DTC P0727 Engine Speed Sensor Circuit No Signal

P0727 - Engine Speed Sensor

Circuit Description
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 (a rib on the torque converter impeller) 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. Using two-wire differential input at the TCM eliminates noise from other sources.

Conditions for Running the DTC
• The test is enabled by calibration.
• For unrealistically low engine speed test:
Turbine speed is at or above 400 rpm.
The ignition key is in RUN.

Conditions for Setting the DTC
DTC P0727 sets when one of the following conditions occur:
• Unrealistic large change in Engine speed. A failure pending is set if the TCM detects a change in Engine speed of more than 1040 rpm.
• Unrealistic low value in Engine speed. A failure is set if engine speed is detected below 61 rpm for 4 seconds.

Actions Taken When the DTC Sets
When DTC P0727 is active, the following conditions occur:
• DTC P0727 is stored in TCM history.
• The TCM does not illuminate the CHECK TRANS light.
• The TCM defaults Engine speed to turbine speed. Turbine speed is used to determine the missing Engine speed.
• The TCM inhibits torque converter clutch (TCC) momentarily.

Conditions for Clearing the DTC/CHECK TRANS Light
Use the Allison DOC™ For PC–Service Tool to clear the DTC 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 and Engine speed sensor. 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 Allison DOC™ For PC–Service Tool and observe 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, damaged torque converter ribs).
• 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.
• Inspect 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 step numbers on the diagnostic table.
2. Tests for proper ignition voltage.
Install the Allison DOC™ For PC–Service Tool.
Start the Engine.
Record the failure records.
Clear the DTCs.
Using Allison DOC™ For PC–Service Tool, measure ignition voltage. Is voltage within specified values? (9–18V for 12V TCM, and 18–32V for 24V TCM).
3. Tests for proper engine speed sensor resistance at the OEM-side of the harness.
Turn ignition OFF.
Disconnect the 80-way connector at the TCM.
Using a DVOM, measure resistance between terminal 39 and terminal 59 at the OEM-side of the 80 way connector. Is the speed sensor resistance within the specified value?
4. Tests for Engine speed sensor resistance.
Disconnect the wiring harness from the output speed sensor.
Using a DVOM, measure the resistance between the speed sensor terminals. Is the speed sensor resistance within the specified value?
→Replace the engine speed sensor (refer to service manual). Is replacement complete?

To verify the repair:
→Clear the DTC.
→Drive the vehicle under normal operating conditions.
→Using Allison DOC™ For PC–Service Tool monitor engine speed sensor operation.
Did the DTC return?

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