear to go "through" the light control unit. So the light switch looks like the source of the problem.

Does it make sense that the switch is broken in the "on" position (circuit closed) yet the dimming action still works when the parking lights are turned on? I can't determine where the extra resistance is introduced to perform that dimming function but hopefully it is NOT inside the light control unit. That part looks like it might cost more than a new light switch.

[babymog]

Could be the switch, I've not had one open to be able to say how it can fail though.

If you look for a used switch, the early ones ('86-'89/'90) without rear fog will not work, different plug/socket.