Checking the wiring and the relay

If the brake light does not light up, check the cable between the central electronic module (CEM) terminal #6B and the rear electronic module (REM) terminal #4A. Check for an open-circuit. Check for a short-circuit to supply voltage.

If the brake light is constantly lit, check the cable between the central electronic module (CEM) terminal #6B and the rear electronic module (REM) terminal #4A. Check for a short-circuit to ground.

HINT: When the brake pedal is depressed, there should be 0 V in the connections, otherwise 12 V in the connections. 

Check the relay (2/79).

HINT: There should be a 12 V supply to the relay switch to terminal #5 and in the supply to coil #2. When the brake pedal is depressed, there should be 12 V in the relay switch #3. 

Check the cable between terminal #5 and fuse 3 in the cargo compartment fusebox (11D/3).

Check that fuse is intact.

Check the cable between relay #3 (2/79) and shunt brake light #3 (20/31).

Check shunt (20/31).

HINT: The voltage must be 12 V at the shunt terminals, i. e. at terminals #3 and #2 when the brake light is active. 

Check the cable between shunt brake light #2 (20/31) and brake light #2B (10/54 for left brake light and 10/48 for right brake light).

Check the ground supply between the brake light terminal #1B and the ground terminal.

If no fault was found, replace the brake light.

Other information: 

• For information on signals:
  • -2004: see Signal specification
  • 2005-: see Signal specification
• To replace the brake light, see Bulb tail lamp/stop lamp/back-up lamp/fog lamp, replacing 
• See Checking wiring and terminals .