Hello I'm Greg and I have been reading this forum and gathering spot on tech info today.
I've tested everything the forum suggested.. fuel heater, lift pump, glow plugs, glow plug relay, fuseable links, fuses(panel) and wiring harness. They all worked when powered up.
Initially I suspected the Glow Plugs, so I checked the relay and no voltage came out of it and later found the yellow wire dead too.
After reading I followed testing procedure and eventually found the Glow Plug fuse in the under hood panel to have no voltage, and the ECM1 fuse blown. It's obvious this has been worked on because the harness had been opened at the large T and all out of order on the bracket. That may have something to do with the ECM1 being blown, not sure.
The wait light doesn't come on and sometimes the service throttle light comes on. Even when I jumper the Glow Plug fuse and the relay works the plugs, as well as the fuel heater, the wait light is still off. Haven't checked the dark blue wire when powered up yet, just saw that now.
I had already bought the glow plugs and some were pretty weak so I replaced those. The T valve wasn't mounted but did have full flow. Thinking ignition switch, but I may have missed something?
The first one like this I've worked on, someone with experience might know.
3500 GMC Please and Thanks

 

I actuallygot the ignition switch idea from this forum and that was the last component to test before the wiring had to be disconnected for inspection. I tried to include every bit of information thinking one of you experienced would see what I hadn't checked.
Was in a hurry then, can't be now.. Paying work has kept it out of the shop.
It is wiring, I got all the way under the dash and it is bad. Added equipment and accessories have literally been tapped into the nearest wire. ALL RED. Taped together and to anything that kept it from showing. reconeccted together too.
I need a wiring diagram for sure. I'll rewire the equipment separately like my 67 dart is. complete chrysler harness and a second race harness. do know of any resources for 1995 GMC 3500 diesel wiring please?
Here's the truck, it has welder hookups and dual trailer harness..some acceries are not there anymore as well. Thank You for helping, only help I got anywhere. I'm a Transmission Builder with a shop, about 32yrs now. Dad was too.
Done Diesel Mods from scratch and products, if you need advice... I'll check the forum and finish up my profile soon.

 

all done. glow plug fuse is not supposed to have power unless the the relay is on.

 

FWIW...