Is it -40F or 250F? The ECM can't tell!

This week we took our coolant temperature sensor and heated it in boiling water with our ohmmeter on the electrical part of the sensor. As the sensor's temperature increased, the resistance values went down. From around 3k (3000) ohms resistance at ambient temperature, it fell to just 300 ohms at close to 212F. The sensor is working as intended.

Next we backprobed the electrical connector which runs from the ECM to the sensor and hooked up a variable resistor. At this point, the truck has no coolant in it, the engine is stone cold and not running. We turn the key on and hook up a scan tool to the truck, going to the data stream where we can look at what the coolant temperature sensor is doing.

With resistance set low, the temperature gauge on the dash of the truck is redlined at 250F, matching what the scan tool data stream reads. The truck's ECM thinks the truck is overheating, though in reality the engine is cold. As resistance is increased, the temperature goes down on the dash gauge and on the scan tool data stream. We can make the truck think it's -40F when in reality it's the middle of summer.

Variable resistor backprobing the ECT sensor's connector.

Variable resistor. With this we can lie to the ECM and make it believe the engine is overheating when in reality it's not even running.

The engine is not running, but the ECM believes it's overheating.

Scan tool data showing what the ECM sees. Hmm interesting to note EGR temperature is -40F?

My reflection:

A modern vehicle uses a wide variety of inputs from sensors all around the vehicle, including engine sensors, transmission sensors, wheel speed sensors for ABS and traction control, the list goes on. It uses these inputs to determine the current operating conditions and tweak as necessary for optimum performance. If any of these sensors is faulty or otherwise giving incorrect information back to the ECM, the ECM will act accordingly.

For instance, if the ECT is faulty and giving incorrect information, similarly to how we lied to it with our variable resistor, the truck may be going down the road and think it is overheating when in reality it is just fine. Most modern trucks today would respond to this by derating the engine and forcing the driver to pull over and shut down to prevent damage. All because a sensor or its wiring was faulty.

The primary lesson to be learned here? Modern technicians will have to understand how these sensors work, as well as the electronic and electrical systems they are incorporated into. Without that knowledge, we cannot properly diagnose a fault with these systems.