Instrument Panel, Gauges and Warning Indicators: Description and Operation

CIRCUIT OPERATION

The operation of an indicator is explained in the operation of the circuit it is a part of. See the circuit referred to for a complete description of that indicator and the other components that work with it.

Tachometer

The Tachometer displays engine speed in rpm. Voltage pulses are taken from the Ignition System and sent to the Tachometer. Solid State circuits process these pulses into a signal that drives the pointer of the meter. The Tachometer responds to the frequency of the voltage pulses. These pulses increase with engine speed.

Fuel Gage

The pointer of the Fuel Gage is moved by the magnetic field of two coils. The coils are at right angles to each other. Battery voltage is applied to the E coil and the circuit divides at the opposite end of this coil. One path continues to ground through the F coil. Another goes to ground through the variable resistor of the Fuel Gage Sender Unit.

When the tank is low, the resistance of the Fuel Gage Sender is low. A large flow of current passes through the E coil and the Fuel Gage Sender resistor. This moves the pointer towards E" on the scale. When the tank is full, the sender resistance is high. More current flows through the F coil, moving the pointer toward "F" on the scale.

With two coils operating the pointer, the Fuel Gage is not affected by changes in the voltage of the system.

Oil Pressure Gage

The engine oil pressure is displayed by the Oil Pressure Gage. The pointer of the gage is moved by two coils, and its operation is similar to that of the Fuel Gage.

With V6 VIN T equipped vehicles, the Fuel Pump/Oil Pressure Switch and Oil Pressure Resistor are connected to the junction of the two coils. The Fuel Pump/Oil Pressure Switch is closed to ground with low oil pressure, causing the pointer of the gage to move to the low range. With oil pressure above 21 kPa (3 psi) the switch opens, and the 68 ohm Oil Pressure Resistor changes the current flow through the coils of the gage. This change in current flow pulls the pointer into the operating range. The magnetic fields of the coils move the pointer to indicate low oil pressure or sufficient oil pressure.

With V8 VIN E, V8 VIN F or V8 VIN 8 equipped vehicles, the Fuel Pump/Oil Pressure Sender/Switch is connected to the junction of the two coils. It has low resistance when the oil pressure is low and 90 ohms resistance when the oil pressure is high. This changing resistance changes the current flow through the coils. The magnetic fields of the coils move the pointer to indicate high or low oil pressure.

Voltmeter

The Voltmeter measures the electrical system voltage with the Ignition Switch in RUN, BULB TEST or START. With the engine stopped, the Voltmeter indicates battery condition. With the engine running, the Voltmeter indicates Charging System operation.

Coolant Temperature Gage

The Coolant Temperature Gage is operated by two coils. Battery voltage is applied to the junction of the two coils. One of the coils is grounded and the other is connected to the Coolant Temperature Sender. With the coolant cold, the sender resistance is high and most of the current passes through the grounded gage coil causing the indicator needle to move toward "100." As the coolant temperature increases, the sender resistance decreases. As the sender resistance decreases, the current through the connected gage coil increases and the indicator moves away from 100," indicating increasing coolant temperature. When the coolant temperature is 38°C (100°F) the sender resistance is approximately 1365 ohms. When the coolant temperature is 127°C (260°F) the sender resistance is 55 ohms. With the Ignition Switch in BULB TEST or START, the gage coil that is connected to the sender is grounded and the indicator needle moves to "260."