Basic Engine Gauge Theory and Testing
The standard Voltmeter type Gauge receives it’s reference voltage from a Sender unit, mounted on the Engine. The Sender is a device, whose resistance varies with changes in Level, Pressure, or Temperature. As the sender resistance increases, the reference voltage transmitted to the Gauge decreases.
1. Remove Sender Wire from “S” Terminal on Gauge, and Turn ON Instruments Circuit.
2. Test Voltage between “I” Terminal (External Power Supply) and Ground Terminal “G”.
Tested voltage must match nominal voltage of the Gauge (ie: 12V Gauge must test between 10 - 16VDC), and the Battery
If no voltage - check Ignition or Instrument Switch(s) and/or Fuses
If tests less than battery
voltage, repair or replace (Violet) wire from Ignition or Instrument Switch.
If a Sender fails “Open” (Infinite Resistance), one or both (if Dual Station) Gauges will read Zero Scale.
If a Sender Fails “Closed” (shorted - Zero Resistance), one or both Gauges will read Full Scale.
Other confusing Ohmmeter results, such as readings lower than expected, could indicate a poor Ground Connection at the Sender Body (Sender to Engine Block). Check for pipe dope or tape on Sender Threads.
1. Disconnect Pink Sender Wire from Gauge, and turn power ON.
The Fuel Gauge should read ZERO Scale (< Empty)
2. Short the Gauge Sender Terminal to Ground.
The Gauge should read FULL Scale (> Full).
3. Measure resistance of Pink Sender Wire to Ground.
American Floats: 240 Ohms (Empty), 103 Ohms (˝ Full), & 33 Ohms (Full)
European Floats: 10 Ohms (Empty), 95 Ohms (Half), & 180 Ohms (Full)
Automotive & other Float Switches may use other resistances such as 0 - 90 Ohm, 73 - 10 Ohm, & etc.
4. Disconnect Pink Wire from Sender on Engine Block.
5. Test continuity of Pink Wire.
Zero Ohms from Block End to Gauge End - If not, repair or replace Wire.
6. Measure Resistance from Sender Terminal (on Block) to Ground on Block.
Should read as per (3) above - If not, replace Sender.