P0726
ALLISON AUTOMATIC TRANSMISSIONDTC P0726
Test 1
Test 2-7
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. Noise from other sources, is eliminated by using two-wire differential inputs at the TCM.
DTC P0726 is a type B DTC.
Conditions for Running the DTC
^ DTC P0716 and DTC P0717 are not active.
^ DTC P0727 is 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
^ DTC P0726 will be stored in the TCM history.
^ The TCM freezes shift adapts (DNA).
^ The TCM inhibits TCC engagement.
^ Illuminates the MIL when active.
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.
^ 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
^ Install a known good speed sensor and see if normal function is restored to rule out an internal short or open in the sensor removed.
^ 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 below 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.