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 Fault Code 22 - Coolant Temperature Sensor Voltage too High or Too Low

OBD1 DTC's for 1994-1997 Rams with Gas Engines        
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Code 22 indicates that the engine coolant temperature (ECT) sensor circuit has sent greater than 4.96 volts or less than 0.51 volts to the computer. These voltages reflect temperatures that are impossible under normal operating conditions. The high voltage represents a temperature of less than -40° F. A false code can be generated in extremely cold weather conditions. The resistance should be between 400 and 300,000 ohms." For a code 22 to be generated, the sensor would almost have to be either open or shorted.

Inside the PCM are two dropping resistors used to feed the ECT sensor. A 10k ohm resistor is used until the coolant temperature exceeds about 120°. Above 120°, the PCM switches to a 909 ohm resistor. When the engine is first started cold, someone monitoring the ECT voltage would see it drop steadily (from about 4.5 volts to about 1.5 volts as the temperature increases) until the coolant reaches 120 °. Then the voltage would would jump to about 4 volts and begin dropping again as temperature continues to increase.

Testing the sensor:

Begin testing the circuit by turning the ignition switch off and disconnecting the coolant temperature sensor wiring harness. Connect a digital voltmeter across the terminals of the harness side of the connection. Turn the ignition switch back on. The voltmeter should read 5 volts.

Wiring harness testing:

With the ignition switch still on, connect the voltmeter between the tan and black wire on the coolant sensor and the battery negative terminal. The voltmeter should read 5 volts. If it does not, disconnect the battery, remove the PCM connector, and check for continuity in the wiring harness between the IAT connector and pin 16 on the PCM connector.

If the 5-volt reference wire tests good, turn the ignition switch off. Connect the digital voltmeter between the positive terminal of the battery and light blue wire on the sensor connector. The voltmeter should read approximately 12 volts. If it does not - disconnect the battery, remove the PCM connector, and check for continuity in the wiring harness between the IAT connector and pin 4 on the PCM connector.

If no wiring problems are found, check the PCM connectors for corrosion, and reconnect. Reconnect the battery and test the circuit again. If the problem still exists, the PCM is probably bad.

Temperature Resistance (Ohms)
°C °F Min Max
-40 -40 291.5k 381.7k
-20 -4 85.8k 108.4k
-10 14 49.3k 61.4k
0 32 29.3k 36.0k
10 50 18.0k 21.8k
20 68 11.3k 13.6k
25 77 9120 10.9k
30 86 7370 8570
40 104 4900 5750
50 122 3330 3880
60 140 2310 2670
70 158 1630 1870
80 176 1170 1340
90 194 860 970
100 212 640 720
110 230 480 540
120 248 370 410

This page would not have been possible without the help of Hank LaViers,
who graciously loaned me his extensive Mopar engine library

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This page was edited on: May 3, 2004