temperature gauge problem
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Fleet Mechanic
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temperature gauge problem
if you are like me, searches, on a site as large as this one, seem to lead to a lot of frustration and dead-ends, so i was delighted to finally find such a good description of how a temperature gauge works, and how to trouble-shoot it ...
many thanks to the writer "smecomark1 for the foregoing. i think it should be a sticky .
am also looking for a similar write-up for the volt-meter (ammeter ? ) for these trucks.
Most temperture gauges are usually whats known as an "RTD" (Resistance Temperature Detector) they work on the principle that Electrical Resistance (Ohms) will change in proportion to a temperature change. So really what you are looking at on the actual gauge is a Voltage Drop Meter that is calibrated for temperature. If the gauge is not reading correctly consult your favorite Service Manual to find the circuit and ID the color(s) of the wires. Perform the following Electrical checks with the battery disconnected
1)Check for contiuity from fuse to gauge, gauge to RTD probe on the engine. If you do not have continuity between either component it indicates a bad wire in that section. The RTD can be checked out of the engine with a thermometer as a referance and a pan of hot water, check Ohms acrossed it cold and Record, heat it up and check it again, the value should INCREASE. If not replace the sensor.
Perform the following checks with the battery CONNECTED
2)Check source voltage at the gauge should be ~12v and record it,
3) With the aid of another person and a set of test jumpers check for voltage at the lead going to the RTD probe (wire should be off the probe) on the engine, you should see source voltage between the hi side of the gauge and the wire at the probe, you should see NO source voltage or very little (<1v) between the lo side of the gauge and the probe wire (STILL OFF PROBE).
With the wire ON the RTD probe check the voltage between the lo side of the gauge and the probe. This value should reflect the actual temperature so it will very. HI volts if its hot, Lo volts if its cold.
If the volts very according to temperature Change but the gauge does not reflect that change REPLACE THE GAUGE.
Hope this Helps
Mark J.
1)Check for contiuity from fuse to gauge, gauge to RTD probe on the engine. If you do not have continuity between either component it indicates a bad wire in that section. The RTD can be checked out of the engine with a thermometer as a referance and a pan of hot water, check Ohms acrossed it cold and Record, heat it up and check it again, the value should INCREASE. If not replace the sensor.
Perform the following checks with the battery CONNECTED
2)Check source voltage at the gauge should be ~12v and record it,
3) With the aid of another person and a set of test jumpers check for voltage at the lead going to the RTD probe (wire should be off the probe) on the engine, you should see source voltage between the hi side of the gauge and the wire at the probe, you should see NO source voltage or very little (<1v) between the lo side of the gauge and the probe wire (STILL OFF PROBE).
With the wire ON the RTD probe check the voltage between the lo side of the gauge and the probe. This value should reflect the actual temperature so it will very. HI volts if its hot, Lo volts if its cold.
If the volts very according to temperature Change but the gauge does not reflect that change REPLACE THE GAUGE.
Hope this Helps
Mark J.
am also looking for a similar write-up for the volt-meter (ammeter ? ) for these trucks.
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