ECM injector pulse width can be as long as a 4 millisecond pulse. To you and I it looks like a sparkplug discharging. A crack and a flash of light but 4 milliseconds is a long time to the ECM. It’s tempting to think of injector pulse width as bolts of electricity that travel from the ECM through the injector wiring harness to the injectors like a projectile. In reality that pulse is a connection between the ECM and the injector. While the ECM is connecting it is also measuring the strength of the injector solenoid when it sends current and makes that connection. I have confirmed in my ECM lab that a DDEC IV can detect the injector solenoid armature getting mechanically jammed in place within the injector body and as soon as the needle valve is mechanically jammed an injector response code is set.  The armature movement in injector solenoids is in the thousandths of an inch. You couldn’t fit a razor blade between the injector armature that moves up and down and the electromagnet that moves it.

 Inside the body of the injector is where electronic control ends and mechanical control begins. When the injector electromagnet is energized a magnetic field lifts the armature’s needle valve off its seat and fuel flow though the injector changes paths. This valve is normally held in its seat by a spring and the electromagnet needs to be strong enough to overcome the strength of that spring. If the electromagnet is able to pull the armature towards itself current rises. The ECM looks for this slight increase in current to verify that all is well with the injector and that the armature and needle valve inside the injector just moved. Now if current is way too high the DDEC IV will show the diagnostic code “injector response time shorted”. This means what you’d think it means. There is a short and the ECM’s pulse found a shortcut to ground before it ever made it to the injector’s electromagnet. This almost always means a pinched or rubbed though wire in the injector harness or the engine harness. A little engine oil in a connector isn’t going to be enough to throw this code. What you’re looking for is copper touching metal somewhere. As with many other shorts you will probably see a little heat wherever this is happening.

When you see injector response time long this means current draw is too low. If the injector isn’t connected to the ECM or the injector’s electromagnet is blown you will see this code. You will also see this code if the armature inside the injector is jammed in the downward position and the electromagnet fails to lift the armature. This will prevent fuel flow to that injector and cause the engine to miss. The cause for the armature jam could be heat, dirty fuel or dirty oil.

I did one more test on the ECM dyno and I have verified that low voltage can also be a contributing factor. The lower the ECM supply voltage gets the more likely it is that the injector’s electromagnet will fail to move the armature. On the flip side when ECM supply voltage is increased the ECM has more voltage to work with and the injector electromagnets can sometimes free a stuck armature. I have verified this on the ECM dyno but I do not recommend going above 15 volts on a live truck. Otherwise other components could be damaged by the high voltage condition.

I wanted to see how low it could go. I found the ECM actually stayed running down to 6 volts but what stopped working were the injector electromagnets. At about 8 volts the ECM lab was quiet and at 6 volts the DDEC IV gave up and shut down.  Think about that this winter when you’re cranking over a cold truck that is too stubborn to start. When the starter engages and voltage drops your ECM might not have enough voltage to lift those armatures and give your engine fuel. 

-Fernando DeMoura

Diesel Control Service LLC.