Speedometer Connection

409

TM 9-2320-312-24-2

Troubleshooting Section

Vehicle speed limiting

The vehicle speed signal is routed directly into

the ECM at ECM vehicle harness connector J1/P1

Idle shutdown

terminal 32 (vehicle speed in positive) and terminal

33 ("vehicle speed in negative").

Operation of the air inlet heater

Speedometer Connection

Secure idle theft deterrent

If the vehicle speedometer is driven by the ECM

Trip data

output, the vehicle speedometer will be connected

to the ECM vehicle harness connector J1/P1

Maintenance data

terminal 36 and/or terminal 37. Some speedometers

require only one of the ECM signal lines to operate.

Other speedometers may require both of the ECM

To begin troubleshooting a vehicle speed problem,

signal lines to operate.

the following information must be determined:

Speedometer Special Test

The problem is electrical or the problem is a

calibration problem.

If the vehicle speedometer is driven by ECM output,

the speedometer circuit can be checked by using

Electrical problems would be an erratic speed

ET. The "55 mph VSP/Speedometer Test" will drive

signal or no speed signal. A calibration problem

the speedometer to approximately 55 mph under

would be a stable ECM speed, but an inaccurate

the following conditions:

ECM speed. Another calibration problem would be

a stable Speedometer speed, but an inaccurate

The "55 mph VSP/Speedometer Test" is activated.

speedometer speed.

The circuit is operating properly.

If the problem is related to vehicle speed calibration,

refer to Troubleshooting, "Vehicle Speed Circuit -

The speedometer is operating properly.

Calibration" for calibrating the vehicle speed in

PPKM (pulses per kilometer) or ppm (pulses per

mile). The ECM speedometer output signal is set to

Note: While the test is active, the odometer will

18,600 PPKM (30,000 ppm) for most configurations.

increment mileage.

For "GM Truck Manufacturer" configurations

with SEP00 or newer software, the output of the

Magnetic Sensors

speedometer is set to 2,485 PPKM (4,000 ppm).

The ECM speedometer output signal cannot be

A magnetic pickup will have only two wires if the

changed. All speedometers that are driven by the

magnetic pickup is a single coil magnetic sensor.

ECM output (J1/P1 terminal 36 and terminal 37)

Also, a magnetic pickup will have four wires if the

must match the fixed output of the ECM for proper

magnetic pickup is a dual coil magnetic sensor. The

operation.

magnetic pickup sensor does not require a power

connection or a ground connection.

The type of vehicle speed sensor that is installed on

the vehicle must be determined.

Vehicles with Electronic Vehicle Speed Sources

Both single coil magnetic sensors and dual coil

For these vehicles, the vehicle speed signal is

magnetic sensors are commonly used. Caterpillar

provided by an electronic source. The signal is

recommends single coil magnetic sensors. Some of

received over a single wire that is connected to

the vehicles may provide the vehicle speed through

ECM vehicle harness connector J1/P1 terminal 32

the Transmission ECM.

(vehicle speed in positive). No other connection

should be made to this line. A speedometer is an

You must determine if the vehicle speedometer is

example of a connection that should not be made

driven from the ECM.

to this line. For this type of signal, the ECM vehicle

harness connector J1/P1 terminal 33 (vehicle speed

The ECM can drive the speedometer through

in negative) should not be connected to the ECM

the connection to ECM connector J1 terminal

in order to prevent introducing electrical noise into

36 (speedometer positive) and/or terminal 37

the circuit.

(speedometer negative).

For vehicle speed problems on trucks that are

Note: The ECM may also drive the speedometer

equipped with either of these vehicle speed

by a connection to the J1587 ATA data link, if the

sources, first inspect the wiring from the electronic

instrument cluster is capable of interpreting the

source to the engine ECM for open circuits or short

signal.

circuits.

Vehicle Speed Circuit Wiring