Porsche 928 Workshop Manual page 604

929 S4
929 GT
Fuel System, Electronic Injection
24
c) Checking control signal with
oscilloscope
Connect DME test lead (Bosch No. 1 684 463
093) between solenoid valve and plug connec-
tion. Connect the engine tester in accordance
with the manufacturer's instructions and adjust
It.
Start the engine and open the throttle slightly
(the idle speed contact must open, engine
temperature must be > 60°C). The following
display is then obtained:
Fuel tank venting
:--:h
As air flow increases, the signal broadens
Test point 21: Alternator, regulator
Engine misfiring may be caused by alternator
voltage peaks.
Remove the drive belt from the alternator and
start the engine.
If this eliminates the fault, check the alternator
and regulator.
Test point 22: Leads K and L
The diagnostic connectino between the LH
control unft and the diagnosis testers is by way
of the two leads K and L. If no diagnosis Is
possible, the following tests must be carried
out:
1. Continuity test (refer to wiring diagram)
!&a&L: Pin 7 (IQ-pole diagnostic socket
Pin 16 (LH plug)
!&adK: Pin 6 (19.pole diagnostic socket
- Pin 12 (LH plug)
2. Ground short test (refer to wiring diagram)
LexU.: With the ignition switched on, the volt-
age at Pin 7 (19.pole diagnostic socket) must
be > 8 V betragen.
!&z&U: With the ignition switched on, the volt-
age at Pin 6 (IQ-pole diagnostic socket) must
be > 8 V betragen.
Possible fault causes:
- Ground short or break in wiring or at plug
connections.
- Faulty control unit connected to these leads
(this may be any control unit with diagnos-
tic capability and must not necessarily be
the LH control unit).
Checking: disconnect plugs in succession at
the control units with diagnostic capability until
the voltage > 6 V. Renew the control unit
which is causing the fault.
- Tester faulty.
- No power supply at the diagnostic plug.
- No ground at the diagnostic plug.
LH-Diagnosis/Troubleshooting
Printed In Germany - XXV, lsso
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