[shmano]

One month ago I had a coolant leak repaired. I had a bad hose. Now my temperature gauge is fluctuating from cold to hot to normal and back again. What would cause this?

[TxGreaseMonkey]

Temperature Gauge Problems

Background:

A temperature gauge is basically a voltage meter, controlled by the Engine Coolant Temperature (ECT) Gauge Sending Unit. The ECT Gauge Sending Unit contains a thermistor (temperature sensitive resistor), which regulates the amount of voltage to the temperature gauge. Since resistance decreases with increasing temperature, it is a negative temperature coefficient (NTC) thermistor. It will vary between zero volts (Cold) and 12 volts (Hot), depending upon the temperature of the engine. For identification purposes, the ECT Gauge Sending Unit has a one-wire connector going to it, while the ECT Sensor, which goes to the ECM, has a two-wire connector.

Troubleshooting:

1. Check all under dash and under hood fuses with a test light or multimeter.

2. Remove the ECT Gauge Sending Unit connector, clean terminal and connector with abrasive cloth, and turn the ignition switch to ON. The temperature gauge should bottom at Cold (0 volts). Next, ground the wire from the sending unit--the needle should go all the way to Hot (12 volts). If it does, the fuse, wiring, and temperature gauge are fine. You will likely, then, need to replace the ECT Gauge Sending Unit. If it doesn't, then you have a fuse, wiring, or temperature gauge problem. Indications that the ECT Gauge Sending Unit may be failing are a fluctuating temperature gauge needle (thermistor breaking down) or a needle that barely moves.

3. An alternative, and possibly safer, way to test the ECT Gauge Sending Unit is to disconnect the electrical connector and attach an ohmmeter between the sending unit terminal and engine ground. When cold, resistance should be around 275 ohms. As the engine warms up, sending unit resistance should drop; at full operating temperature, it should read 18 to 20 ohms.