the glow plugs on these trucks are run in a series, and the glow plug controller has a circuit breaker made into it. when one or more glow plugs go bad then there's more voltage going to the other glow plugs. too much voltage will cause the circuit breaker in the glow plug controller to "pop" or what your hearing the "click,click,click,click", if it was me then id check all your glow plugs to see if there are any bad ones. also if the glow plugs aren't getting a good connection then it will do the same thing.
Actually the glow plugs are in Parallel meaning that if one goes out the others will still function. In a series circuit if one burns out then none of them see voltage or current.
You are correct in that one glow plug burned out will shorten your burn time this is because of this.....
The glow plug controller monitors the voltage that is dropped across the zig zag resistor to make a determination on how long to keep the glow plugs on. How this happens is as the glow plugs heat (either by way of ambient temp or current flow through them) they gain resistance causing more voltage be be dropped across them - when this voltage reaches a set point the glow plug controller knows that they are at optimal temp for starting. At this point the glow plug controller turns off the wait to start lamp in the cabin.
Now keep in mind that when you have 8 glow plugs in PARALLEL that the total resistance for the circuit is extremely low, but 7 glow plugs in PARALLEL will have a higher resistance and will then because of OHMS LAW drop more voltage across it causing the glow plug controller to think the system is warmer than it actually is.... So what does that mean to us? Well quite simply put it will cause shorter ON times for the glow plugs which means that they (glow plugs) won't heat as hot as they should causing a hard or no start condition. 6 good plugs and two bad ones will almost render the system non functional for days colder than say 50 degrees F. She will if you are lucky fire the glow plugs for 1 or 2 seconds at best before going into After Glow.
Now what is After Glow you ask.... Well that is the portion of the programming that the glow plug controller says to it's self "Ok the plugs are hot and ready to burn up, so I better turn them off but I need to keep them up to temp so that truck can start and help with keeping it running until the engine can generate enough excess heat to ignite the next combustion cycle".... How it keeps the glow plugs up to proper temp is by turning them on for a half second then turning them off for a half second - it will do this for around (I'm going from memory here) maybe 10 seconds at best, so this is when you hear the classic clicking and seeing the voltage gauge dance.
The reason you see the voltage gauge dance is because the current draw is far above what the battery can really support... I can go into the math but it's really not needed - I can if any one wants me to... but a cold glow plug will have an ohmic value of around 1-2 ohms and a hot plug will ring in around 20-30 ohms... so you can dig up on google the formulas for figuring 8 glow plugs in PARALLEL and then apply ohms law using 12 volts to find the current draw.
In my experience you need to check the glow plugs them selves to see if they are good and then if they check out - fall back to the connections on the Glow plug controller and Relay strapped to it's back. Remember I was talking about voltage drops and bad connections can cause extra voltage drops which can fool the glow plug controller into thinking the plugs are hotter than they really are, so you need to make sure that you connections are CLEAN and TIGHT concerning the glow plug SYSTEM.... Then includes the ground for the controller. All measurements are based off the ground being good.
sorry man
