Brake Light Switch: Testing and Inspection

Because the Diesel injection system uses a Throttle Position sensor which could be faulty, for reasons of safety, the engine is regulated when the brakes are applied. The Engine Control Module requires signals from the brake light switch and the brake pedal switch to accomplish this.
If the brakes are operated while the accelerator pedal is held at a constant position, engine speed is immediately reduced to idle speed. Incorrectly adjusted switches could lead to unnecessary control.

Special Tools, Testers And Auxiliary Items:
- VAG 1551/1552 Scan Tool with VAG 1551/3 adaptor cable
- VAG 1598/18 Test Box
- Fluke 83 multimeter
- VW 1594 adaptor kit
- Wiring diagram

Test Sequence:
- Connect VAG 1551/1552 Scan Tool
- Switch ON ignition
- Press 0 and 1 buttons to select address word 01 "engine electronics"
Display will appear as shown:

Rapid data transfer HELP
select function XX

- Press 0 and 8 buttons to select Function 08 "Read measuring value block"
- Press Q button to enter input
Display will appear as shown:

Read measuring value block HELP
Input display group number XXX

- Press 0, 0 and 6 buttons to select display group 6
- Press Q button to enter input
- Observe display in display zone 2

Read measuring value block 6 ->
0 km/h 0 0 0 00000 255

Specification: 0 0 0
- Operate brake pedal slowly

Read measuring value block 6 ->
0 km/h 0 1 1 00000 255

Specification: 0 1 1

NOTE: Both displays must change from 0 to 1 at same time.
- Center display = Brake pedal switch
- Right-hand display = Brake light switch

- Release brake pedal slowly into rest position again

Read measuring value block 6 ->
0 km/h 0 0 0 00000 255

Both displays must change from 1 to 0 again

If one or both displays do not change:
- Adjust brake pedal switch point by turning the switch
- Adjust brake light switch point by moving the plunger

If center display does not change:
- Press -> button
- Press 0 and 6 buttons to select Function 06 "end data transfer"
- Press Q button to enter input
- Switch OFF ignition





- Disconnect harness connector from brake pedal switch -1-
- Measure resistance between switch terminals
Specification:
Brake pedal not depressed: less than 10 Ohms.
Brake pedal depressed: Infinite Ohms.

If specification not obtained:
- Replace brake pedal switch F47
- Display the readiness code

If DTC memory was erased:
- Create readiness code again

If specification obtained:





- Connect VAG 1598/18 Test Box to ECM harness connector





- Check wiring between Test Box and harness connector for open circuit using Wiring diagram:
terminal 1 + socket 20
terminal 2 + socket 33
Wire resistance: max. 1.5 Ohms.

- Check wires for short to one another
Specification: Infinite Ohms
Right-hand display does not change

- Press -> button
- Press 0 and 6 buttons to select Function 06 "end data transfer"
- Press Q button to enter input
- Switch OFF ignition
- Switch Fluke 83 multimeter to 20 Volt range
- Connect multimeter between Test Box sockets 44 + 1 using jumper wires from VW 1594 adaptor kit and observe voltage display
Brake pedal not depressed: 0 Volts.
Brake pedal depressed: approx. battery voltage

No voltage displayed:
- Check whether brake light functions

Brake light OK:
- Check wiring to brake light switch for open/short circuit using wiring diagram

Brake light not OK:
- Replace brake light switch F
- Display the readiness code

If DTC memory was erased:
- Create readiness code again