Instrument Cluster / Carrier: Description and Operation
When the ignition switch is in ON (II) or START (III), battery voltage is supplied from fuse 15 to the gauges in the gauge assembly.Speedometer and Odometer
With the ignition switch in ON (II) or START (III), the odo/trip meter driver and speedometer driver in the gauge assembly are supplied battery voltage from fuse 15. The odometer and speedometer drive circuits receive pulses from the vehicle speed sensor (VSS). The pulse rate increases as the car accelerates. The frequency and duration of these input pulses are measured and displayed by the speedometer odometer and tripmeter.
Tachometer
Turning the ignition switch to ON (II) or START (III) turns on the tachometer driver in the gauge assembly. The tachometer driver receives pulses from the ignition control module (ICM) in the distributor assembly The solid-state tachometer then displays these pulses as engine speed. For each 200 pulses per minute from the ignition control module (ICM), the tachometer displays 100 RPM.
Engine Coolant Temperature Gauge and Fuel Gauge
The engine coolant temperature gauge has two intersecting coils wound around a permanent magnetic rotor. Voltage applied to the coils, through Fuse 15, generates a magnetic field. The magnetic field, controlled by the coolant temperature sending unit, causes the rotor to rotate and the gauge needle to move. As the resistance in the sending unit varies, current through the gauge coils changes. The gauge needle moves toward the coil with the strongest magnetic field. The fuel gauge works the same way.
The engine coolant temperature (ECT) sending unit's resistance varies from about 137 ohms at low engine temperature to between 30 - 46 ohms at high temperature (radiator fan running).
The fuel gauge sending unit's resistance varies from about 2 - 5 ohms at full, to about 110 ohms at empty. When the ignition switch is turned to lock (0), the gauge remains at the last reading until you turn the ignition switch to ON (II) or START (III) again.