The 6.5L V8 turbo diesel engine uses an oil pressure and coolant temperature gauge for vehicle driver information (refer to Figure 8-31). Power from the ignition switch in the CRANK and RUN position moves through the GAGES fuse to the gauges mounted in the instrument cluster. A CHECK GAUGES warning lamp and its control module are also in the cluster and alert the vehicle driver when low oil pressure and/or high coolant temperature conditions exist.
The position of the indicator needle for the oil pressure gauge is controlled by the varying resistance of the oil pressure switch/sending unit, located in a lubrication system passage at the rear of the cylinder case. The position of the coolant temperature gauge is controlled by a sending unit threaded into the coolant passage at the front of the left cylinder head.
The oil pressure switch/sending unit provides low resistance for low gauge indication and high resistance for high gauge readings, while the temperature sending unit acts in the opposite way. Diagnosis includes opening or grounding the sending unit wire while observing gauge indicator needle movement.
The 6.5L V8 turbo diesel engine also uses an instrument cluster-mounted low coolant lamp to alert the vehicle driver (refer to Figure 8-32). Power from the ignition switch in the CRANK and RUN position moves through the GAGES fuse to the lamp mounted in the instrument cluster. A control module mounted behind the instrument panel above the ash tray also receives power through the GAGES fuse and has a circuit ground wire.
A probe for the low coolant lamp circuit threads into the right tank of the radiator and provides a varying path to ground as an input for the control module circuit. If the probe senses the absence of coolant (indicating a loss of fluid), the control module completes the circuit for the warning lamp. After repairs have been performed to correct the loss of coolant, the control module detects a changed probe circuit resistance and opens the warning lamp circuit.
The low coolant probe provides a low resistance that results in the warning lamp remaining OFF, and high probe circuit resistance causes the warning lamp to illuminate. Part of the steps of diagnosis includes opening or grounding the probe wire while observing the warning lamp status.
