GEAR AS 5I7686 - Caterpillar

System Operation Description:Use this procedure under the following situation:Use this procedure to troubleshoot the system only under the following conditions:
There is an active diagnostic code.
There is an easily repeated diagnostic code that is associated with the vehicle speed circuit.Refer to Troubleshooting, "Vehicle Speed Circuit - Calibrate" if the vehicle speed is being calibrated in ppm (pulses per mile) or PPKM (pulses per kilometer).The following background information is related to this procedure:The vehicle speed circuit consists of the vehicle speed source and associated wiring. The vehicle speed source is installed by the vehicle OEM. Usually, a sensor reads movement of the transmission output shaft from the teeth on a chopper wheel. The ECM converts the signal from the vehicle speed source into vehicle speed for the following functions:
Cruise control
Fast idle
Idle speed control
Progressive shift
Speedometer
PTO operation
Vehicle speed limiting
Idle shutdown
Operation of the air inlet heater
Secure idle theft deterrent
Trip data
Maintenance dataTo begin troubleshooting a vehicle speed problem, the following information must be determined:The problem is electrical or the problem is a calibration problem.Electrical problems would be an erratic speed signal or no speed signal. A calibration problem would be a stable ECM speed, but an inaccurate ECM speed. Another calibration problem would be a stable Speedometer speed, but an inaccurate speedometer speed.If the problem is related to vehicle speed calibration, refer to Troubleshooting, "Vehicle Speed Circuit - Calibration" for calibrating the vehicle speed in PPKM (pulses per kilometer) or ppm (pulses per mile). The ECM speedometer output signal is set to 18,600 PPKM (30,000 ppm). The ECM speedometer output signal cannot be changed. All speedometers that are driven by the ECM output (J1/P1 terminal 36 and terminal 37) must match the fixed output of the ECM for proper operation.The type of vehicle speed sensor that is installed on the vehicle must be determined.Both single coil magnetic sensors and dual coil magnetic sensors are commonly used. Caterpillar recommends single coil magnetic sensors. Some of the vehicles may provide the vehicle speed through the Transmission ECM.You must determine if the vehicle speedometer is driven from the ECM.The ECM can drive the speedometer through the connection to ECM connector J1:36 (speedometer positive) and/or J1:37 (speedometer negative). Note: The ECM may also drive the speedometer by a connection to the J1587 ATA data link, if the instrument cluster is capable of interpreting the signal.Vehicle Speed Circuit WiringThe vehicle speed signal is routed directly into the ECM at ECM vehicle harness connector J1/P1:32 (vehicle speed in positive) and J1/P1:33 (vehicle speed in negative). Speedometer ConnectionIf the vehicle speedometer is driven by the ECM output, the vehicle speedometer will be connected to the ECM vehicle harness connector J1/P1:36 and/or J1/P1:37. Some speedometers require only one of the ECM signal lines to operate. Other speedometers may require both of the ECM signal lines to operate.Speedometer Special Test If the vehicle speedometer is driven by ECM output, the speedometer circuit can be checked with the Caterpillar Electronic Technician (Cat ET). The "55 mph VSP/Speedometer Test" will drive the speedometer to approximately