Cummins ISB CM550 (1990-94) Fault Code: 241 PID: 84 SPN: 84 FMI: 2 Vehicle Speed Sensor Circuit

Posted by PerDiezel Solutions on January 16, 2026

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) Disconnect the vehicle speed sensor connector that connects to the OEM speedometer,
or trip recorder, and move the truck. If the fault goes inactive, there is probably electrical noise being fed into the vehicle speed sensor circuit from the OEM device.
2) Verify the vehicle speed sensor wires in the OEM harness are twisted-pairs.
3) Check brake switch operation with INSITE™. Fault could be caused by rear-wheel lockup
without brake indication to ECM.
4) Verify the speed sensor is installed properly and its locking nut has not vibrated loose.

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Cummins ISB CM550 (1990-94) Fault Code: 234 PID: 190 SPN: 190 FMI: 0 Engine Overspeed Circuit

Cummins ISB CM550 (1990-94) Fault Code: 222 PID: 108 SPN: 108 FMI: 4 Ambient Air Pressure Sensor Circuit

Cummins ISB CM550 (1990-94) Fault Code: 221 PID: 108 SPN: 108 FMI: 3 Ambient Air Pressue Sensor Circuit

Cummins ISB CM550 (1990-94) Fault Code: 191 PID: 50 SPN: 876 FMI: 11 Air Conditioner Clutch Supply Circuit

Cummins ISB CM550 (1990-94) Fault Code: 155 PID: 105 SPN: 105 FMI: 0 Intake Manifold Air Temperature Sensor – Engine Protection Circuit

WARNING!