Understanding Your ECM

It’s been 19 years since the first computer-controlled electronic diesel engine for semi trucks has been released.  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).  And each year, these electronics become more and more integrated in the truck.  Just being good with a wrench doesn’t cut it anymore.  Today’s technicians need to truly understand the electronics and the ECM.

Think of your truck’s ECM and its electrical system being similar in function to your body’s electrochemical nervous system.  The ECM is the brain and can “feel” problems through its wires and nerve-like sensors.  If sensors are the ECM’s nerves, then solenoids (including injector solenoids) can be thought of as the “muscles” that the ECM controls.  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.  Each sensor typically makes a single signal circuit that feeds into a pin on the ECM.  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.  If the voltage drops too low or goes too high, the ECM will let you know by turning on a warning light.  The ECM will tell you where to look by giving you a code that identifies a specific circuit and condition.  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.

Compared to your nervous system, an ECM is very dumb.  Most ECMs have no way to verify what the truck is actually doing.  The ECM is simply reading a voltage and making an assumption.  Because of this, ECMs are easily fooled.  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.  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.  When ECMs do crazy things, like read 1,600 degrees at idle, don’t assume the ECM is bad.  There is a big difference between an ECM that is bad and an ECM that is simply being fooled.  The ECM that is being fooled is doing its job correctly.  What is not working correctly, in this case, is the electrical circuit or sensor the ECM is monitoring.

Among technicians in the trucking industry, the most widely misunderstood part of the truck is the ECM.  As a result, the ECM has become a scapegoat for almost any problem the technician fails to identify.  A mechanical problem that a technician fails to recognize (or recognizes but fails to fix properly) is often blamed on the ECM.  I have had people blame the ECM for everything from bad valve timing to CB radio interference.  Here is a hint: if there isn’t an electrical solenoid or sensor involved with the system having the problem, then the ECM has no control over that system and you’re barking up the wrong tree!

From time to time, ECMs do fail.  ECMs have a limited lifespan.  All ECMs have some sort of flash memory and, over the years, this memory will deteriorate and ECM-related circuits on the truck will fail.  Also, some ECMs rely on a battery that powers the memory chip, and when the battery wears out, your ECM will soon fail.  Sometimes you will get a “check engine light” and sometimes you won’t.  However, the ECM will almost never leave you stranded.  Once a problem begins to develop, you will have a warning, and then several days of driving time left to get to a shop.  These batteries usually last about 10 to 12 years, but we recently had a 2005 Cat Acert in the shop and the ECM had failed.  So, occasionally, they will fail before their time.  This ECM was only six years old and needed to be replaced.  Reprogramming an ECM that is about to fail or has other hardware problems is not the answer – it will not fix the issues.

In the trucking industry, technicians tend to take shortcuts when working on electrical problems.  Instead of tracking down the source of the problem, many mechanics just start making assumptions and changing parts.  It doesn’t matter whether the technician doesn’t understand how to diagnose an electrical problem or is just too lazy to do it – you are the one who’s buying the parts, so their assumptions end up costing you.  These guys will typically tell you that the ECM, a sensor, a wiring harness or even an injector is faulty without bothering to test the part and verify that it has actually failed.  These parts are expensive, and the technician telling you to replace these parts isn’t buying them – you are – so beware of any technician that doesn’t have a well-used multi meter sitting in his toolbox.

Electrical problems are not always easy to find – especially if they are intermittent.  Some problems only show symptoms under a load, at a specific rpm or when it’s cold outside.  Most intermittent electrical problems come and go with no observable reason at all.  Thankfully, if your ECM does produce a “check engine light” it will log which circuit produced the problem, then you can find it.  If a wire is severed or a sensor fails, the ECM only knows it’s getting zero volts back from the sensor.  You may just have a dirty, loose, corroded, wet or filled with oil connector.  To find out, you have to ohm out the circuit to find the break with a good old-fashioned multi meter.  Diagnostic software won’t do this for you – a multi meter is the only way to be sure.

The relative simplicity of our Pittsburgh Power Computer allows us to diagnose some problems over the phone, but that is where it stops.  These problems are very difficult to find unless you have good diagnostic software and a multi meter.  We get many phone calls about electrical problems but we will not attempt to diagnose an electrical problem over the phone.  Phone calls like this only make both of us frustrated.  You must take your truck to a good shop that is competent with electronics.  If you have any comments or questions, I can be reached at Pittsburgh Power in Saxonburg, PA at (724) 360-4080 or by sending an e-mail to bruce@pittsburghpower.com.