Allison 3000 & 4000 Gen 4 Fault Code: P0726 Engine Speed Sensor Circuit Performance

DIAGNOSTIC RESPONSE:
Default to turbine speed

CIRCUIT DESCRIPTION:
The speed sensors are variable reluctance devices which convert mechanical motion to an AC voltage. Each sensor consists of a wire coil wrapped around a pole piece that is adjacent to a permanent magnet. These elements are contained in a housing which is mounted adjacent to a rotating ferrous member. Two signal wires extend from one end of the housing and an exposed end of the pole piece is at the opposite end of the housing. The permanent magnet produces lines of flux around the pole piece. As a ferrous object (such as a gear tooth) approaches and passes through the gap at the end of the pole piece, an AC voltage pulse is induced in the wire coil. The Transmission Control Module (TCM) calculates the frequency of these AC pulses and converts it to a speed value. The AC voltage generated varies from 150mV at low speed to 15 volts at high speed. The signal wires from the sensor are formed as twisted pairs to cancel magnetically induced fields. The cable is also shielded to protect from voltage-related fields. Noise from other sources is eliminated by using two-wire differential inputs at the TCM.

CONDITIONS FOR RUNNING THE DTC:
1. Engine speed is above 600 rpm for more than 1 second.
2. Shifts complete and range attained not neutral.

CONDITIONS FOR SETTING THE DTC:
DTC P0726 is set when one of the following conditions occur:
1. Engine speed change greater than 600 rpm detected within a 0.15 second span.
2. Engine speed change greater than 650 rpm detected 5 or more times within a 2 second span.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0726 is active, the following conditions occur:
1. DTC P0726 is stored in the TCM history.
2. The TCM does not illuminate the CHECK TRANS light.
3. If ignition is OFF, then engine speed is set to 0. If turbine speed is greater than the engine speed calibration default value, then substitute turbine speed as 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 the DTC recurring.

DIAGNOSTIC AIDS:
NOTE: When performing any resistance checks for troubleshooting, the current production sensors will have a nominal coil resistance value of 340 Ohms (± 25 Ohms) at an ambient temperature of 20°C (68°F), as compared to the former sensors which had a nominal resistance value of 300 Ohms (± 25 Ohms) at an ambient temperature of 20°C (68°F).

DTC P0726 may be caused by:
1. W139 and/or W159 shorted to another wire in chassis harness or to ground.
2. W139 and/or W159 high resistance or open circuit.
3. Broken or defective connector assemblies allowing pushed back pins to short together.
4. Unlocked connectors, missing seal plugs allowing moisture in connectors.
5. Fatigued wires at speed sensor connectors (broken strands).
6. Incorrect pin crimps or expanded terminals.
7. Defective engine speed sensor.
8. Defective TCM.
9. Wire pair not twisted or not sufficiently twisted.

NOTE: Inspect that the speed sensor wiring consists of twisted pairs at the rate of 12 to 16 twists per 300 mm. These twists must extend the entire length of the wiring harness to within at least 50mm of the speed sensor connector.

NOTE: Select SpeedSignalsDatalntegrityList in DPID configuration prior to connecting with the diagnostic tool in order to strip-chart close to real time speed signal data. Refer to the OEM Technician’s Library for additional information about DPID configuration and data collection.

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