P1785

DIAGNOSTIC TROUBLE CODE P1785
THROTTLE POSITION SIGNAL FROM THE FUEL SYSTEM, SIGNAL WRONG

Fault Symptom.
Warning lamp CHECK GEARBOX lit.
System's emergency program activated.

Conditions.
Throttle position signal missing or is outside permitted limits for more than 4 seconds.

Diagnostic procedure
The throttle position signal comes from a throttle position sensor on the throttle via the ECM and states the throttle's opening angle.

The signal is a 175 Hz PWM and the Transmission Control Module (TCM) uses the signal to control gear shifting steps and system pressure, among other things.

1. If the CHECK ENGINE lamp is on, read and remedy the diagnostic trouble code in the ECM system.

If the trouble code remains, go to step 2.

2. Connect ISAT scan tool to the car's data link connector. Check the throttle position signal by initiating THROTTLE POS on ISAT menu READ FUNCTIONS.

The nominal value when the accelerator pedal is not activated is about 9% and it then changes in proportion to the depressing of the accelerator pedal up to about 90% with the accelerator pedal fully depressed.

If the measurement result is correct, go to step 4.

If the measurement value is wrong, go to step 3.





3. With the ignition switched OFF, connect a BOB between the TCM and the wiring. Check the throttle position signal by measuring the voltage. Measure between pins 25 and 35. Nominal value is about 1.2 V at idling and about 2 V at 2500 rpm.

If the measurement result is correct, go to step 4.

If the measurement result is wrong, carry out continuity measuring of the cable between the TCM and the ECM. Check with ohm measurement between pin 25 to Motronic ECM pin 52. Nominal resistance is <1 ohm.

If the measurement result is wrong, check the wiring and remedy any faults.

If the measurement result is correct, go to step 4.

4. Clear the fault code, start and test drive the car and check if the fault code reappears.

If a fault code is recorded, go to "Actions Before Changing a TCM".