Brake Light Switch and Brake Vacuum Vent Valve Switch, Checking
Brake light switch and brake vacuum vent valve switch, checkingBecause the Diesel injection system uses a throttle position sensor that could malfunction, for reasons of safety, the engine is regulated when the brakes are applied. The Diesel Direct Injection (DFI) Engine Control Module (ECM) requires signals from the brake light switch and the brake vacuum vent valve switch for this function.
If the brakes are operated while the accelerator pedal remains in a constant position, engine speed is immediately reduced to idle. Incorrectly adjusted switches could lead to unnecessary control.
Special tools, testers and auxiliary items:
VAG1551 or VAG1552 scan tool (ST)
VAG1551/3 adaptor cable
VAG1598/18 test box
Multimeter (Fluke 83 or equivalent)
VW 1594 connector test kit
Wiring diagram
Test Sequence:
Connect VAG1551 or VAG1552 scan tool
Switch ON ignition
Press -0- and -1- buttons to insert "Engine Electronics" address word 01.
Display will appear as shown:
Rapid data transfer HELP
select function XX
Press -0- and -8- buttons to select "Read Measuring Value Block" function 08
Press -Q- button to confirm input
Display will appear as shown:
Read measuring value block HELP
Input display group number XXX
Press -0-, -0- and -6- buttons to input display group No. 6 (006)
Press -Q- button to confirm input
Check value in display field 2
Read measuring value block 6 ->
0 km/h 0 0 0 00000 255
Specified display: 0 0 0
Center display element = Brake vacuum vent valve switch
Right display element = Brake light switch
Press brake pedal slowly
Read measuring value block 6 ->
0 km/h 0 1 1 00000 255
Specified display: 0 1 1
NOTE: Both displays must change from 0 to 1 at same time.
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 vacuum vent valve switch activation point by turning the switch
Adjust brake light switch point by moving plunger
If center display does not change:
Press -> button
Press -0- and -6- buttons to select "End Output" function 06
Press -Q- button to confirm input
Switch OFF ignition
Disconnect harness connector from brake vacuum valve vent switch -1-
Measure resistance across switch terminals
Specifications:
Brake pedal not depressed: less than 10 Ohms.
Brake pedal depressed: Infinite Ohms (no continuity)
If resistance is not as specified:
Replace brake vacuum vent valve switch -F47-
Display the readiness code
If DTC memory was erased:
Create readiness code again
If resistance values are OK:
Connect VAG1598/18 test box to Diesel DFI ECM harness connector
Check wiring for open circuit between test box and harness connector, using wiring diagram:
terminal 1 + test box socket 20
terminal 2 + test box socket 33
Wire resistance: max. 1.5 Ohms.
Check wiring for short circuit between connector terminals
Specification: Infinite Ohms (no continuity)
If the right display element in display field 2 (brake light switch) does not change
Press -> button
Press -0- and -6- buttons to select "End Output" function 06 and press -Q- button to confirm input
Switch OFF ignition
Switch multimeter to 20 Volt range
Connect multimeter using adaptors from VW1594 connector test kit, and check voltage between test box sockets 1 and 44.
Specifications:
Brake pedal not depressed: 0 Volts.
Brake pedal depressed: approx. battery voltage
If there is no voltage:
Check brake lights
If brake lights are OK:
Check wiring to brake light switch for open or short circuit using wiring diagram
If the brake light do not come on:
Replace brake light switch -F-
Display the readiness code
If DTC memory was erased:
Create readiness code again