CIRCUIT DESCRIPTION:The park/neutral (P/N) switch contacts are a part of the neutral start switch and are closed to ground in park or neutral and open in drive ranges. the ECM supplies ignition voltage through a current limiting resistor to Orn/blk wire and senses a close switch when the voltage on the orn/blk wire drop to less than one volt.
The ECM uses the P/N signal as one of the inputs to control:
* Idle speed (IAC)
* Exhaust gas recirculation (EGR)
* Spark Timing
* Fuel control
code 31 symptoms:
* The orn/blk circuit indicates P/N (grounded) while in drive range, 4th gear TCC engaged, EGR would be inoperative, resulting in possible detonation.
* The orn/blk circuit indicates drive (open) at start up, a dip in the idle may exist when the gear selectors moved into drive range.
* Trans axle 4th gear switch has an intermittent open circuit (light blue wire) , the ecm thinks vehicle is in 4th gear and may set a code 31.
TEST DESCRIPTION:number(s) refer to circled number(s) on the diagnostic chart.
1. Check for close switch to ground in park position. different makes of "scan" tools will read P/N differently. Refer to "tool operator`s" manual for type of display used for a specific tool.
* circuit (orn/blk wire) indicates an open for 4 consecutive starts.
* circuit (orn/blk wire) indicates a ground circuit when transmission is in 4th gear, TCC engaged and no code 29 for 10 seconds.
2. checks for an open switch in drive range.
3. be sure "scan" indicates drive, even while wiggling shifter, to test for an intermittent or misadjusted switch in drive or overdrive range.
SOURCE: GM SERVICE MANUAL.
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