components of glow plug system

[CharlieC]

Howdy, I'm curious if someone can help me continue to track down my glow plug problem. Since I've owned this truck (4 yrs now), it fries 4-6 plugs / year. I have replaced the controller twice, the valve cover wiring harnesses, and now the external harness plugs (what plugs into the wiring harness).

Not sure if it's related, but I also have an intermittent low voltage issue. I suspect that the plugs may be getting stuck on, thus lowering available voltage and burning the plugs out.

Assuming the controller is working properly, what tells the plugs to cycle on / stay on? is there something that might be causing the system to stick on and burn the plugs out?

FWIW, I use motorcraft plugs, so no autolite issues here.

[madpogue]

There is no "controller", except for the PCM. It cycles the glow plugs for anywhere from about 30 seconds to two minutes. There are a couple of ways that the relay could malfunction and run the glow plugs continuously.

Maybe it's the relay you've replaced so many times? Are you sure you've gotten a glow plug relay, one that is NOT grounded through the mounting base like a starter relay is? What brand of relay is it?

Is it the same glow plugs (in the same positions) that fry every time?

One way to test to see if your low voltage issue is glow-plug-related -- next time it happens, disconnect the big terminal on the relay that leads to the plugs. Be VERY careful, as this terminal may be energized at the time; DON'T let your wrench touch any metal part of the truck. If the voltage returns to normal when you disconnect it, then the plugs/relay/wiring are suspect.

[CharlieC]

The controller is a glow plug controller, not a starter relay. Thanks for the info on the PCM.

I hooked up an in-cab led to the glow plug controller so I could see when it's on. Doesn't appear to be sticking on.

I'm off on a 1200 mile trip tomorrow, so that will give me plenty of opportunity to see.

I am seeing some fairly significant voltage drop at the fuse box in the cab. For example, alternator puts out 14.5, I get about 14 at the batteries, then only about 12.5 at the fuse panel in the cab. Drops to 12 when I turn the headlights on. Is that normal or am I experiencing too much voltage loss?

Thanks!

[Turbostang500]

Not normal to me. Battery voltage should be the same through out the vehicle [bold] IF[/bold] it is suppose to be bat voltage. I think the fuse box should be bat voltage. Hopefully another member can agree or disagree with that.

This is an OBS right? If so, it has a glow plug relay, not a controller.

[CharlieC]

Yeah, sorry, glow plug relay (not controller). It is an OBS. Do you know where that fuse panel in the cab is grounded? One guess is I have a loose / bad ground coming off that panel.

[Turbostang500]

I am not well versed on OBS trucks.

Are you familar with checking voltage drop?