Porsche 928 Workshop Manual page 603

24
Fuel System, Electronic Injection
926 s4
929 GT
If the flap does not open:
At the LH control unit plug, connect terminals
17 and 21 with the test leads. Separate the
plug connection at the solenoid valve. Conned
voltmeter to plug socket (2) and ground.
Display: battery
voltage
No display:
Check power supply to solenoid valve by way
of LH relay XXV according to the wiring dia-
gram. Check that there is no break in the cir-
cuit from LH control unit terminal 34 to the
solenoid.
d) Checking the control signal with the
oscilloscope
Attach the LH control unit plug. Disconnect the
plug at the solenoid valve. Connect the engine
tester with the test leads to the sleeve side of
the solenoid valve plug in accordance with the
manufacturer's insfructions, and adjust (bei
Bosch MOT 300/400. but select screen range
of 20 V). While starting the engine and if the
throttle is opened suddenly when it Is running,
the following picture must be visible on the
tester:
1 /
1 1 I t
Test point 20: Fuel tank venting (V)
When the engine is at regular operating
temperature and the throttle valve is opened
(idle speed contact), the solenoid valve is acd-
vated by the LH control unit with an intermit-
tent signal to ground. The greater the airffow at
the air mass sensor, the longer the solenoid
valve remains open.
a) Checking solenoid valve activation
Connect terminals 17 and 21 with test leads at
the LH control unit plug. Use another test lead
to connect terminal 27 to ground (on body):
the solenoid valve is energized.
If the valve does not open:
Check power supply and continuity of circuil
from terminal 27 fo solenoid valve according to
the wiring diagram.
b) Checking power supply 01)
At the control unit plug, connect terminals 17
and 21 with the test leads. Disconnect the sole-
noid valve plug and connect the voltmeter be-
tween the plug connection and ground with
the aid of test leads.
Display: battery voltage
No display:
Check power supply through LH relay accord-
ing to wiring diagram.
35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19
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D 24 - 26
LH-Diagnosis/Troubleshooting
a
Printed in Germany - xxv, ,990