Allison 1000 & 2000 Gen 4 Fault Codes: P0727 Engine Speed Input Circuit No Signal

Allison 1000 & 2000 Gen 4 Fault Codes: P0727 Engine Speed Input Circuit No Signal

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 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.

CONDITIONS FOR RUNNING THE DTC:
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, or P0726 is not active.
3. Unrealistically low engine speed detected.
4. Engine is running.
5. Transmission turbine speed is at or above 400 rpm.
6. Ignition is in the ON position.

CONDITIONS FOR SETTING THE DTC:
DTC P0727 is set when one of the following conditions occur:
1. Unrealistic large change in engine speed. Failure pending is set if an unrealistic change in engine speed is detected at or above 1140 rpm.
2. Unrealistic low value in engine speed. Failure is set if engine speed is detected below 61 rpm for 4 seconds.

ACTION TAKEN WHEN THE DTC SETS:
1. The TCM defaults engine speed to turbine speed. Turbine speed is used to determine missing engine speed.
2. TCM inhibits TCC momentarily.
3. DTC P0727 is stored in TCM history

CONDITIONS FOR CLEARING THE DTC/CHECK TRANS LIGHT:
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.

DIAGNOSTIC AIDS:
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.

Install a known good TCM, if available. If the DTC does not return, reinstall the old TCM to verify the repair.