GM Service Manual Online
For 1990-2009 cars only

Transmission Range Circuit Description

The transmission range indicator indicates the gear selection for the automatic transmission. The transmission range indicator receives the input code through 4 wires from the transmission range selector. The transmission range selector is part of the park/neutral position switch. A gear selection indication light emitting diode (LED), e.g. Reverse (R) illuminates depending upon which combination of the 4 wires to the transmission range indicator ground. The indications are separate from the transmission inputs to the vehicle control module.

Fuel Gauge Circuit Description

The magnetic fields of the 2 coils moves the pointer of the fuel gauge. The coils are at right angles to each other. The E-coil (empty) supplies the battery voltage. The circuit divides at the opposite end of the coil. One path continues to ground through the F-coil (full). Another path goes to ground through the variable resistance of the level sensor in the fuel pump and sender. When the tank is low, the resistance of the level sensor is low. A large current flow passes through the E-coil. The pointer moves toward E on the fuel gauge. When the tank is full, the fuel level sensor resistance is high. Less current passes through the E-coil. The pointer moves toward F on the scale. With 2 coils operating the pointer, changes in the system battery voltage does not affect the fuel gauge.

The fuel level sender operates in a range from around 40ohms when the tank is empty and a 250ohms when the tank is full. If the VCM receives a signal from the fuel level sender either higher or lower than the specified range it will automatically send a signal to the fuel gauge to show empty.

The fuel gauge does not operate in the same range as the sender. When the signal is sent to the VCM it is modified to a 0ohms to 90ohms scale. When it sees 0ohms at the VCM it shows empty. When it sees 90ohms it shows full.

Oil Pressure Gauge Circuit Description

The oil gauge displays the engine oil pressure. The current flows through the 2 coils moves the pointer of the oil gauge. This is similar in operation to the fuel gauge. The oil pressure switch and sender senses the engine oil pressure. The oil sender has a low resistance, approximately 0ohms when the oil pressure is low (0 psi). The oil sender has a high resistance, approximately 90ohms when the oil pressure in high (80 psi).

Temperature Gauge Circuit Description

The temperature gauge displays the engine coolant temperature. The current flow through the 2 coils moves the pointer of the temperature gauge. This is similar in operation to the fuel gauge. The engine coolant temperature sender has a low resistance at higher temperatures. The engine coolant temperature sender has a resistance of approximately 49.5ohms at 123°C (260°F). The resistance increases at lower temperatures. The resistance is approximately 1305ohms at 47°C (100°F). The resistance increase causes the current through the sensor to increase as the coolant temperature increases. The temperature gauge pointer also moves toward the hot end of the scale.

Voltmeter Gauge Circuit Description

The voltage gauge displays the battery voltage. The current flow through the gauge coils moves the pointer of the voltage gauge. The voltage reading is supplies from the GAUGES fuse through CKT 39 (PNK) cavity B17 at the cluster. Cavity B13 at the cluster to grounds G105/G107 supplies the ground.

Speedometer and Odometer Circuit Description

An electronic speedometer is standard equipment on all models. Integrated circuits control the air core speedometer and the stepper motor odometer. Removable bulb and socket assemblies provide the lighting. The socket assembly inserts into the back of the instrument cluster.

The speedometer system consists of the following components:

    • The instrument cluster
    • The vehicle speed sensor (VSS) calibrator
    • The VSS
    • The related wiring

The VSS calibration is part of the VCM that changes the analog output from the VSS to a digital signal. The digital signal contains 4000 pulses per mile (PPM) for the instrument cluster. The VCM sends the signal to the speedometer through CKT 389. The VSS calibration matches to the final drive and the tire size of each vehicle. Reprogram the VCM with the proper VSS calibration if the final drive or tire size changes for any reason. The signal will not be accurate for the proper speedometer/odometer operation if the necessary change is not made. An incorrect VSS calibrator can also affect the following components:

    • The antilock brakes
    • The vehicle control module (VCM)
        The cruise control systems

The VSS calibration is programmed into the VCM. Do not service the VSS calibrator separately.

The VSS is a permanent magnet signal generator that sends an analog signal proportional to the propeller shaft speed to the VCM. The VSS mounts to the transmission tailshaft extension housing on the two-wheel drive model. The VSS mounts to the transfer case output shaft housing on the all-wheel drive models.

Check Gauges Lamp Circuit Description

The check gauges lamp lights when either the engine oil pressure is low or the engine coolant temperature is high. The instrument cluster check gauges lamp driver solid state logic monitors the input signals to the oil gauge and the temp gauge. With the ignition switch in the RUN or the START position, the check gauges lamp driver grounds the check gauges lamp. The check gauges lamp grounds when either of the input signals exceeds the setpoint. Exceeding the setpoint causes the lamp to light. The lamp normally illuminates when the ignition switch moves to the RUN position. The lamp extinguishes when the engine starts and the oil pressure increases above the setpoint.