P026

Code PO26:

ISC Motor Circuit:

DESCRIPTION:
The throttle position switch is part of the Idle Speed Control (ISC) motor. The PCM provides a 5 volt reference on CKT 427 to the throttle position switch. When the throttle contacts the ISC plunger, the throttle position switch closes. When the throttle position switch is closed the input voltage is low. Code P026 sets when the PCM sees a Throttle Position (TP) sensor signal between 20 and 75 degrees (accelerator being applied) but the throttle position switch remains closed.

TEST CONDITIONS:
1. Codes P021 and P022 not set.

2. Throttle position between 20 and 87 degrees.

FAILURE CONDITIONS:
Throttle position switch input to the PCM is LO (switch closed) for 1.8 seconds.

ACTION TAKEN:
1. PCM turns 'ON' the 'SERVICE ENGINE SOON' MIL.

2. PCM disables EGR.

3. PCM assumes the throttle position switch is closed if brake is applied and Throttle Position (TP) sensor reads 18 degrees or less.

4. PCM assumes the throttle position switch is open if brake is not applied or if Throttle Position (TP) sensor reads greater than 18 degrees.


NOTES ON FAULT TREE:
1. PCM input P172 should cycle from 'LO' to 'HI' to 'LO' when accelerator is depressed and released.

2. If P172 changes to 'HI' when the ISC motor is disconnected, the fault is with the ISC motor. If P172 still stays 'LO', check CKT 427 for a short to ground.

3. Checking if CKT 427 is shorted to ground or if the PCM is bad.