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Allison 1000 & 2000 Gen 4 Fault Codes: P0726 Engine Speed Input Circuit Performance

Allison 1000 & 2000 Gen 4 Fault Codes: P0726 Engine Speed Input Circuit Performance

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 15V 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.

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. DTC P0716, P0717, P0721, or P0727 is not active.
3. Engine speed is above 600 rpm for 1 second.
4. Shift is complete and range attained is not neutral.

DTC P0726 is set when one of the following conditions occur:
1. Unrealistic large change in engine speed. Failure is set if an unrealistic change in transmission engine speed is detected at or above 600 rpm.
2. Noisy input speed signal. Noise is determined with two counters. A low counter is incremented when engine speed change is below 650 rpm for 2.5 seconds. A high counter is incremented when engine speed change is above 650 rpm. When both counters accumulate 5 events, a failure is set.

1. DTC P0726 is stored in the TCM history.
2. The TCM defaults engine speed to turbine speed. Turbine speed is used to determine the missing engine speed.

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.

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.

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:
1. Intermittent wiring connection.
2. Excessive vibration (driveline or engine torsionals).
3. Irregular sensor gap (loose sensor, loose tone wheel, or damaged tone wheel.

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.