P1785

INCORRECT THROTTLE POSITION SIGNAL FROM THE FUEL SYSTEM





Fault Symptom.
CHECK GEARBOX indicator lamp on. The system's emergency operation program has been activated.

Conditions.
The "throttle position absent or outside limits for longer than 4 s" signal.

Diagnostic Help.
^ The throttle position signal can be read using an ISAT scan tool.
Select "READ FUNCTIONS".
Select "THROTTLE POSITION".
The nominal value with the accelerator not activated (closed throttle) is about 9%. It then changes proportionally as the accelerator is activated until it is about 90% with the throttle fully depressed (wide open throttle).

^ 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 gearshifting 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.

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.





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?

YES: Obtain a readout of the diagnostic trouble code, if any, in the engine management system and take the appropriate remedial measures in the engine management system for the system in question. Then proceed to point 4.
NO: Continue with point 2.





2. Check the engine management system's output signal
- Plug a BOB into the transmission control module's connector, but do not connect the control module.
- Start the engine.
- Take a reading across pins 25 and 35.
- The signal should be:
at idling speed: about 1.2 V 100 9%(+)
at 2,500 ± 50 rpm: about 2.0 V 100 15 %(+).

Is the signal OK?

YES: Proceed to point 4.
NO: 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 OK?
YES: Continue fault diagnosis in the engine management system and then continue with point 4.
NO: 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.

Has the diagnostic trouble code recurred

YES: Continue as described under "Before Changing A Control Module". Before Changing A Control Module
NO: The remedial action taken was correct.