ZF 4HP 16 - Automatic Transmission (European and N. American Emissions)

DTC P0721 (European and N. American Emissions)

Circuit Description

The vehicle automatic transmission (A/T) output speed sensor (OSS) is a magnetic inductive pickup that relays information relative to vehicle speed to the transmission control module (TCM). Vehicle speed information is used by the TCM to control shift timing, line pressure, and torque converter clutch (TCC) apply and release.

The output speed sensor mounts in the case at the speed sensor rotor, which is pressed onto the spur gear. An air gap of 0.1-1.3 mm (0.004-0.05 in) is maintained between the sensor and the teeth on the spur gear teeth. The sensor consists of a permanent magnet surrounded by a coil of wire. As the differential rotates, an AC signal induces a higher frequency and voltage measurement at the sensor. The sensor resistance should measure infinity at 20°C (68°F). The sensor can measure from 20-8,000 hertz.

Conditions for Setting the DTC

* The transmission speed is less than 8,160 RPM.
* The increasing rate of output speed is more than 45,000 RPM/second.
* The system voltage is greater than 8.5 volts.
* The engine is running.
* DTCs P0705, P0725, and P0727 are not set.
* The TCM or engine control module (ECM) is defective.

Action Taken When the DTC Sets

* The malfunction indicator lamp (MIL) will illuminate.
* The TCM will record operating conditions at the time the diagnostic fails. This information will be stored in the Failure Records buffer.
* The running vehicle remains in actual gear.
* Open the lock-up clutch.

Conditions for Clearing the MIL/DTC

* The MIL will turn OFF when the malfunction has not occurred after 3 ignition cycles.
* A history DTC will clear after 40 consecutive warm-up cycles without a fault.
* Clear the DTCs with a scan tool.

Diagnostic Aids

When DTC P0721 sets, the possible cause of fault could be the OSS.