Allison LCT1000 No Shift Speedometer Erratic/Inop P0721 C0237 P0500 C0055 - keywords ABS brake cruise gauge ipc

This PI is being superseded to update models. Please discard PIT3155A.

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

Vehicles equipped with the Allison LCT1000 Automatic Transmission may exhibit one or more of the following conditions:

DTCs P0721, C0237, P0500, C0055

Speedometer inoperative, erratic or drops out

Cruise control inoperative

Radio speed control volume (SCV) inoperative

Transmission shifting concerns

ABS light illuminated

The following diagnostic approach may be helpful in repairing any of the above conditions.

The testing should begin at the VSS sensor located in the rear of the transmission on 2WD models and in the transfer case on the 4WD models. An erratic signal caused by a loose or damaged reluctor wheel can cause any of the above concerns.

On vehicles equipped with the Allison LCT1000 Automatic Transmission, the VSS signal is sent to the Transmission Control Module (TCM) via Circuits 821 and 822.

The signal from the VSS to the TCM is an A/C signal that produces approximately 28-34 HZ per 1 mile per hour. This signal can be checked using a Fluke 87 (DVM) set on the A/C voltage scale with hertz function activated and connecting leads to Circuit 821 and 822.

The TCM converts the VSS signal from an A/C sign wave to a digital wave. The TCM sends out approximately 4.5 volts on Circuit 400 to the PCM/ECM. With the vehicle stopped, the DVM will read approximately 4.5 volts on circuit 400, but once the vehicle begins to move the voltage will drop to near 0 volts. To check the voltage with the DVM with the vehicle moving, the min/max function should be set to 1 ms, the max should be 4.5 volt and the minimum should be -4.5 volts. The reason the reading goes negative is because the TCM converts the A/C signal from the VSS to the exact digital wave, therefore when checking the HZ signal on circuit 400 the fluke meter can be set on the A/C or D/C signal with hertz mode active. Circuit 400 can be checked by backing out the terminal at the ECM/PCM connector and placing the red lead of the meter to Circuit 400 and the black lead to ground. The signal from the TCM on circuit 400 will be the same HZ reading as the VSS approximately 28-34 HZ per 1 mile per hour.

The vehicle speed signal to the IPC, radio, cruise control module, and body control is provide by the ECM/PCM on Circuit 817.

Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.