Circuit Operation - Gauges and Warning Indicators

SPEEDOMETER
The Speedometer applies voltage to CKT 389. Each time the Vehicle Speed Sensor Buffer (VIN D) or the Powertrain Control Module (PCM) (VIN L) grounds CKT 389 the Speedometer sees one "pulse." The Vehicle Speed Sensor Buffer grounds CKT 389 to produce a square wave at the rate of 4000 pulses per mile. The Speedometer converts the frequency of the square wave to vehicle speed. A high frequency square wave indicates a high vehicle speed, while a low frequency square wave indicates a low vehicle speed.

TACHOMETER
As the field in the primary coil of the Ignition Coil (VIN D) or Ignition Control (IC) Module rises and collapses, an AC signal is produced. This signal is sent through CKT 121 directly to the Tachometer. The Tachometer converts the AC signal to engine speed.

VOLTMETER
The Voltmeter measures the vehicle's electrical system voltage. When the Engine is running, the Voltmeter indicates Charging System operation. When the Engine is not running and the Ignition Switch is in "RUN," the Voltmeter indicates Battery condition.

FUEL GAGE
A potentiometer is built into the Fuel Sender Assembly. When the Fuel Tank is full, the resistance throughout the potentiometer is approximately 88 W, and at empty, the resistance is approximately 1.0 ohm. The Fuel Gage applies voltage through CKT 30 to the potentiometer. As the resistance of the potentiometer decreases, the Fuel Gage senses a smaller voltage drop across the potentiometer, indicating a lower fuel level. As the resistance of the potentiometer increases, the Fuel Gage senses a larger voltage drop across the potentiometer, indicating a higher fuel level.

OIL PRESSURE INDICATOR (CHEVY)
A two-position switch opens with oil pressure greater than approximately 24 kPa (3 psi), interrupting the ground path for the indicator, thus turning it off. If the oil pressure drops below 24 kPa (3 psi), the switch closes and the indicator lights, indicating low oil pressure.

OIL PRESSURE GAGE (VIN D) (PONT, OLDS)
A two-position switch opens with oil pressure greater than approximately 24 kPa (3 psi), directing current flow in CKT 331 through a 68 ohm resistor to ground. This causes the Oil Pressure Gage to deflect to approximately 3/4 scale.

OIL PRESSURE GAGE (VIN L) (PONT, OLDS)
A variable resistor is part of the Oil Pressure Sender/Fuel Pump Switch. With no oil pressure, the resistance of the variable resistor is approximately 1.0 ohm. At an oil pressure of 550 kPa (80 psi) the resistance of the variable resistor is approximately 88 ohms. The Oil Pressure Gage applies voltage through CKT 331 to the variable resistor in the Oil Pressure Sender/Fuel Pump Switch. As the resistance of the variable resistor increases, the Oil Pressure Gage senses a larger voltage drop across the variable resistor, indicating high oil pressure. As the resistance of the variable resistor decreases, the Oil Pressure Gage senses a smaller voltage drop across the variable resistor, indicating low oil pressure.

ENGINE COOLANT TEMPERATURE GAGE
The Engine Coolant Temperature Gage applies voltage through CKT 135 to the Engine Coolant Temperature Sender and Ignition Switch. The Engine Coolant Temperature Sender is a variable resistor. At 405C (1005F) the resistance of the Engine Coolant Temperature Sender is approximately 1365 ohms. At 1255C (2605F) the resistance of the Engine Coolant Temperature Sender is approximately 55 ohms. As the resistance of the Engine Coolant Temperature Sender decreases, the Engine Coolant Temperature Gage senses a smaller voltage drop across the Engine Coolant Temperature Sender, indicating higher coolant temperature. As the resistance of the Engine Coolant Temperature Sender increases, the Engine Coolant Temperature Gage senses a larger voltage drop across the Engine Coolant Temperature Sender, indicating a lower coolant temperature. When the Ignition Switch is turned to "BULB TEST" or "START," CKT 135 is grounded through the Ignition Switch, causing the gage to indicate high coolant temperature.

"GATE AJAR" INDICATOR
Voltage is applied to the "GATE AJAR" Indicator when the Ignition Switch is in "RUN," "BULB TEST" or "START," through the Gauges Fuse and CKT 39. The RH and LH Liftgate Jamb Switches are closed when the Liftgate is open. When either the LH or RH Liftgate Jamb Switch is closed, the "GATE AJAR" Indicator is grounded through the Jamb Switch and turns "ON."