Allison

Allison 1000 & 2000 Gen 4 Fault Codes: P0873 Transmission Pressure Switch Solenoid E Circuit High

CIRCUIT DESCRIPTION:
The Pressure Switch Manifold (PSM) is a multiple-switch assembly made up of three normally open (N/O) and one normally closed (N/C) pressure switches. The E pressure switch monitors E shift valve positioning and relays it to the TCM. When E pressure switch is in the open state, E shift valve should be in the destroked position.

CONDITIONS FOR RUNNING THE DTC:
1. Components are powered and the ignition voltage is greater than 5V.
2. DTC P0561, P0562, or P0563 is not active.
3. E shift valve is commanded to the ON (stroked) position.
4. Transmission temperature is above -25°C (-13°F).
5. Vehicle shutdown is not in process.

CONDITIONS FOR SETTING THE DTC:
DTC P0873 sets during steady state operation when E shift valve is in the stroked state (commanded ON) and E pressure switch status is detected as destroked (commanded OFF). Steady state is defined as attaining a valid range. The intent of this code is to detect an open condition in the pressure switch wiring circuit or the pressure switch.

Later software levels incorporate retry logic that allows the TCM to command E solenoid OFF and ON. If the pressure switch state does not change, and remains in the OFF state, DTCs P0873 and P0871 will set.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0873 is active, the following conditions will occur:
1. If the failure occurs while in a forward range, the transmission shifts to another forward range.
2. If the shift selector is moved to N (Neutral), the transmission will shift to neutral.
3. If the shift selector is moved to R (Reverse), the transmission shifts to reverse.
4. If the shift selector is moved to a forward range or reverse and the transmission is compromised by overspeeding or direction change, the transmission shifts to neutral.

DTC P0873 is stored in TCM history.

The CHECK TRANS light illuminates.

The TCM inhibits TCC engagement.

The TCM freezes shift adapts (DNA).

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 only a DTC P0873 is set, look for an intermittent open condition at the pressure switch circuit or an intermittent hydraulic failure (sticking valve). This code is set when the condition is present more than 3 times during a current drive cycle. See Intermittent Fault information.

When a P0873 and P0871 are set in combination, this may indicate an open circuit condition is present at the pressure switch circuit or the shift valve is stuck in the destroked state. See Troubleshooting Process information.

When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change. It may be necessary to check for an open condition at individual wires within a harness to isolate an intermittent condition. Refer to Wire Check information.

You may have to drive the vehicle in order to experience a fault. Use the data obtained from failure records to determine transmission range and/or certain vehicle operating variables such as temperature, run time, etc. This data can be useful in reproducing the failure mode when the DTC was set.

Multiple inactive pressure switch DTCs may be due to a plugged Control Main Filter. Ensure that the initial 8000 km (5,000 mile) filter change was performed.

Allison 1000 & 2000 Gen 4 Fault Codes: P0875 Transmission Reverse Pressure Switch Circuit Malfunction

CIRCUIT DESCRIPTION:
The Pressure Switch Manifold (PSM) is a multiple switch assembly made up of three normally open (N/O) and one normally closed (N/C) pressure switches. Fluid pressure is supplied to the reverse pressure switch, holding it open, when the manual selector valve is in any position except reverse. When the manual selector valve is moved to Reverse, pressure to the reverse pressure switch is cut off, allowing the switch to close.

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. Engine speed is greater than 200 rpm and less than 7500 rpm for 5 seconds.
3. TFT is greater than 0°C (32°F).
4. The hydraulic system is pressurized.
5. DTC P0708 is not active.

CONDITIONS FOR SETTING THE DTC:
DTC P0875 sets when P (Park), N (Neutral), or a forward range is selected and the reverse pressure switch state remains in the mechanically closed/electrically ON position for a period of time that is calibration dependent.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0875 is active, the following conditions will occur:
1. Calibration Dependent – The transmission will either lock-to-neutral or shift to 3rd or 5th range.
2. If the shift selector is moved to N (Neutral), the transmission will shift to neutral.
3. If the shift selector is moved to R (Reverse), the transmission shifts to reverse.
4. If the shift selector is moved to a forward range or reverse and the transmission is compromised by overspeeding or direction change, the transmission shifts to neutral. If an output speed fault or NSBU fault is present when this test fails, then hydraulic default is implemented.

DTC P0875 is stored in TCM history.

The CHECK TRANS light illuminates on the second occurrence.

The TCM freezes shift adapts (DNA).

The TCM inhibits TCC engagement.

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. It may be necessary to check for shorting to ground at individual wires within a harness to isolate an intermittent condition. Refer to Wire Check information.

You may have to drive the vehicle in order to experience a fault. Use the data obtained from failure records to determine transmission range and/or certain vehicle operating variables such as temperature, run time, etc. This data can be useful in reproducing the failure mode when the DTC was set.

This DTC could indicate a hydraulic leak path exhausting pressure from the reverse pressure switch.

Multiple inactive pressure switch DTCs may be due to a plugged Control Main Filter. Ensure that the initial 8000 km (5,000 mile) filter change was performed. To help reduce multiple pressure switch DTCs from setting due to a plugged control main filter or slow pump prime, later software levels have a time delay up to 30 seconds before a pressure switch test is enabled.

If a P0875 (previously P1713) is shown in failure records as being logged first on the failure list followed by P0872 (previously P1711) combined with P0708, this generally indicates a NSBU internal switch failure. Often when the codes are cleared they do not return. This may be due to moisture that was present at the time the code was logged, causing an internal short. Inspect the NSBU switch linkage for proper adjustment and for signs of damage. If the shift linkage is properly adjusted and no damage is noted, replacement of the switch may be indicated.

Allison 1000 & 2000 Gen 4 Fault Codes: P0876 Transmission Reverse Pressure Switch Circuit Stuck Open

CIRCUIT DESCRIPTION:
The Pressure Switch Manifold (PSM) is a multiple switch assembly made up of three normally open (N/O) and one normally closed (N/C) pressure switches. Fluid pressure is supplied to the reverse pressure switch, holding it open, when the manual selector valve is in any position except reverse. When the manual selector valve is moved to Reverse, pressure to the reverse pressure switch is cut off allowing the switch to close.

CONDITIONS FOR RUNNING THE DTC:
TFT is greater than 0°C (32°F).

DTC P0706, P0708, or P0875 is not active.

There are two cases where this DTC runs.
Case 1: Ignition voltage is above 9V (12V TCM) or 18V (24V TCM). The PRNOL state and the reverse pressure switch state do not agree.
Case 2: Ignition voltage is under 9V (12V TCM) or 18V (24V TCM). All speed sensors are under 50 rpm.

CONDITIONS FOR SETTING THE DTC:
DTC P0876 sets in two cases:
1. When PRNDL and reverse pressure switch do not agree. When reverse range is selected and the reverse pressure switch is detected in the mechanically open/electrically OFF state for more than 1 second.
2. When engine shutdown and reverse pressure switch do not agree. When engine shutdown is in process and the reverse pressure switch is detected in an improper state, mechanically open/electrically OFF, for more than a period of time that is temperature dependent. The time period is 5 seconds at 35°C (95°F) to 30 seconds at -20°C (-4 °F).

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0876 is active, the following conditions will occur:
1. Calibration Dependent – The transmission will either lock-to-neutral or shift to 3rd or 5th range.
2. If the shift selector is moved to N (Neutral), the transmission will shift to neutral.
3. If the shift selector is moved to R (Reverse), the transmission shifts to reverse.
4. If the shift selector is moved to a forward range or reverse and the transmission is compromised by overspeeding or direction change, the transmission shifts to neutral.
5. If an output speed fault or NSBU fault is present when this test fails, then hydraulic default is implemented.

DTC P0876 is stored in TCM history.

The CHECK TRANS light illuminates on the second occurrence.

The TCM freezes shift adapts (DNA).

The TCM inhibits TCC engagement

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. It may be necessary to check for shorting to ground at individual wires within a harness to isolate an intermittent condition. Refer to Wire Check information.

You may have to drive the vehicle in order to experience a fault. Use the data obtained from failure records to determine transmission range and/or certain vehicle operating variables such as temperature, run time, etc. This data can be useful in reproducing the failure mode when the DTC was set.

Multiple inactive pressure switch DTCs may be due to a plugged Control Main Filter. Ensure that the initial 8000 km (5,000 mile) filter change was performed. To help reduce multiple pressure switch DTCs from setting due to a plugged control main filter or slow pump prime, later software levels have a time delay up to 30 seconds before a pressure switch test is enabled.

Allison 1000 & 2000 Gen 4 Fault Codes: P1779 Engine Torque Delivered To TCM Signal

CIRCUIT DESCRIPTION:
The GM 8.1 liter gasoline engine is used in some medium duty applications. The 8.1 liter engine uses a 3-wire system to communicate engine torque data/requests between the powertrain control module (PCM) and the transmission control module (TCM).

Managed engine torque is sent from the PCM to the TCM over wire 116 (GM wire 464).

CONDITIONS FOR RUNNING THE DTC:
1. The components are powered and ignition voltage is greater than 9V and less than 18V (12V TCM).
2. Engine speed is greater than 200 rpm and less than 7500 rpm for 5 seconds.
3. Pulse Width Modulation (PWM) driver demand torque option is selected in transmission calibration.
4. No DTC is active at the time of this test.

CONDITIONS FOR SETTING THE DTC:
DTC P1779 sets if managed engine torque (net) signal (circuit 464) is under 1.5 percent or over 98.5 percent for more than a 2 second period.

ACTION TAKEN WHEN THE DTC SETS:
1. DTC P1779 is stored in the TCM history.
2. The CHECK TRANS light illuminates on the second occurrence.
3. The TCM defaults to a calculated managed torque (net) input value using throttle and engine speed.
4. The TCM defaults to a calculated unmanaged torque (gross) input value using throttle and engine speed.

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:
An Intermittent condition may be the cause of the problem. Return the vehicle to the OEM for further troubleshooting.

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.

Allison 1000 & 2000 Gen 4 Fault Codes: P1892 Throttle Position Sensor Pulse Width Modulation (PWM) Signal High 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. Engine speed is greater than 200 rpm and less than 7500 rpm for 5 seconds.

CONDITIONS FOR SETTING THE DTC:
DTC P1892 sets if the TCM is calibrated to receive the PWM signal and the throttle percentage exceeds 95 percent for 5 seconds.

ACTION TAKEN WHEN THE DTC SETS:
1. DTC P1892 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.

Allison 1000 & 2000 Gen 4 Fault Codes: P2771 Four-Wheel Drive Switch Circuit

CIRCUIT DESCRIPTION:
Detects abnormal conditions for the four-wheel drive indication switch input. Four wheel-drive statuses are detected using speed and ratios.

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. Engine speed is greater than 200 rpm and less than 7500 rpm for 5 seconds.
3. DTCs P0721 and P0722 (output speed sensor circuits) are not active.
4. Output speed is above 60 rpm.
5. Transmission fluid temperature it between 20°C (68°F) and 130°C (266°F).
6. Shift is complete and the range attained is not neutral.

CONDITIONS FOR SETTING THE DTC:
P2771 sets when one of the following conditions occurs:
1. The transfer case switch indicates high range, and the calculated range is low range for 5 seconds or more.
2. The transfer case switch indicates low range, and the calculated range is high range for 5 seconds or more.

ACTION TAKEN WHEN THE DTC SETS:
1. DTC P2771 will be stored in the TCM memory.
2. The CHECK TRANS light illuminates on the second occurrence.
3. The TCM inhibits TCC engagement.

CONDITIONS FOR CLEARING THE DTC/CHECK TRANS LIGHT:
A Scan Tool can 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 a failure.

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.

Inspect for any transmission DTCs that may have reset.

When testing for a short or ground, the transmission should be in a range where the switch or solenoid is in the OFF state. When testing for an open, the transmission should be in a range where the switch or solenoid is in the ON state.

Allison 1000 & 2000 Gen 4 Fault Codes: P2773 Torque Control Request Ignored – ECM/TCM

CIRCUIT DESCRIPTION:
Shift Energy Management (SEM) allows the Transmission Control Module (TCM) to request torque reduction from the engine controller. By reducing torque during upshifts, shifts can be made quicker and at a more consistent output torque. This increases clutch life by reducing clutch engagement temperatures.

CONDITIONS FOR RUNNING THE DTC:
1. DTC U2105 CAN bus error is not active.
2. The components are powered and ignition voltage is greater than 9V and less than 18V (12V TCM) or greater 18V and less than 32V (24V TCM).
3. Engine speed is greater than 200 rpm but less than 7500 rpm for 5 seconds.
4. SEM has been activated for this vehicle.

CONDITIONS FOR SETTING THE DTC:
DTC P2773 sets when the TCM detects one of the following conditions for a minimum of four upshifts (consecutive or nonconsecutive) during one drive cycle:
1. Engine ECM is not responding to torque reduction signal request from the TCM.
2. A nonapproved J 1939 device is interfering with the torque reduction signal request.

ACTION TAKEN WHEN THE DTC SETS:
1. The CHECK TRANS light illuminates.
2. All 4-5 upshifts and full throttle 3-4 upshifts are inhibited.

CONDITIONS FOR CLEARING THE DTC/CHECK TRANS LIGHT:
A Scan Tool may be used to clear the code from the TCM history. The DTC is cleared if, for one ignition cycle, actual engine torque follows TCM commanded torque for 20 consecutive upshifts. To be counted as one upshift, engine torque demand must be above 80 N-m (59 lb ft).

DIAGNOSTIC AIDS:
1. It will be necessary to drive the vehicle with heavy to moderate throttle settings for at least four upshift cycles in order to set a DTC P2773.
2. If DTC P2773 becomes active shortly AFTER an engine update and the troubleshooting procedure for P2773 has been performed, see TCM Parameters to Support Engine information.