P0726

ALLISON AUTOMATIC TRANSMISSION

DTC P0726
Circuit Description for Allison Automatic Transmission
The speed sensors are variable reluctance devices that 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 that 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 150 millivolt 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. Using two-wire differential inputs at the TCM eliminates noise from other sources.

DTC P0726 is a type B DTC.

DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P0726 Engine Speed Circuit Performance

Conditions for Running the DTC
^ DTC P0716, P0717 and P0727 are not active.
^ The components are powered and ignition voltage is greater than 9.0 volts and less than 18 volts.
^ The input speed is greater than 600 RPM for 1.0 second.

Conditions for Setting the DTC
DTC P0726 sets when the TCM detects a large unrealistic engine speed or if excessive noise is present in the engine speed sensor circuit.

Action Taken When the DTC Sets
^ The TCM illuminates the malfunction indicator lamp (MIL) during the second consecutive trip in which the Conditions for Setting the DTC are met.
^ DTC P0726 will be stored in TCM history.
^ The TCM freezes shift adapts.
^ The TCM inhibits torque converter clutch (TCC) engagement.

Conditions for Clearing the DTC
A scan tool can clear the code from TCM history. The TCM automatically clears the DTC from TCM history if the vehicle completes 40 warm-up cycles without failure.

Diagnostic Aids
^ Inspect the wiring for poor electrical connections at the TCM. Inspect for the following conditions:
- A bent terminal
- A backed-out terminal
- A damaged terminal
- Poor terminal tension
- A chafed wire
- 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 condition.
^ If the condition is intermittent, connect the scan tool and select the speed sensor indicated by the code. If the signal is erratic, investigate and eliminate the following:
- Intermittent wiring connection
- Excessive vibration, such as driveline or engine torsionals
- Irregular sensor gap, such as a loose sensor, a loose tone wheel or a damaged tone wheel
- Inspect that the speed sensor wiring consists of twisted pairs at the rate of 12-16 twists per 300 mm (12 inch). These twists must extend the entire length of the wiring harness to within at least 50 mm (2 inch) of the speed sensor connector.

Test Description





The numbers below refer to the step numbers on the diagnostic table.
2. This step tests ignition voltage.
3. This step tests the wiring at the TCM.
4. This step tests engine speed sensor resistance.