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P1785

Symptom: CHECK GEARBOX warning lamp on. TCM in Limp-Home mode.

P1785

Throttle Position. Signal Incorrect.


Fault symptoms

CHECK GEARBOX warning lamp on.

TCM in Limp-Home mode.

Conditions

The “Throttle Position” signal is absent or outside tolerances for more than 4 s.

Diagnostic help




• The throttle position sensor can be read with the diagnostic tool.

Select Data Display / Output Control (New)"".

Select " Throttle Position (Engine Running)".

The nominal value with the accelerator pedal released is approx. 9%. The value then changes proportionally to how far the accelerator pedal is depressed until it reaches 90% with fully depressed pedal.

• The throttle position signal comes from a position sensor on the throttle via Motronic or Trionic and provides information on the throttle opening angle.

The signal is 100 Hz PWM and is used by the control module to control the shifting points and the system pressure amongst other things.

A defective throttle position sensor will generate a DTC in the engine management system and turn on the CHECK ENGINE warning lamp.


Check the wiring

Jiggle the wiring harness at several points and in different directions to reveal intermittent breaks and short circuits in the various leads. Observe the multimeter, test lamp or diagnostic instrument while carrying out this check.

Diagnostic procedure

1 Check the DTCs in the engine management system


Check whether the CHECK ENGINE lamp is on.

Is CHECK ENGINE on?

Yes

Read and rectify the DTC in the engine management system if one is found, see Service Manual 2:7 "Engine Management System" for the respective systems. Then continue with step 3.


no

Continue with step 2.


2 Check the output signal from the engine management system





- Connect the BOB to the transmission control module connector but do not connect the control module.

- Start the engine.

- Take a reading between pin 25 and pin 35. The signal should be:

at idling speed: approx. 1.2 V 100 Hz 9% (+)

at 2500 ± 50 rpm: approx. 2.0 V 100 Hz 15% (+).


Is the signal correct?

Yes

Change the control module.


no

Rectify the relevant lead and continue with step 3.


3 Final check



- Clear all diagnostic trouble codes in the system.

- Perform a driving cycle:

• Drive the car at varying loads and engine speeds for 5 minutes.

- Evaluate the driving cycle:

• Make sure the trouble code has not recurred.


Has the trouble code recurred?

Yes

Continue with “ Diagnostic procedure before replacing control module "


no

The steps taken to rectify the fault were correct.