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.

Engine Coolant Temperature Sensor

This week we pulled the engine coolant temperature sensor off the Series 60. It is located on the right side (passenger) toward the rear of the head and block. It is an NTC-type thermistor which measures engine coolant temperature by changing resistance (temperature goes up, resistance goes down) and sends a voltage signal back to the ECM, which uses the information, along with that from other sensors, to adjust to a wide variety of engine operating conditions.

The sensor can be tested by comparing ambient temperature to resistance. One could also heat up a small container of water, stick the sensor in and measure resistance as long as the temperature of the water is known.

Coolant temp sensor and specifications for temperature and resistance.

Coolant temp sensor.

My reflection:

Removal and testing of this sensor shouldn't be any big deal on a typical engine, what might present a problem is finding specifications for it. It took longer to find specifications (appreciation to our instructor for finding the correct ones) than it did to pull the sensor and make measurements.

The moral here? Sometimes the hardest part of the job isn't turning a wrench, it's finding information! Without information, we would have little idea what correct resistance values are at what temperatures, and be unable to reliably diagnose this sensor.

Organization & Cleanliness

They say that "cleanliness is next to godliness." This week our Electronic Diesel Engine Controls class was cancelled to due illness, so I will use this week's blog post to focus on a general issue which may be a problem for some automotive and diesel shops.

If you were to walk into our shop right now, you might find things like this:

I don't think this is where we put the grease gun after use.

This is just sloppy and unprofessional.

My reflection:


Without an organized shop, tools will be harder to find, may end up lost, stolen, or even present a tripping or other hazard if left on the floor. Leaving tools where it is most convenient instead of where they belong after use is lazy and unprofessional.

Cleaning up messes is part of the job. Oil, coolant, and other fluid spills are not only an inconvenient mess, they are a safety hazard. Clean them up ASAP! This means more than throwing floor dry on it and driving off.

Finally, does leaving trash laying around everywhere really need an explanation?

It reflects poorly on the professionalism of a shop and the character of its technicians when they cannot be organized, clean up messes, or even clean up their own trash. It would be the first and last time I would ever let a shop work on my vehicle where the technicians used it as a coffee table.

Thermostat Housing On The Series 60

This week my team was assigned to work on the FLD Freightliner which has a Series 60 engine in it. Our first task was the testing of various sensors, starting with the engine coolant temperature sensor. This meant draining the coolant and pulling the thermostat housing off the engine.

The coolant is drained either at the driver's side radiator drain valve on the bottom side of the radiator, or a drain plug on the bottom radiator pipe. We drained it at the latter, and my suggestion if draining it here is to wear safety glasses and be ready to get out from under the truck quickly as it will pour out fairly violently. Coolant doesn't taste very good, for the record.

Next we began pulling hoses off the thermostat housing. There is one hose which goes to the upper radiator, another which goes in the side of the thermostat housing from the water pump, and another small hose with a 14mm fitting that runs out from the top to a fitting on top of the radiator.

A lot of hoses and pipes!

Drain pans, not a very good picture. That tire step makes climbing up on the truck easy!

Thermostat housing with the upper radiator hose removed. Note heat shrink clamps, this is the first time I encountered one.

My reflection:

I have worked on this engine before but one thing that struck me was the replacement of the old, often-seen worm and spring clamps with a new heat shrink type. I had never encountered this type of clamp before. To remove them you have to heat them up with a heat gun and cut them off with a knife.

These clamps, although not reuseable, are supposed have a better clamping force than traditional clamps. Will we perhaps see more and more of these on newer trucks and use them as replacements whenever we work on the hoses of older trucks using traditional clamps? Time will tell, but I definitely learned something new this week!