Reversing lights failed

[jbrodr]

On my E300 auto estate, both reversing lights no longer work. Both bulbs are OK. With the bulbs removed, there's about 9V across the terminals when reverse gear is engaged. With one bulb in, this goes down to about half a volt. The earth is OK.
Does anyone know which electronics box controls the reversing lights, and where is it located?
I've looked at the WIS, which, strangely, doesn't include reversing lights on any of the wiring diagrams.

 

[television]

Hello and welcome, is your car a 124 or 210

 

[Bolide]

What car are we talking about?

Can you explain again what voltages are at each bulb?

Do the reversing lights bring up a bulb failure warning when "on"? The Bulb Failure Warning Unit is like a Wheatstone Bridge - the two sides of the car are compared by the BFWU and the light is triggered when the current differs from side to side

Have you check the voltages at the internal plug & socket for the rear lights with the lights disconnected? The bulb holders tend to get corroded and make bad contact but the plug & socket assembly is silver-plated and normally doesn't corrode

Nick Froome
www.w124.co.uk

 

[jbrodr]

What car are we talking about?

Can you explain again what voltages are at each bulb?

Do the reversing lights bring up a bulb failure warning when "on"? The Bulb Failure Warning Unit is like a Wheatstone Bridge - the two sides of the car are compared by the BFWU and the light is triggered when the current differs from side to side

Have you check the voltages at the internal plug & socket for the rear lights with the lights disconnected? The bulb holders tend to get corroded and make bad contact but the plug & socket assembly is silver-plated and normally doesn't corrode

Nick Froome
www.w124.co.uk

Sorry Nick & Malcolm, should have mentioned, it's a 210.225, E300 Turbodiesel. R reg.
I disconnected both lights at the internal connectors: same problem. The open-circuit voltage is about 9V. I tried connecting a bulb (resistance about 10 Ohms) across the terminals, and the voltage then drops to about 0.8V.
I'd guess that there must be a high resistance somewhere in the line, either a fault in the harness between the light control unit and the tailgate, or a blown transistor (I'm guessing here!) in the control unit. I'm an electronics engineer, so I can repair this type of thing, but I need to know which box it is; I can probably trace it from there.
There is no BFWU warning indication when in reverse.
I can't find a wiring diagram anywhere in the WIS that includes the reversing lights: is there a drawing number that you can point me at?
Many thanks // Jim

 

[television]

Its in the gear selector housing

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