Coolant Temperature Sensor

by Bob O’Neill and Bob Lincoln

I have used your site with much success over the years. Recently I had a problem with my 1997 Ram 1500 with the 3.9l V6 and 87,000 miles. It was getting harder to start as it got colder. The symptoms were exactly like in the old carburetor days when the choke quit: long cranking, firing but dying immediately, eventually running but badly with no power, worse as it got colder; once it warmed up all was well and it ran well, like normal. A plug, wire, rotor, and cap change made no difference. I even changed the fuel pump due to some advice, no difference. Then I noticed an article about what the water temperature sensor's function is and what symptoms it has when it fails. I replaced that $12 part and it's cured! The guts of the sensor had actually come loose from the brass fitting. If anyone else has these same symptoms, especially if it runs well after warm-up and the way it starts gets better or worse as the outside temperature changes, I would check the coolant temperature sensor (2 wires) first. Thanks, Larry Hitze

The coolant sensor, not surprisingly, measures the engine coolant temperature, and the rate of the coolant temperature change. The logic module uses this information to control when the engine enters closed loop (using the oxygen sensor to adjust fuel usage) and when to turn on the radiator fan for 1985 and later models.

The coolant sensor is located in the head near the thermostat. This is where the top heater hose is attached. You’ll find it on the side of the thermostat “box.”

In the 1984 model there is a thermistor/fixed resistor combination measuring 5,290 to 5,610 ohms at 77°, a combination which provides better sensor accuracy at high temperatures. For the 1985 and later models the design of the sensor is a thermistor. This thermistor measures 9,120 to 10,880 ohms at 77°. To provide the same accuracy as the thermistor/fixed resistor combination in the 1984 model, the latter used a fixed resistor in the logic module. This is needed so the logic module can control the radiator fan by turning it on and off at high temperatures.  This configuration also can read very cold temperatures measured during a cold start. The voltage from the sensor should be 2.5v at 77°.

• If the sensor output voltage is less than 0.157v or more than 3.76v for 1984 models, the computer will trigger a code 22.
• Code 22 will be triggered if the sensor output is less than 0.51v or more than 4.96v on 1985 and later models.  

When code 22 is triggered, the power limited light will be lit and the system will enter limp mode. When this happens the system uses the air charge temperature sensor to estimate the coolant temperature for 1984 models and the radiator fan is turned on for 1985 or more models. 

If the sensor falls back into normal range for 1988 and later Turbo 1 engines, the power limit light will go out and the system will keep code 22 stored but will no longer be in limp mode. For all other models will use the air charge temperature sensor to estimate coolant temperature.

If the sensor reads a temperature of -20 to 212° when starting the engine, a twenty minute timer is started. When the timer expires, if the temperature is not at least 160°, code 17 is triggered.  An eight minute timer is started for 1987 models 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, code 17 is triggered.

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

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

Disclaimer: The information provided in this and linked pages are intended for informational use and are provided as accurate to the best of our knowledge.  Neither the author nor Allpar.com is to be held responsible for their contents or the use of the information by you the reader. THE INFORMATION IS OFFERED “AS IS” FOR INFORMATIONAL PURPOSES AND WITHOUT WARRANTIES OF ANY KIND EITHER EXPRESSED OR IMPLIED. IF YOU USE THIS INFORMATION YOU RELEASE THE AUTHOR AND ALLPAR.COM FROM ANY LIABILITY WHATSOEVER.