W124 electrical issues

[16simon]

Hi all, was hoping for some help getting an electrical issue on my W124 sorted out....

There's a red light on the centre console that flashes when the ignition circuit isn't live (presumably it's meant to look like a car alarm). This worked as expected until recently when I fitted a radio to the car (I bought it without one installed but the connectors were there). I noticed that the red light would flash faintly when the radio was on.... I then also noticed that it did the same when the glovebox light was on.

I then noticed that opening the glovebox (so that the light came on) would make the radio cut out.... it would come back on when the glovebox was closed. Now the glovebox light isn't working at all.

Any idea what might be causing this? I'm presuming that all these things are related, and that if I can find out what it is I can - hopefully - fix it AND will have learnt something new about how the car electrics work.

BTW I opened the fusebox and cleaned the ends of relevant fuse but it didn't make any difference.

[r129sl]

Irritatingly, all cars from this period were fitted with an alarm at the dealer. Why they didn't use the factory fit system is anyone's guess. Presumably someone at MBUK did a deal with Scorpion, the manufacturer of the alarm. It isn't a very good alarm system in my experience. I always take it out whenever I get a car with one in. Anyway, the LED is part of the alarm system; there is usually a disabling key lock in the glovebox; a siren under the bonnet; and a control unit in the dashboard, just above the headlamp switch; it is plumbed into the interior light circuitry. I assume yours is having a meltdown: loose wire or bad connection somewhere. That's my guess, anyway.

[16simon]

At first I thought the light was part of an alarm but there's no siren under the bonnet (I had a previous W124 with a Scorpion alarm so know what to look for) and no disabling key in the glove box either.

Is the LED maybe just a fake alarm light that flashes when there is no power to the ignition circuit?

[Tino]

It sounds like they have wired it to an ignition live feed that goes to earth when the ignition is switched off. I wired a few of these in back in the day.
It's possible that for some reason the radio circuit has taken its negative feed from the light circuit. You may just be able to find another earth point.
Probably not a difficult fix, and if you fancy popping to ealing i'll have a look for you.

[16simon]

Ah, though it was something like that.

Thanks for the offer Tino, but cleaning up the earth points and replacing all the fuses seems to have sorted the issue.