Originally Posted by argve

Kevin,

Here is how the system works.

When you turn on the key the controller is activated and it closes the relay that is mounted on top of the controller then it reads how much voltage is dropped across the Glow Plugs. As current flows through the system it has a fixed resistance in which to use (the big zig zag piece of metal on the controller) so it waits until the voltage on the glow plugs comes up to a set point. We don't know what this voltage set point is and really we don't need to know.

What happens is as the glow plugs heat they change resistance value - they actually increase in ohmic value so as they increase in ohms they become more of a restriction to current flow and so they will increase their voltage drop. Once the voltage drop gets high enough the controller knows that they are HOT HOT HOT and so it switches to a pulsing action so to keep them at that temp and not burn out. Once it switches to the pulsing it will turn off the WAIT TO START LAMP in the cab to let you know to start the truck. Once it has pulsed the glow plugs with power for I think it's 10 seconds or so it will shut down and wait for another key cycle before they will work.

By key cycle I mean OFF and then back ON.

In 70 degree weather the glow plugs are sitting at a higher temp from the get go than in 30 degree weather so the controller will not run the glow plugs for as long because remember as the glow plugs heat they gain resistance and since they are already warmer (it's summer time now) the controller will see it's magical set point voltage sooner because the plugs don't have to heat as long as say they do when the outside temp is at 30 degrees.

Now if you have at least one glow plug that is not working this will fool the controller in that it thinks the plugs are warmer than they actually are.

This is because the glow plugs are in parallel so we know from OHM's law that you as remove resistors in a parallel circuit that the overall resistance increases for that section of the circuit. Well because the resistance increased then so will our voltage drop across that section of the circuit, so the controller is looking at the voltage from backside of the zig zag resistor to ground and it will see that magical voltage sooner so the plugs did not get enough time to heat fully.

This is what causes short cycling as some call it. Where the controller only ran the wait to start lamp for a very short period of time and will go right into pulsating the plugs. So then you have a hard start condition.

Typically you can get away with one bad glow plug and not really see too much of difference unless you really pay close attention - with two glow plugs not working your gonna have a hard time getting her to start and you'll know there is a problem. With 3 or more bad glow plugs if you get it started in the cold you got dang lucky.