System Operation
SYSTEM OPERATIONCoolant Temperature Gauge
The coolant temperature gauge gives an indication of the engine coolant temperature. The coolant temperature sending unit is a thermistor-type sensor that changes its internal electrical resistance with changes in the engine coolant temperature. The sending unit is installed in a threaded hole that penetrates a coolant passage of the engine.
High sending unit resistance causes low coolant temperature gauge readings. Low sending unit resistance causes high coolant temperature gauge readings. The sending unit resistance values are shown in a chart in Specifications.
Fuel Gauge
The fuel gauge gives an indication of the level of fuel in the fuel tank. The fuel gauge sending unit is mounted to the electric fuel pump module located inside the fuel tank. The sending unit has a float attached to the end of a swing-arm. The float moves up or down within the fuel tank as the fuel level changes. As the float moves, an electrical contact on the pivot end of the swing-arm wipes across a resistor coil, which changes the internal electrical resistance of the sending unit.
High sending unit resistance causes low fuel gauge readings. Low sending unit resistance causes high fuel gauge readings. The sending unit resistance values are shown in a chart in Specifications.
Gear Selector Indicator
The gear selector indicator gives an indication of the position of the automatic transmission gear selector lever. The indicator is mounted to the instrument cluster housing.
The indicator is mechanically actuated by a cable connected to the PRNDL driver of the gear selector lever mechanism on the steering column. This covers only the removal and installation of the gear selector indicator. Refer to Transmission and Drivetrain for the diagnosis and adjustment of this component.
Odometer And Trip Odometer
The odometer and trip odometer each give an indication of the distance the vehicle has travelled. How- ever, by depressing the trip odometer reset knob on the face of the speedometer/odometer gauge unit, the trip odometer number rolls can be reset to zero.
The odometer and trip odometer number rolls are controlled by the same circuitry and vehicle speed sensor pulse signals as the speedometer. The odometer and trip odometer are serviced only as a part of the speedometer/odometer gauge unit.
Oil Pressure Gauge
The oil pressure gauge gives an indication of the engine oil pressure. The combination oil pressure switch/sending unit contains a flexible diaphragm and a variable resistor coil. The diaphragm moves in response to changes in the engine oil pressure, which changes the internal electrical resistance of the sending unit.
The sending unit is installed in a threaded hole that penetrates an oil passage of the engine. High resistance on the gauge side of the sending unit causes low oil pressure gauge readings. Low resistance on the gauge side of the sending unit causes high oil pressure gauge readings. Sending unit resistance values are shown in a chart in Specifications.
Speedometer
The speedometer gives an indication of the current vehicle speed. The speedometer circuitry controls the gauge pointer position. The proper gauge pointer position is calculated by the speedometer circuitry based upon a vehicle speed pulse signal from the Vehicle Speed Sensor (VSS).
The electronic integrated circuit contained within the speedometer gauge unit reads and analyzes the vehicle speed pulse signal. It then adjusts the ground path resistance of one electromagnet in the gauge to control the speedometer needle movement. The frequency values for the pulse signal are shown in a chart in Specifications.
The VSS is a hall-effect sensor that is installed in the transmission, and is driven by the output shaft through a speedometer pinion gear. Incorrect tire size, incorrect axle ratio, a faulty or incorrect speedometer pinion gear, or a faulty VSS can each result in inaccurate speedometer readings.
Tachometer
The tachometer gives an indication of the engine speed in revolutions-per-minute (rpm). The tachometer circuitry controls the gauge pointer position. The proper gauge pointer position is calculated by the tachometer circuitry based upon an engine speed pulse signal received by the tachometer from the Powertrain Control Module (PCM).
The PCM calculates the engine speed based upon an input from the crankshaft position sensor. The PCM then sends the proper pulse signal to the tachometer circuitry. An electronic integrated circuit contained within the tachometer unit reads and analyzes this pulse signal. It then adjusts the ground path resistance for one of the electromagnets in the gauge to control the tachometer needle movement.
The crankshaft position sensor is a hall-effect sensor installed on the engine, where it is aimed at a trigger wheel on the crankshaft. The frequency values for the pulse signal are shown in a chart in Specifications.
Voltmeter
The voltmeter gives an indication of the electrical system voltage. The voltmeter is connected in parallel with the battery With the ignition switch in the On position, the voltmeter indicates the battery voltage or the generator output voltage, whichever is greater.
Airbag Indicator Lamp
The airbag indicator lamp gives an indication when the airbag system is faulty or inoperative. The lamp is turned on by the Airbag Control module (ACM) for about seven seconds when the ignition switch is turned to the On position as a bulb test. After the bulb test, the lamp is controlled by the ACM based upon the results of the airbag system self-tests that are performed by the ACM.
The ACM continually monitors the airbag system circuits and sensors to decide whether the system is in good operating condition. The ACM then switches the lamp ground circuit to turn the lamp on or off. If the ACM turns the lamp on after the bulb test, it indicates the ACM has detected a system malfunction and/or that the airbag system has become inoperative.
Anti-Lock Brake System Lamp
The Anti-Lock Brake System (ABS) lamp gives the driver an indication when the standard Rear Wheel Anti-Lock (RWAL) or optional all-wheel ABS system is faulty or inoperative. See the following description that applies to the brake system used on the vehicle being serviced.
Rear Wheel Anti-Lock
On vehicles equipped with the standard RWAL brake system, the ABS lamp is switched to ground by the RWAL control module. The RWAL control module lights the lamp when the ignition switch is turned to the Start position as a bulb test. The lamp will stay on for about four seconds after vehicle start-up to indicate that a system self-test is in progress.
If the ABS lamp remains on after start-up, or comes on and stays on while driving, it may indicate that the RWAL control module has detected a system malfunction and/or that the RWAL system has become inoperative.
Anti-Lock Brake System
The optional all-wheel ABS lamp is grounded through the normally closed contacts of the de-energized ABS lamp relay When the ignition switch is turned to the Start position, the lamp is lighted as a bulb test. The lamp will stay on for about four seconds after vehicle start-up to indicate a system self-test is in progress. If the self-test is successfully completed, the ABS lamp relay is energized by the Controller Anti-lock Brake (CAB) module, which turns the lamp off.
The CAB module energizes the ABS lamp relay to turn the ABS lamp off, and de-energizes the ABS lamp relay to turn the ABS lamp on. If the ABS lamp remains on after start-up, or comes on and stays on while driving, it may indicate that the CAB module has detected a system malfunction and/or that the ABS system has become inoperative.
Brake Warning Lamp
The brake warning lamp gives an indication when the parking brake is applied, when the pressures in the two halves of the split brake hydraulic system are unequal, or when certain problems are detected in the Rear Wheel Anti-Lock (RWAL) or Anti-Lock Brake Systems (ABS). The lamp is turned on when the ignition switch is turned to the Start position as a bulb test. After the bulb test, the lamp is controlled by hard-wired inputs from the parking brake switch, the brake warning switch, or the RWAL control module/Controller Anti-lock Brake (CAB).
The brake warning switch closes to ground when it senses unequal hydraulic pressures in the two halves of the split brake hydraulic system, possibly due to low brake fluid level or brake fluid leakage. The parking brake switch closes to ground when the parking brake is applied. The RWAL control module or CAB can also turn on the brake warning lamp if certain brake system faults are detected.
Cluster Illumination Lamp
When the park or head lamps are turned on, the cluster illumination lamps light. Illumination brightness is adjusted by rotating the headlamp switch knob (clockwise to dim, counterclockwise to brighten). The instrument cluster illumination lamps receive battery feed from the panel dimmer rheostat in the headlamp switch through a fuse in the fuseblock module.
Each of the illumination lamps is located on the instrument cluster circuit board. Each lamp has a replaceable bulb and bulb holder.
Four-Wheel Drive Indicator Lamp
On models equipped with the four-wheel drive option, a four-wheel drive indicator lamp in the instrument cluster lights any time the front axle is in the four-wheel drive operating mode. Ignition switched battery voltage is supplied to one side of the indicator lamp bulb. A switch on the front axle disconnect housing is hard-wired to the other side of the indicator lamp bulb.
The normally open, plunger-type, four-wheel drive switch is threaded into the front axle disconnect housing. The switch is connected in series between the indicator lamp bulb and ground. The plunger of the four-wheel drive switch is actuated by the front axle disconnect shift fork when the vacuum-controlled front axle disconnect mechanism is actuated, opening or closing the switch and the ground path for the indicator lamp.
Generator Warning Lamp
The generator warning lamp gives an indication of a charging system problem. One side of the bulb is connected to an ignition-switched battery feed. The other side of the bulb is switched to ground by the Powertrain Control Module (PCM).
The lamp should light when the ignition switch is turned to the On position, but should go out whenever the engine is running. If the lamp comes on and stays on while the engine is running, it indicates that the PCM has detected a charging system malfunction and/or that the charging system has become inoperative.
Headlamp High Beam Indicator Lamp
The headlamp high beam indicator lamp gives an indication when the headlamp high beams are turned on. The lamp is controlled by the headlamp dimmer (multi-function) switch.
One side of the high beam indicator lamp bulb is grounded at all times. The other side of the bulb receives a battery feed through the contacts of the dimmer switch when the multi-function switch stalk is actuated to turn on the headlamp high beams.
Low Fuel Warning Lamp
The low fuel warning lamp will light when the fuel level in the fuel tank has fallen below about one- eighth of a full tank, as registered on the fuel gauge. One side of the low fuel warning bulb is grounded at all times. A low fuel warning circuit attached to the rear of the gauge cluster provides ignition-switched battery voltage to the lamp when the fuel level is low.
The low fuel warning circuit monitors the signal from the fuel level sending unit. When the circuit senses about 65 ohms or more from the fuel level sending unit for eight to thirty continuous seconds, the indicator lamp will light. The indicator lamp will remain on until the circuit senses about 65 ohms or less from the fuel sender for eight to thirty continuous seconds.
The fuel gauge sending unit is mounted to the electric fuel pump module inside the fuel tank. The sending unit has a float attached to the end of a swing- arm. The float moves up or down within the fuel tank as the fuel level changes. As the float moves, an electrical contact on the pivot end of the swing-arm wipes across a resistor coil, which changes the resistance of the sending unit.
Low Oil Pressure Warning Lamp
The low oil pressure warning lamp gives an indication when the engine oil pressure is low. The lamp lights with the ignition switch in the On position and the engine not running as a bulb test. The lamp should be off when the engine is running.
Battery voltage is supplied to one side of the indicator bulb when the ignition switch is turned to the On position. The warming lamp side of the combination oil pressure switch/sending unit resistor coil is connected to the other side of the bulb.
When normal engine oil pressure is applied to the sending unit, resistance to ground on the warning lamp side of the resistor coil is high and the lamp goes off. When the engine oil pressure is too low, resistance to ground on the warning lamp side of the resistor coil is low, which causes the bulb to light.
Low Washer Fluid Warning Lamp
The low washer fluid warning lamp gives an indication when the fluid level in the washer fluid reservoir is too low. The washer fluid level sensor uses a float in the reservoir to monitor the fluid level.
The up and down action of the float opens or closes the switch within the washer fluid level sensor that provides a ground path to the warning lamp bulb.
Malfunction Indicator Lamp
The Check Engine or Malfunction Indicator Lamp (MIL) gives an indication when the Powertrain Control Module (PCM) has recorded a Diagnostic Trouble Code (DTC) for an On-Board Diagnostics II (OBDII) emissions-related circuit or component malfunction.
The lamp is controlled by the PCM. The PCM sends a lamp-on signal for about three seconds when the ignition switch is turned to the On position as a bulb test. Following the bulb test, the PCM uses inputs from many emissions-related circuits and sensors, along with its internal programming, to decide whether a condition exists that requires the MIL lamp to be turned on.
The MIL lamp can also be used to display a stored DTC by flashing on and off. Refer to Powertrain Management/Computers and Control Systems for more information on the PCM or the PCM inputs.
Seat Belt Reminder Lamp
The seat belt reminder lamp gives a visual reminder to the vehicle occupants to fasten their seat belts. The lamp is turned on by a timer circuit within the chime/buzzer module for about seven seconds when the ignition switch is turned to the On position.
The chime/buzzer module also receives a hard- wired input from the driver found side seat belt switch. If the driver side seat belt switch is closed (seat belt is not buckled), the chime/buzzer module will generate an audible warning for the duration of the seat belt reminder lamp illumination. The audible warning will stop when the driver side seat belt switch is open (seat belt is buckled).
Service Reminder Indicator Lamp
The Maintenance Required or Service Reminder Indicator (SRI) lamp is used on certain vehicles equipped with a heavy-duty emissions cycle package. This lamp is intended to provide a reminder that certain scheduled vehicle emissions services and maintenance must be performed, as required by federal emissions laws.
This lamp is not intended to indicate a warning, or that a state of emergency exists. However, when the lamp has been activated, the required services and maintenance must be performed before the lamp can be legally reset.
Transmission Oil Temperature Warning Lamp
The transmission oil temperature warning lamp gives an indication that the transmission oil is overheated. One side of the lamp bulb receives battery voltage when the ignition switch is turned to the On position. When the Powertrain Control Module (PCM) senses that the transmission oil temperature is too high, it provides a ground path to the other side of the bulb.
A transmission oil temperature sensor located within the transmission sends a signal to the PCM, indicating the transmission fluid temperature. This lamp is only used on vehicles equipped with an automatic transmission.
Turn Signal Indicator Lamp
The left and right turn signal indicator lamps give an indication when the turn signal circuits are activated. These lamps are completely controlled by the turn signal and hazard warning (multi-function) switches.
The indicator lamps are grounded at all times and receive battery feed through the contacts of the multi-function switch when the turn signal lever (multi-function switch stalk) or hazard warning button are actuated to the On position.
Upshift Indicator Lamp
Certain vehicles equipped with a transmission have an upshift indicator lamp. The upshift indicator lamp gives an indication when the driver should shift to the next highest gear for the best fuel economy One side of the indicator lamp bulb receives ignition-switched battery feed. The ground path for the other side of the lamp bulb is switched by the Powertrain Control Module (PCM).
The PCM will turn the lamp off after three to five seconds if the upshift is not performed. The lamp will remain off until the vehicle stops accelerating and is brought back into the range of lamp operation, or until the transmission is shifted into another gear.
The indicator lamp is normally on when the ignition switch is in the On position, and is turned off when the engine is started. The PCM uses inputs from many sensors and its internal programming to decide whether the engine speed and load conditions are proper for a transmission upshift.
Wait-To-Start Lamp
Vehicles equipped with an optional diesel engine have a wait-to-start lamp. The wait-to-start lamp gives an indication that the conditions for easiest starting of the diesel engine have not yet been achieved. The wait-to-start lamp bulb in the message center is lighted by the Powertrain Control Module (PCM), after the ignition switch is turned to the On position.
One side of the wait-to-start lamp bulb receives battery voltage when the ignition switch is turned to the On position. The PCM switches the ground path for the other side of the bulb based upon several inputs and its internal programming.
The wait-to-start lamp lets the driver know that the intake manifold air heater grid has had sufficient time to warm the intake air for a good quality start. The intake manifold air preheat cycle is controlled by an electronic air heater control module. The lamp will be turned off by the PCM when the heater control module cycle is completed, or if the driver turns the ignition switch to the Start position prior to the end of the heater control module cycle.
Water-In-Fuel Lamp
Vehicles equipped with an optional diesel engine have a water-in-fuel lamp. The water-in-fuel lamp gives an indication when the water contamination in the diesel fuel exceeds a certain level. The lamp in the message center is turned on as a bulb check by the Powertrain Control Module (PCM) for two seconds, when the ignition switch is first turned to the On position.
One side of the water-in-fuel lamp bulb receives battery voltage when the ignition switch is turned to the On position. The other side of the bulb is switched to ground by the PCM.
After the bulb check is complete, the PCM uses an input from the water-in-fuel sensor in the fuel filter/ water separator to determine that excess water has accumulated in the diesel fuel. The PCM will turn the lamp off when the excess water has been drained.