Chart C-1A Park/Neutral Switch Diagnosis

CIRCUIT DESCRIPTION
The Park/Neutral switch contacts are a part of the neutral start switch. They are closed to ground in park or neutral and open in drive ranges.

The ECM supplies voltage through a current limiting resistor to CKT 434 and senses a closed switch when the voltage on CKT 434 drops to less than one volt.

The ECM uses the P/N signal as one of the inputs to control idle air and EGR.

If CKT 434 indicates P/N (grounded), while in drive range, the EGR would be inoperative, resulting in possible detonation.

If CKT 434 indicates drive (open), a dip in the idle may exist when the gear selector is moved into drive range.

TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.
1. Checks for a closed 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.
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.