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Coolant Temperature Sensor

newer coolant sensorThe coolant sensor measures both the temperature of the engine coolant (“antifreeze”), and the rate of the temperature change. The computer needs this to control when the engine uses the oxygen sensor to adjust the fuel mixture, and when to turn on the radiator fan for 1985 and later models.

The coolant sensor is in the engine head, near the thermostat, roughly where the top heater hose is attached — on many cars, on the side of the thermostat “box.” (In some of the carburetor years, they put it on the radiator itself.)

My 1997 Ram 1500 was getting harder to start as it got colder: long cranking, firing but dying immediately, eventually running badly; once it warmed up all was well and it ran well. ... I replaced the water temperature sensor, and it's cured! The guts of the sensor had come loose from the brass fitting. Thanks, Larry Hitze

In the 1984 cars, a thermistor/fixed resistor combination measures 5,290 to 5,610 ohms at 77°, for high sensor accuracy at high temperatures. Starting in 1985 cars, they switched to a thermistor, measuring 9,120 to 10,880 ohms at 77°; it had a fixed resistor in the logic module to increase accuracy, so the car could turn the radiator fan on and off as needed. It can also read very cold temperatures. The voltage from the sensor should be 2.5v at 77°.

On cars using two-digit error codes:

If the sensor’s output voltage is less than 0.157v or more than 3.76v (1984), or is less than 0.51v or more than 4.96v (1985 and up), the computer will register a code 22, light the “power limited” light, and go into limp mode, keeping the transmission to first and second gear. 1984 cars use the air charge temperature sensor to estimate the coolant temperature; later models turn on the fan and leave it on.

If the sensor goes back to normal in 1988 and later Turbo 1 engines, the power limit light goes out and the car leaves limp mode. It still stores the code. All other models used the air charge temperature sensor to estimate coolant temperature.

If the sensor reads a temperature of -20 to 212° when starting the engine, the system waits 20 minutes, and then, if the temperature is not at least 160°, it triggers code 17.  Staring with 1987, the system also starts an eight minute timer, 12 minutes after the engine has started. If the engine temperature is below 174° and the car is moving faster than 28 miles per hour, and code 22 is not stored, it stores code 17.

Coolant sensor failures: avoiding the nightmare

About ten months ago, my mechanic did a computer test and told me my coolant sensor was bad. It is screwed into the side of the “water box” on the cylinder head, where the thermostat is. The sensor is brass, with two threaded posts coming out of the Bakelite or black plastic end of the sensor.

newer coolant sensorOne of the posts was broken off. NAPA’s catalog showed that the correct sensor was part # ECH TS5005, and I installed it. I presumed it was the updated design, since they showed it as being for all cars from 1984 onwards. I also had to order a new connector.

My troubles got worse, but not right away. The broken post had already been tripping off the “Power Loss” light. When I ran fault codes I got all kinds of readouts, which may or may not have been accurate. I replaced both engine and fuel computer modules, oxygen sensor (twice), and all of the vacuum lines (which was good, I found a couple of broken ones).

The car would not start all the time, and when it did, it would sputter and belch out dark gray smoke. When it didn’t start, I pulled the spark plugs and they were wet with gasoline. I had the injectors re-built, nothing got better. And I was getting 10 mpg.


I had a conversation with a friend in Great Autos of Yesteryear and he asked me about sensors. After some research, I found information on which explained that the 1984 sensor, which I originally had, had an ohm value in the mid 5s, while the 1985 and newer sensor is designed for a different computer and has an ohm value of 9. The new sensor was wrong, and resulted in dumping much more gasoline into the cylinders.

I also realized that the air charge sensors are the same as the coolant sensors, so I just ordered the air charge sensor from NAPA (part # ECH TS5009) and installed it (see below — it doesn’t have the covered pins of the 1984-only sensor, and was also used in the Mopar V8, V10, and Mitsubishi V6 engines — according to NAPA).

coolant - air charge sensor

This fixed the problem. The car starts instantly, is getting over 20 mpg, and the “Power Loss” light has not come on since.

1984 was the first year of the turbo, and has many unique parts for the engine management system. Since auto parts companies don’t really know much about a 32 year old car that had limited numbers, this kind of mistake can occur often. Still, it was expensive, and made my car barely drivable for almost ten months. I’m sharing this so you won’t go through a similar nightmare.

Check the sensor type, and make sure you replace it with the same type of sensor.

LINKS: Overheating | Heat issues | Code 17 | Code 22

Sensors, Switches, and Other Systems | Main Repairs Page | EEKs

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