Checking signal from brake light switch F and brake pedal switch F47 . — VW

Checking signal from
brake light switch
F
and
brake pedal switch
F47
.
Function
Because the injection system operates with an accelerator pedal sender (potentiometer) which may be defective, the engine is regulated for reasons of safety when the brakes are operated. To do this the control unit requires signals from the brake light switch and also the brake pedal switch. This means that if the brakes are operated when the accelerator pedal is held at a constant position the engine is immediately reduced to idling speed. Incorrectly adjusted switches may lead to unrequired regulating action
Special tools and workshop equipment required
Hand multimeter
V.A.G 1526C
or
multimeter
V.A.G 1715
Fault reader
V.A.G 1551
or
vehicle system tester
V.A.G 1552
with
diagnosis cable
V.A.G 1551/3
Adapter set
V.A.G 1594C
Adapter cable, 121-pin
V.A.G 1598/31
Current flow diagram
Test prerequisites
Fuses must be OK.
The battery voltage must be at least 11.5 V.
Test procedure
Connect
fault reader
V.A.G 1551
or
vehicle system tester
V.A.G 1552
and select engine electronics control unit with
“address word 01”
. The ignition must be switched on for this purpose. (Connecting
fault reader
V.A.G 1551
and selecting engine electronics control unit
Chapter
).
Indicated on display:
Rapid data transfer HELP
Select function XX
Press keys
0
and
8
for function
“Read measured value block”
and confirm entry with
Q
key.
Indicated on display:
Read measured value block
Input display group number XXX
Press keys
0
,
0
and
6
for
“Display group number 6”
and confirm entry with
Q
key.
Observe display in display zone 2.
Specification: 0 0 0
Read measured value block 6 ->
0 km/h 0 0 0 000000 255
Middle display =
brake pedal switch
F47
.
Right display =
brake light switch
F
.
Operate brake pedal slowly. Both displays must simultaneously switch from 0 to 1.
Specification: 0 1 1
Read measured value block 6 ->
0 km/h 0 1 1 000000 255
Release brake pedal slowly into rest position again.
Both displays must change from 1 to 0 again.
Read measured value block 6 ->
0 km/h 0 0 0 000000 255
Press
→
key.
Press keys
0
and
6
for function
“End output”
and confirm entry with
Q
key.
Switch off ignition.
If one or both displays do not change:
Remove cover in footwell (driver side).
Pull 4-pin connector off
brake light switch
F
and
brake pedal switch
F47
.
Measure resistance between switch contacts 1 + 4.
Specification: Brake not depressed: ∞ Ω∼(no continuity), brake depressed: Max. 1.5 Ω
Measure resistance between switch contacts 2 + 3.
Specification: Brake not depressed: Max. 1.5 Ω, brake depressed: ∞ Ω∼(no continuity)
If the specifications are not attained:
Renew
brake light switch
F
and
brake pedal switch
F47
.
If the specifications are attained:
Connect
adapter cable, 121-pin
V.A.G 1598/31
to control unit wiring harness. The engine control unit remains disconnected.
Check wiring between
121-pin adapter cable
V.A.G 1598/31
and 4-pin connector for open circuit according to current flow diagram. Contact 2 + socket 1 Contact 2 + socket 2 Contact 3 + socket 65 Contact 4 + socket 32.
Cable resistance: max. 1.5 Ω.
Check the line between 4-pin connector contact 1 and central electrics according to current flow diagram:
Current flow diagrams, Electrical fault finding and Fitting locations
Also check wiring for short to one another, short to vehicle earth and short to battery positive.
Specification: ∞ Ω.
If no fault in lines is detected:
Renew engine control unit
Chapter
.