Cummins ISB CM550 (1990-94) Fault Code: 242 PID: 84 SPN: 84 FMI: 10 Vehicle Speed Sensor (VSS Circuit

Circuit Description
The vehicle speed sensor (VSS) uses two separate coils of wire (some applications use a
single-coil sensor) to count gear teeth as they pass in front of the sensor. One coil is used by
the electronic control module (ECM) to sense vehicle speed. The other coil is sometimes used by the OEM to send a vehicle speed signal to the speedometer.

Component Location
The VSS is most commonly installed in the rear of the transmission.

Shop Talk
1) Verify that the feature settings for VSS antitampering (Fault Code 242), application type
and automatic transmission are set correctly. If any of these are set incorrectly, Fault code 242 could occur erroneously.
NOTE: Driving techniques, such as driving for extended periods of time in lower gears, could
log Fault Code 242.
2) Fault Code 242 can be logged if the driver attempts to defeat the road speed governor by
repeatedly cycling the keyswitch.
3) Interview the driver to discover what occurred when the fault code was logged. Explain
the driver actions that can cause Fault Code 242 to be logged.
4) When deactivating the fault, verify that the vehicle is stopped and the engine is shut
down.
5) Verify that the keyswitch has been cycled and has remained in the ON position for 30
seconds after the invalid signal has been corrected. This fault will remain active until the
keyswitch is cycled and the ECM sees zero vehicle speed and zero engine speed for 30
seconds.