P0726

ALLISON AUTOMATIC TRANSMISSION

DTC P0726

DTC P0726 Step 1 - 6:

DTC P0726 Step 7 - 8:

Circuit Description for Allison Automatic Transmission
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 150 mV at low speed to 15 V 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 P0726is a type B DTC.

Conditions for Running the DTC
^ DTC P0716 and DTC P0717 are not active.
^ DTC P0727is not active.
^ The components are powered and ignition voltage is greater than 9 V and less than 18 V.
^ The input speed is greater than 800 RPM for 3 seconds.

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 the TCM history.
^ The TCM freezes shift adapts (DNA).
^ The TCM inhibits TCC engagement.

Conditions for Clearing the MIL/DTC
A scan tool can 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 failure.

Diagnostic Aids
^ Inspect the wiring for poor electrical connections at the TCM. Look 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 and watch the test equipment for a change.
^ You may have to drive the vehicle in order to experience a fault.
^ 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 - driveline or engine torsionals
^ Irregular sensor gap-loose sensor or damaged torque converter
^ Check 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 50 mm of the speed sensor connector.

Test Description
The numbers refer to the step numbers on the diagnostic table.
2. This step tests ignition voltage.
3. This step tests the wiring harness for opens/shorts.
4. This step tests engine speed sensor resistance.