Thread: MY14 D4 Intermittent Brake Light

Originally Posted by cjc_td5

My D4 has started an intermittent fault with the RH brake light. Press the brake pedal with the engine off and all three brake lights will illuminate correctly. Start the engine and all brake lights will work correctly for about 5-10 seconds then the RH light will go out. The other 2 lights (LH and upper window) work fine. If you leave your foot on the brake and press the button to shut down the engine the RH brake light will come on again and work correctly. The main lights setting has no effect on this fault.

I assume the brake light might be dropping out when the alternator charging kicks in? I wonder if it voltage related? Earthing? Any ideas?

Another but possibly related fault I have noticed recently is bad interference through the AM radio band when braking, but only when moving. When you braked to a stop the interference would disappear when the vehicle stopped moving...

Complete stab in the dark but... The antennas for the radio are on the rear side windows - I don't know which side for AM/FM. I am guessing that there is a short close to the antennas which is causing interference on the AM band. The fact that it is only apparent when moving indicates that it is the movement of the vehicle that is making and breaking the short.

I would look for any wiring issues from the light back into the rear cargo space. Good luck tracking down whatever it is.

Remove the RH light cluster,which is very easy to do,and while someone pushes brake pedal,wait for light to go out and check voltages.

Also wiggle wiring harness behind RH light cluster,electrical plugs,etc,and see if fault can be located.

Completely off topic,one of the work vans LED above the tailgate would work intermittently.
The fault is in the LED,with 12V there it will work sometimes,and not other times.A tap on the LED will also make it work at times.

Voltage when off, 0v.
Voltage when light on 11.5-12v when engine not running. When the engine is started, the voltage will stabilise at about 12-12.5v for 5-10 seconds, then drop to 0v when the light goes out.
No improvement when all connections jiggled about.