P1786 , P1787 , P1788
Throttle position signal from the Traction Control System, open circuit or shorted to ground (P1787) or B+ (P1788) and signal incorrect (P1786).
Fault symptom
Harsh gear-shifting when TCS activated.
Condition
P1786:
Generated if the TCS throttle position signal from the TCS control module is incorrect, outside limits, for longer than 4 seconds.
P1787:
Generated if the TCS throttle position signal from the control module is shorted to ground or the circuit has no continuity.
P1788:
Generated if the TCS throttle position signal from the TCS control module is shorted to the positive battery terminal.
Diagnostic help
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Throttle position signal readings can be obtained with the diagnostics instrument. Select "READ FUNCTIONS". Select "TCS THROTTLE POS".
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The signal is obtainable
only
on cars equipped with a 6-cylinder engine. It is a 122 Hz PWM signal which provides information on the TCS throttle position (opening angle). A faulty throttle position sensor will cause a diagnostic trouble code to be generated in the TCS system and the TCS indicator lamp 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 diagnostic trouble codes in the TCS
Check to see whether the TCS indicator lamp is on.
Did the lamp light up?
Obtain a readout of the diagnostic trouble codes, if any, in the TCS. Continue fault diagnosis as described in Service Manual 2:5 "Traction Control System" and then proceed to point 4.
Continue with point 2.
2 Check the TCS output signal
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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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Connect the Scan tool diagnostics instrument.
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Turn the ignition switch to the ON position.
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Select "BUTTERFLY CLOSING". The TCS butterfly can be opened and closed with diagnostics instrument keys F1 and F2.
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Connect a voltmeter to pins 15 and 35. For "BUTTERFLY CLOSING ON", the voltmeter reading should be: about 1.6 V 122 Hz 13%% (+). For "BUTTERFLY CLOSING OFF", the voltmeter reading should be: about 8.5-11.0 V 122 Hz 70-92%% (+).
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Is the signal OK?
Continue with point 4.
Continue with point 3.
3 Check the wiring
Check the wiring between pin 15 of the transmission control module connector and pin 4 of the TCS engine control module.
Is the wiring OK?
Continue fault diagnosis as described in Service Manual 2:5 "Traction Control System" and then proceed to point 4.
Repair or replace the wiring 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.
