P1785
Symptom: CHECK GEARBOX indicator lamp on. The system's emergency operation program has been activated.
Incorrect throttle position signal from the fuel system.
Fault symptom
CHECK GEARBOX indicator lamp on. The system's emergency operation program has been activated.
Condition
The "throttle position absent or outside limits for longer than 4 s" signal.
Diagnostic help
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A throttle position signal reading can be obtained with the diagnostics instrument. Select "READ FUNCTIONS". Select "THROTTLE POSITION". With the throttle pedal not actuated, the nominal value is about 9%%. This value then changes proportionately to the depression of the throttle pedal up to about 90%% when the pedal is fully depressed.
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The throttle position signal comes from a position sensor on the throttle via the Motronic 4.1 or Saab Trionic (4-cyl.) or Motronic 5.2 (6-cyl.) and provides information on the opening angle of the throttle. The signal is a 100 Hz PWM signal. The transmission control module uses it to control gear-shifting points and system pressure, among other things. A faulty throttle position sensor will cause a diagnostic trouble code to be generated in the engine management system and the CHECK ENGINE lamp (MIL) to light up.
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Check the wiring
Intermittent faults may occur as a result of temporary short circuits and breaks in the wiring. Jiggle the leads at several places and in different directions to reveal any faults in the wiring harness, including connectors. Observe the multimeter, test lamp or ISAT scan tool while carrying out this check.
Diagnostic procedure
1 Check the diagnostic trouble codes in the engine management system
Check whether the CHECK ENGINE lamp (MIL) is on.
Is the CHECK ENGINE lamp on?
Obtain a readout of the diagnostic trouble code, if any, in the engine management system and take the appropriate remedial measures, see Service Manual 2:7 "Engine management system" for the system in question. Then proceed to point 4.
Continue with point 2.
2 Check the output signal from the engine management system.
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Plug a BOB into the transmission control module's connector, but do not connect the control module.
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Take a reading across pins 25 and 35. The signal should be
at idling speed:
about 1.2 V 100 Hz 9%% (+)
at 2500 ± 50 rpm:
about 2.0 V 100 Hz 15%% (+).
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Is the signal OK?
Continue with point 4.
Continue with point 3.
3 Check the wiring
Take a resistance reading across pin 25 of the transmission control module and pin 54 of the Motronic 5.2 (6-cyl.), pin 52 of the Motronic 4.1 (4-cyl.) or pin 57 of the Saab Trionic. The resistance should be less than 1 ohm.
Is the resistance correct?
Continue fault diagnosis as described in Service Manual 2:7 "Engine management system" and then continue with point 4.
Check and if necessary replace or repair the wiring or connectors and continue with point 4.
4 Final check
Clear the diagnostic trouble code, drive the car on test at varying engine loads and speeds for 5 minutes and obtain readouts of any diagnostic trouble codes that are generated.
Does the diagnostic trouble code recur?
Continue with
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The remedial action taken was correct.
