The ECM that is being fooled is doing its job correctly. There is a big difference between an ECM that is bad and an ECM that is simply being fooled. When ECMs do crazy things, like read 1,600 degrees at idle, don’t assume the ECM is bad. It didn’t actually know that the temperature was actually 1,600 degrees – it was only going by what voltage it received from what it thinks was its temperature sensor.
I once had an ECM tell me it was reading a turbo compressor outlet temperature of 1,600 degrees – while at idle! Remember, the ECM is just a machine. The ECM is simply reading a voltage and making an assumption. Most ECMs have no way to verify what the truck is actually doing. Your brain does this as well, and will quickly let you know if you’re touching something that’s outside of your comfort range – like a hot exhaust stack.Ĭompared to your nervous system, an ECM is very dumb. The ECM will tell you where to look by giving you a code that identifies a specific circuit and condition. If the voltage drops too low or goes too high, the ECM will let you know by turning on a warning light. The ECM powers the sensor through a sensor supply and sometimes a sensor return circuit, and then monitors the sensor signal wire by watching the voltage range the sensor sends back. Each sensor typically makes a single signal circuit that feeds into a pin on the ECM. Sensors are designed to produce a specific range of either electrical resistance or voltage that changes with temperature, pressure or in the presence of an electromagnetic field. If sensors are the ECM’s nerves, then solenoids (including injector solenoids) can be thought of as the “muscles” that the ECM controls. The ECM is the brain and can “feel” problems through its wires and nerve-like sensors. Think of your truck’s ECM and its electrical system being similar in function to your body’s electrochemical nervous system. Today’s technicians need to truly understand the electronics and the ECM. Just being good with a wrench doesn’t cut it anymore. And each year, these electronics become more and more integrated in the truck. These days, just about everything on the engine is monitored and controlled by the engine computer, also commonly referred to as the ECU (Electronic Control Unit) or ECM (Electronic Control Module). It’s been 19 years since the first computer-controlled electronic diesel engine for semi trucks has been released.
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