DTC P0722 Output Speed Sensor Circuit No Signal

P0722 - Output 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 tooth of the tone wheel) 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 components are powered and ignition voltage is greater than 9V and less than 18V (12V TCM) or greater than 9V and less than 32V (24V TCM).
• For fire truck vocation, pump not pumping.
• For unrealistically large change in output speed test:
Output speed is at or above 600 rpm for more than 1 second.
• For low output speed test:
Shifting complete
Reverse-to-Neutral shift not in progress
Engine is running
No hydraulic default condition due to loss of ignition voltage
Transmission fluid temperature greater than –1.1ºC (30ºF)
Transmission turbine speed greater than or equal to 600 rpm

Conditions for Setting the DTC
DTC P0722 sets when one of the following three conditions occur:
• Unrealistic large change in output speed. A failure pending is set if the TCM detects a change in output speed of more than 600 rpm. A failure is set if range attained is Neutral.
• Unrealistic low value in output speed. A failure pending is set if output speed is detected below 61 rpm.
A failure sets when output speed is below 61 rpm in third, fourth, or fifth range for more than 1 second.
A failure is also set when output speed is below 61 rpm in second range for more than one 1 second when net Engine torque is +/- 1 Nm or turbine speed is greater than 800 rpm.
Figure 1.1 - DTC and General Information

Actions Taken When the DTC Sets
When DTC P0722 is active, the following conditions occur:
• If failure occurs while in a forward range and a shift has been completed, the transmission will remain in the current range.
• If failure occurs while in a forward range and a shift is in progress, the transmission will return to the previous range, except in post-shift state, then the transmission will continue to the commanded range.
• DTC P0722 is stored in TCM history.
• The CHECK TRANS light illuminates.
• The TCM freezes shift adapts (DNA).
• The TCM inhibits TCC engagement.

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 service brake switch. 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, 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.

#Test Description
The numbers below refer to step numbers on the diagnostic table.
2. Tests 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 output 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 40 and terminal 60 at the OEM-side of the 80-way connector. Is the speed sensor resistance within the specified value?
4. Tests for output speed sensor resistance.
Disconnect the wiring harness from the output speed sensor.
Using a DVOM, check the resistance between the speed sensor terminals. Is the speed sensor resistance within the specified value?
Replace the output speed sensor (refer to appropriate service manual). Is replacement complete?

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

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