Allison 3000 & 4000 Gen 4 Fault Code: P1891 Throttle Position Sensor (TPS) PWM Signal Low Input

DIAGNOSTIC RESPONSE:
Use default throttle values

CIRCUIT DESCRIPTION:
The Transmission Control Module (TCM) can be calibrated to receive a Pulse Width Modulation (PWM) TPS pedal position signal.

NOTE: A throttle source can be specified by:
1. Autodetect of the throttle source (prior to MY 09 new release calibrations). Calibrations that have rolled forward from a previous software release will continue to have the autodetect source available in later CIN versions.
2. Use of CMC Override Throttle Source with the diagnostic tool to set throttle source (PWM, SAE J1939, SAE J1587, PWM, or Autodetect). Choose “Autodetect”‘ from the CMC list only if the calibration is prior to MY 09 or the autodetect feature has rolled into a later CI N version.
3. Communication link type selected during calibration request process for MY 09 and later calibrations.

NOTE: New MY 09 and later calibrations that did not maintain autodetect, i.e. did not roll forward from earlier software, should not be re-programmed by the CMC Override Throttle Source. This disables some datalink features for the calibration’s specified communication link type. If a different throttle source is needed, calibrate with a new CIN that enables the desired throttle source.

The TCM is connected to the TPS by:
PWM TPS signal (W144)

CONDITIONS FOR RUNNING THE DTC:
1. A PWM TPS must be the throttle source.
2. After TCM initializes normally and engine speed is above 200 rpm for more than 5 seconds, this DTC test will run at 0.10 second intervals.
3. DTC P1891 not active.

CONDITIONS FOR SETTING THE DTC:
1. DTC P1891 sets if the TCM has been autodetected, forced, or calibrated to receive a PWM TPS signal, and the PWM throttle frequency is less than 4.9% for more than 5 seconds.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P1891 is active, the following conditions occur:
1. The TCM does not illuminate the CHECK TRANS light, unless January 2010 software or later.
2. DTC is stored in TCM history.
3. Default value used for% throttle is based on engine torque and engine speed.

CONDITIONS FOR CLEARING THE DTC/CHECK TRANS LIGHT:
Use the diagnostic tool to clear the DTC from the TCM history. The TCM automatically clears the DTC from the TCM history if the vehicle completes 40 warm-up cycles without failure.

DIAGNOSTIC AIDS:
DTC P1891 indicates the TCM has detected a PWM throttle percentage in the low error zone, and may be caused by the following:
1. Open or short to ground in W144 circuit.
2. PWM TPS source set in calibration or has been autodetected in non-PWM TPS applications.
3. PWM TPS source set by CMC Override Throttle Source using diagnostic tool in non-PWM TPS applications.
4. Missing, defective, or unplugged PWM TPS.
5. Signal wire (W144) open or grounded.
6. Broken or defective connector assemblies allowing pushed back pins to short together.
7. Unlocked connectors, missing seal plugs allowing moisture in connectors.
8. TCM issue.
9. TPS not adjusted. Check with OEM for PWM TPS specifications and adjustment procedures.

Review applicable information in Control System and Transmission Specifications and the Appendices to find additional circuit specifications, system and connector diagrams, and troubleshooting tips.

PWM may be monitored using data monitor in the diagnostic tool version 9.2 or higher. Absolute throttle % should increase and decrease from Closed Throttle (CT) to Wide Open Throttle (WOT).

PWM throttle frequency should increase and decrease from Closed Throttle (CT) to Wide Open Throttle (WOT) if using a DMM. Refer to Troubleshooting with a Digital Multimeter (DMM) for instructions on measuring frequency with a DMM.