Temperature Senders are often Bi-Metalic Thermo-Couple devices, whose resistance decreases as Temperature rises; resulting in a higher reference voltage at the gauge.
All Gauges:
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 Voltage.
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.
Temperature Gauges:
1. Disconnect Tan Sender Wire from Gauge (Terminal “S”) and turn Instrument
Power ON.
The Temperature Gauge should indicate ZERO Scale (< Low Temperature).
2. Short the Gauge Sender Terminal “S” to Ground Terminal “G”.
The Gauge should deflect to FULL Scale (> High Temperature).
3. Measure Resistance from Tan Sender Wire (disconnected from Gauge) to Ground.
American Temp. Senders will read: 450 Ohms (Engine Cold @ 1000F)
or 29.6 Ohms (Engine Hot @ 2500F )
European Temp. Senders will read: 281 Ohms (Cold @ 400C) and 22 Ohms (Hot @ 1200C)
4. Disconnect Tan Wire from Sender on Engine Block.
5. Test continuity of Tan 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.
Excerpted from:
Basic Engine Gauge Theory and Testing
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