Circuit Description
The 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, 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 millivolt at low speed to 15 volts 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 inputs at the TCM eliminates noise from other sources.
DTC P0721 is a type A DTC.
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P0721 Output Speed Sensor Performance
Conditions for Running the DTC
| • | DTC P0716, P0717 and P0722 are not active. |
| • | The components are powered and ignition voltage is greater than 9.0 volts and less than 18 volts. |
| • | Engine input speed is greater than 200 RPM and less than 7,500 RPM for 5 seconds. |
| • | The output speed is greater than 200 RPM. |
Conditions for Setting the DTC
DTC P0721 sets when the TCM detects an unrealistically large change in output speed or if excessive noise is present in the output speed sensor circuit.
Action Taken When the DTC Sets
| • | The TCM illuminates the malfunction indicator lamp (MIL). |
| • | When failure occurs in a forward range and shifting is complete, the transmission remains in the current range. When failure occurs while in a forward range and the shift is in process, the transmission returns to the previous range, except in post-shift state, where the transmission will continue to the commanded range. When failure occurs in other conditions, the transmission shifts to 1st, 3rd or 5th. While diagnostic response is active, if the shift selector is moved to NEUTRAL or REVERSE or any other forward range, then the transmission will lock in NEUTRAL. |
| • | DTC P0721 will be stored in TCM history. |
| • | The TCM freezes shift adapts. |
| • | The TCM inhibits torque converter clutch (TCC) engagement. |
Conditions for Clearing the DTC
A scan tool can clear the code from TCM history. The TCM automatically clears the DTC from TCM history if the vehicle completes 40 warm-up cycles without failure.
Diagnostic Aids
| • | Inspect the wiring for poor electrical connections at the TCM. Inspect 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 |
| • | When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change. |
| • | You may have to drive the vehicle in order to experience a condition. |
| • | 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, such as driveline or engine torsionals |
| - | Irregular sensor gap, such as a loose sensor, a loose tone wheel or a damaged tone wheel |
| - | Inspect that the speed sensor wiring consists of twisted pairs at the rate of 12-16 twists per 300 mm (12 in). These twists must extend the entire length of the wiring harness to within at least 50 mm (2 in) of the speed sensor connector. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
This step tests ignition voltage.
-
This step tests the wiring at the TCM.
-
This step tests for proper resistance at the output speed sensor resistance.
Step | Action | Value | Yes | No |
|---|---|---|---|---|
Schematic Reference: Automatic Transmission Controls Schematics | ||||
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | Go to Diagnostic System Check - Vehicle in Vehicle DTC Information |
Important: Before clearing the DTC, use the scan tool in order to record the Freeze Frame and Failure Records for reference. The Clear Info function will erase the data. Is the voltage within the specified range? | 9-18 V | Go to Step 3 | Go to DTC P0562 for low voltage or DTC P0563 for high voltage | |
Is the speed sensor resistance within the specified values? | 2WD: 2,600-3,160 ohms 4WD: 1,420-2,140 ohms | Go to Diagnostic Aids | Go to Step 4 | |
Is the speed sensor resistance within the specified values? | 2WD: 2,600-3,160 ohms 4WD: 1,420-2,140 ohms | Go to Step 5 | Go to Step 6 | |
5 | Repair the engine wiring harness. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 7 | -- |
6 |
Important: Do not rotate the speed sensor in the retaining bracket. Orientation is fixed, and if changed, may cause improper operation. Replace the output speed sensor. Refer to Speed Sensor Replacement . Did you complete the replacement? | -- | Go to Step 7 | -- |
7 | Perform the following procedure in order to verify the repair:
Has the test run and passed? | -- | System OK | Go to Step 1 |
