Allison 1000 & 2000 Gen 4 Fault Codes: P1891 Engine Throttle Position Sensor Pulse Width Modulation (PWM) Signal Low Input

Allison 1000 & 2000 Gen 4 Fault Codes: P1891 Engine Throttle Position Sensor Pulse Width Modulation (PWM) Signal Low Input

CIRCUIT DESCRIPTION:
The Transmission Control Module (TCM) can be calibrated to receive throttle information from a Pulse Width Modulation (PWM) signal.

CONDITIONS FOR RUNNING THE DTC:
1. The components are powered and ignition voltage is greater than 9V and less than 18V (12V TCM) or greater than 18V and less than 32V (24V TCM).
2. The components are powered and engine speed is within a 5 rpm range for 5 seconds.

CONDITIONS FOR SETTING THE DTC:
DTC P1891 sets if the TCM is calibrated to receive the PWM signal and the throttle percentage falls below 5 percent for 5 seconds.

ACTION TAKEN WHEN THE DTC SETS:
1. DTC P1891 is stored in the TCM history.
2. The TCM uses the default throttle value.
3. The TCM freezes shift adapts (DNA).
4. The TCM inhibits TCC engagement.
5. MY ’00 through ’02 – no CHECK TRANS light. MY ’03 CHECK TRANS light is illuminated.

CONDITIONS FOR CLEARING THE DTC/CHECK TRANS LIGHT:
A Scan Tool may be used to clear the code from the TCM history. The TCM automatically clears the DTC from the TCM history if the vehicle completes 40 warm-up cycles without the DTC recurring.

DIAGNOSTIC AIDS:
Inspect the wiring for poor electrical connections at the TCM. Look for the following conditions:
1. A bent terminal.
2. A backed-out terminal.
3. A damaged terminal.
4. Poor terminal tension.
5. A chafed wire.
6. A broken wire inside the insulation.

When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change.

You may have to drive the vehicle in order to experience a fault.