Moved the truck yesterday to a convenient spot to change coolant.
Drained radiator and filled with water after work today and went to get her running...^#*!? no WTS light, no clicking. Dark already so just checked the fuses in power distribution box.
Will go study the sticky again.
Any suggestions where to look first?

 

Does the engine start, or does it just crank and crank without firing?

 

Cranked for a few seconds, didn't start of course.

Went to diagnose the problem next day. Turned the key - same thing, no light.
So pulled the air filter housing, grabbed my multimeter, got some wires and bypass switch ready...
went to the cab to turn the ignition on so I could check for power to the relay...
WTS light came on!
Spent some time wondering around with wires and connectors, put the whole thing back together and was back to flushing my cooling system.

 

Right on! Such relief when a problem is solved! Just keep in mind that any small, brittle, important wires may be fuse links... I once did a nice splice only to turn the wire over and see that it was a fuse link; off to NAPA we go lol

 

Well, I wouldn't consider this a solved problem. Hate misteries like this.

 

mine once had a problem with the yellow wires that supply power to the gp relay, bypassed that with a piece of 4ga direct from the battery. if you see any sign of melting where those wires go through the main harness connection, you'll want to repair that. also, you might want to temporarily wire up a pushbutton to manually force the glow plugs on, at least if you think it might be a controller issue. i just did that to mine last week because the controller wasn't acting reliably

 

You can always buy a new one, install and if the problem doesnt go away, return the Glow plug relay. I did this earlier this week. Got my $85 back. But my problem was non electrical related, we just assumed it was. Make sure the ground wire from the relay (the black one) is connected. If its not, it will neither click nor light as I found out this week.

 

When I put a missing CDR valve and changed glow plugs a couple of weeks ago I checked all wiring and connectors, cleaned all connections on relay and greased with dielectric grease, reinstalled the original plastic cover which was just hanging there backwards. Everything looked good except for #7 gp connector which has broken plastic at the bottom. Felt the big difference when I started the truck after that. 2 sec of cranking after wts light went off and she fired up.
I have manual gp switch on my old Bobcat from the factory and kind of like it. Turn spring loaded key counterclockwise, hold for x seconds depending on the temp outside, then release, turn clockwise for ignition on and crank. Never use gp during the day, just for the first cold start. Since I got in the habit to crank right away when engine is warm I did start warm engine in the truck not paying attention to wts light a few times already. Not sure if that will affect gp life, hope not much if any.

 

I would still get a new relay and install it just to test it out. Takes 20min to swap. Sometimes the relay looks perfect outside, but internally something might be off. My 87 6.9 came with a button, I swapped to a relay because it just knows exactly when it's time to crank. The problem with a button is the glow plugs might not be hot enough when trying to turn over, then you have to push again etc... I trust the set up IF everything is to spec. I can't imagine using a button with autolite glow plugs (not saying that you have that, but I'm trying to give the worst case as an examole). Make sure you have good glow plugs, check the wiring, gp control relay and leave it at that.

With that being said. I never removed my manual set up (just unhooked the attachments to the relay) just in case.

 

I will get the relay and wait for another failure. It happened once, I did nothing, and then it worked again. Still works...
And yes, I got a new set of ZD-9. PO had put new plugs a month before he sold the truck to me...Autolites....had to cycle 2-3 times and crank for like 10 seconds at 90* outside to get her running.

 

Yeah. Been there. Many 6.9 and 7.3 idi trucks, mostly uhauls. Both my trucks have the 87-94 style glow plug relay set up. They start right up. My 87 tales about 20 seconds of light/buzzard, but starts right up on 1st try. Might need to change glow plugs. My 94.uhaul takes about 8 seconds, starts right up. Los Angeles 6am weather. 65 degrees on average.

I believe once you install those zd9 plugs, you will see a completely different start up. Also, if you get a local parts store glow plug controller, and it turns out that way the problem, I would still return it and get a Motorcraft relay as a permanent replacement.