Code 15 - Coolant Sensor Signal Volt Hi (A/F/J/N/P Body)
The Coolant Temperature Sensor uses a thermistor to control signal voltage to the ECM. The ECM applies voltage on circuit No. 410 to the sensor. When the engine is cold the sensor (thermistor) resistance is high. Therefore the ECM will see a high signal voltage. As the engine warms, the sensor resistance becomes less and voltage drops. At normal engine operating temperature, voltage will be about 1-1.5 volts at terminal 4 of ECM.
Code 15 will set if signal voltage indicates a coolant temperature less than -24°F (-35°C) for more than 4 seconds or if time since engine start is more than one (1) minute. Coolant temperature is one (1) of the inputs used to control fuel delivery, engine timing (EST), idle (IAC), and converter clutch (TCC).
If coolant circuit No. 410 opens with ignition off, the ECM will see -40°F (-40°C) and deliver fuel for this temperature. If actual temperature is above 20°F (-7°C), the engine will not start due to rich mixture unless "Clear Flood" is used by fully depressing accelerator. Engine will start using "Clear Flood". However, "SERVICE ENGINE SOON" light will not come on and code will not be stored, until engine has run for one (1) minute.
- If voltage is above 4 volts, the ECM and wiring are okay. If location of sensor makes it hard to check, disconnect Black ECM connector and check resistance between connector terminals 4 and 11.