60-7 HOW THE CIRCUIT WORKS When the IGNITION SWITCH is in start or run the Gasoline and Diesel Engine Gauges and several indicators are powered by fuse 18 through the 640(R/Y) wires. The clusters include Fuel, Oil, Tempera ture, and optional Tachometer Gauges. The gauges consist of a coil, a magnet and a pointer, which moves in direct proportion to the output of its sender. The Diesel Clusters use Gauges (except Tachometer) that include two coils. When the Gauges are powered, a magnetic field is pro duced which varies in direction and strength according to the resistance of the sender. Fuel Gauge and Fuel Sender The pointer position of the magnetic Fuel Gauge is controlled by the resistance of the Fuel Sender. The sender has a reSistance of 145 OHMS when the fuel tank is full and 22.5 ohms when the tank is empty. Coolant Temperature Gauge and Sender The pointer position of the magnetic Cool ant Temperature Gauge is controlled by the resistance of the Coolant Temperature Sender. The sender provides 74 ohms of re- sistance when the engine coolant is cold and 9.7 ohms when the engine is hot. The Diesel engines Coolant Temperature Gauge uses an overheat switch that closes at appproximately 247 F. INSTRUMENT CLUSTER Oil Pressure Gauge and Oil Pressure Switch The pointer position of the magnetic Oil Pressure Gauge is controlled by an Oil Pres sure Switch. The Oil Pressure Switch closes under normal operating conditions of the en- gine. The Oil Pressure Switch is open with the engine off and no oil pressure. Engine Warning Indicator (Gas withTachometer and Diesel) The Engine Warning Indicator Module (part of Cluster Assembly) will light the Engine Warning Indicator when the module senses a high coolant temperature condition or a low oil pressure condition. Tachometer The Tachometer is an electrically operated gauge that indicates engine speed in rpm. In Gasoline Engines the Tachometer receives its voltage pulses from the ignition coil when the engine is running. In Diesel Engines the En gines Speed is indicated by the Engine RPM Sensor. Voltmeter With the Ignition Switch in START or RUN. the Voltmeter measures voltage to indicate the Battery state of charge. For further diagnostic information, refer to section 33-01 of the shop manual.