1994 w124 estate tail lights not working

[DITTRICH]

I got the bulb warning light on the dash.
Got home and checked the rear tail light clusters.
Only the bottom most lights (not sure what ones these are) work when I turn on the headlights.
The tailgate number plate lights are fine.
The headlamps dipped and main beam are okay.
The parking light settings don't seem to work anymore (turning the headlamp rotary dial to the left).
In the interior of the car the roof illumination light seems to have gone but the reading light still works.
I have checked the fuses in the bonnet fuse box visually AND removed #5 which it says controls the stop lights and sandpapered the fuse contacts and replaced.
I have checked the tail lights visually and they all seem to be okay and anyway I refuse to believe that they all blew at once.
The tail is wired up for towing and I checked the fuse on the additional setup visually and it seems okay. There seems to be some sort of relay connected to the towing wires and each time my son presses the brake, the unit clicks, so I assume a voltage is being applied.
The car will have to in on Monday pronto as non functioning brake lights are a bit of a concern. People's experiences and advice gratefully received.

 

[boomtings]

Wiring diagrams are here http://www.mbclub.co.uk/forums/electronics/93178-w124-wiring-diagrams.html

I've had failures caused by the light control module in the fusebox. You'll need to use a multimeter if your going to fault find the problem, otherwise the best you can do is clean any connections and the bulb contacts.

I'd hook a bulb up to the tow brake wire and see if that's functioning. Also might be worth isolating the tow bar wiring to eliminate it.

 

[Deadly Dave]

I had intermittent warning problems and the problem was caused by the relay box cover not being screwed down properly.

 

[DITTRICH]

I purchased a multimeter today and some cable/plug/clamp from maplins. I have a wiring diagram of sorts.
All the lights work except the brake ones.
I see only a brake switch and the lighting control relay on the wiring diagram I have, so tomorrow I shall see whether or not there is voltage at various points in the chain. Annoyingly, I seem to have mislaid the brake light switch which I removed from the car yesterday with the intention of testing it. My money is on the lighting control relay or the brake light switch. Never tried electrical fault finding before or reading circuit diagrams but how hard can it be?

 

[boomtings]

Good luck with the fault finding.
Let us know how you get on.

 

[DITTRICH]

Well, I managed to work out a few things...
1 with the ignition key turned to the position before the position at which the starter motor engages, I verified that all fuses were okay and I was getting 11.7 ish volts at all the terminals - that and fiddling a bit with the control stick and the rotary light dial.
I also verified that I was getting 11.7v before and after the brake light switch down by the brake pedal. After the brake light switch, the wiring diagram indicated a trip to the lighting control relay, and after opening up the relay box, I did not feel confident enough to prise out the relay as it is very big and quite firmly seated - if I knew where all the clips etc were... I also verified that I was getting no voltage at both tail lights which supposedly feed direct from the lighting control relay. I need to get the relay out in order to see what kind of voltage I am getting on the outputs for the tail brake lights...to find out whether or not the fault is in the wiring or the relay. Going to look for pics of the relay tonight. The book I used was a workshop manula w1`24 1985-1995 by Brooklands Books inside which is a wiring diagram for at 230TE which I assume is similar in terms of the braking circuitry. If I can't isolate the problem tomorrow, its going in to the garage on Wednesday am. But, having never read a wiring diagram before or used a multimeter, I'm feel a little more confident that I have narrowed down the problem.

 

[balge]

The lamp control just 'lifts' straight up - may require gentle encouragement, but it's just straight pins so if you keep the pressure straight up it should come out easily enough, iirc the pins are not symmetrical, it only goes in one way round?

Tow hitch and relay test


http://www.w124performance.com/service/w124CD2/Program/Chassis/54-6070.pdf


I would replace all fuses with brass contact ones instead of aluminium, they are prone to being rubbish...

 

[DITTRICH]

Thanks - will try tomorrow am - will be interesting!

 

[DITTRICH]

I will also double check voltage before and after fuses to see if any need attention.