Hey guys I’m new to this forum and new to the idi’s! I’ve been doing a lot of reading and talking to some people about everything with these motors. I picked myself up a nice flatbed dump truck with a 7.3 idi. It is a 1994 f450.

my current issue is that I think my glow plugs are not cycling. I replaced all of them with zd9’s and also replaced all the plug connectors because they were completely shot. With the key off, my relay is reading 12 volts. With the key on, I’m pretty sure the controller is reading 12 volts, sometimes hard to find a good ground but I do get steady readings at 12 volts. The new connectors I put on for the glow plugs read 11.4 volts with the key turned on. The battery voltage on the dashboard does not move at all when I turn the key it just stays at whatever voltage it usually is for starting. It used to go down to like 8 volts when I turned the key. My wait to start like comes on and I can hear the relay click once when I turn the key and then click again about five seconds after the wait to start light goes out.

so what exactly should the glow plug connectors be reading voltage wise when I turn the key? I thought they should read something like 7 volts, but they’re reading 11.4 volts. If that is wrong, then what could the problem be? Controller? Relay? I’m not too sure so hope you guys can help!

 

https://www.dieselhub.com/maintenanc...eshooting.html

Maybe this will help

 

welcome to FTE.
how does the truck start cold?
if it fires rite up, don't worry about the voltage at the glow plugs.
wait to start light should stay on for about 15 seconds on a cold engine.

oh and by the way, you have a 94 F-SuperDuty, NOT a F-450.
first year for the F-450 was 1999

 

I assume you are seeing this with the glow plugs CONNECTED. In other words you are not seeing the voltage drop at your volt meter gauge in the gauge panel in the truck when key is in the 'ON' position, when WTS light is supposed to be on.


Unless there is a faulty connection or defect in your wiring to the controller, or from the controller to the glow plug connectors portion of the harness, I think the controller is not working properly, or the controller relay is defective. Most likely I think it's the wiring because you've recently worked with that, so lets examine that first.

My primary suspicion is that some of the new glow plug connector connections you made in the glow plug harness are defective or the connection of the whole completed glow plug harness to the controller is defective . Lets examine this in more detail:

How it works:
When the controller relay is engaged in the 'RUN' position of the ignition key, (during the 'wait to start' light ON sequence) the battery voltage crosses over to the secondary side of the controller relay and THAT is the large relay post were the flat bar bent in a zig-zag fashion is attached. That bar IS A RESISTER. It heats up very quickly and as it heats the resistance in the circuit to the glow plugs GOES UP very quickly and this resistance drops the voltage to the glow plugs to a nominal 6 volts which is the (nominal) specified operating voltage of the ZD9 glow plugs. Instantaneously they'll receive the full battery voltage but that voltage will drop very quickly as that resister bar heats up. This voltage drop (down to 8 volts or so) is what you used to observe on your volt meter inside the truck which is not occurring now so that resister is not heating up. There is not enough current flowing through it to heat up the resistor or the glow plugs.
The BOTTOM end of the bar IS (supposed to be) GROUNDED through the controller mounting bolt on that side. There also is where your glow plug harness (both sides) attaches and there is also a smaller wire with a ring terminal that attaches there and goes back to the controller. Through that smaller wire the controller senses the resistance feedback from all 8 glow plugs collectively and a computer chip in the controller then determines the temperature state of the glow plugs based on that resistance feedback. In this way (and with a timing circuit) the controller determines when to disconnect/reconnect voltage to the glow plugs to maintain proper cylinder temperatures for start up. When the WTS light goes out, the upper cylinder temperatures are right for a quick start-up. The afterglow cycle or cycling of the controller/relay after engine start-up is to REDUCE smoke and other emissions during initial warming of the engine. (I love that feature - my truck doesn't smoke at all on start up).

So where/what is the problem?
1) I would closely examine the connections at the bottom end of that bar to make sure they are clean and making good contact. That would be at the controller mounting bolt. Did you install a new eye or 'ring' terminal on the glow plug harness at that location? CRIMPED ON CONNECTORS ARE ALWAYS SUSPECT. (in my opinion) For the 10 gauge wires in the entire glow plug circuit I always solder the ring terminals on, then shrink wrap, because those connections can at times handle quite a lot of current and crimped on connectors aren't as reliable. (I did use high quality crimp-on connectors for these 10 ga. wires to bypass the engine harness connector which had over heated and melted at the glow plug wiring side of the plug.)

2) What type of 'new' GP connectors did you put on the harness? In fact what exactly were they and where did you get them? If they were connectors with pig-tails (already connected to a length of wire) how did you connect those into the old harness. Any of those connections could be suspect and causing issues especially if there were more than 2 or 3 inadequate connections.

It could be a failed controller or a failed controller relay.

What was the state of the glow plug circuit and function BEFORE you did this work.

I would examine every connection that you manipulated during this glow plug circuit repair and look for something that's not right, something that you forgot to reconnect, etc. BTW, as I recall, my 1987 has a small black ground wire in the passenger side glow plug harness that comes out of the loom on the passenger side near the controller.
That's as much as I am able to concentrate on right now.
I hope it turns out to be something SIMPLE to find

 

Mr. Fixinit, thanks for that little blurb, answered quite alot of questions for me on my 89 F250 IDI current GPR maintenance.