Just chiming in that I had the same problem... I'd like to somewhat apologize for resurrecting a zombie thread but no matter how many ways I google problems I always seem to stumble back upon this one.
So its a 91 6.2l, was running like a charm and decided to clean it out real well one day. Drove it to a car wash, sprayed it all down, fired right up and got home. 30 minutes later I went back out and the controller (nartron) was making weird spaz noises (griding, whirring noises and if i smacked it, the noise would change). I figured I knocked a wire loose cleaning so I started looking, sure enough I found a glow plug wire dangling loose and the tip snapped off (in the harness). Decided to just replace all 8 glow plugs, put in a grounding harness, and she fired right up when I did so.
Stupid me then decided to tidy up the engine bay and I noticed I had the ground to the gpc (natron) upside down, it just wasn't sitting flush and I was irritated I did that so I unscrewed it and decided to clean the contacts on the nartron since I was at it. The only problem was that I forgot to unplug the batteries. Put it all back together and I got the quick flash wait light!
Now I was kicking **** in the garage and just relatively pissed at myself presuming I fried the box so I ordered a new KDS and TSU (matched pair from Erik's).
Slapped em in (after unplugging the battery) and bam, still quick flash wait light. Tried taking out the tsu, same problem. Batteries are good tested them and threw a jump pack on just to be sure. Now I'm super pissed because it may well not have been the Nartron but I guess I have a back up now. I pulled 2 plugs and the second was swollen, so now I'm off to buy an ohm-meter and start ordering some new plugs (again).
Purely speculating that I did fry the nartron and in doing so fried the plugs and now the new KDS is doing the quick flash due to the faulty plugs.
tldr; new plugs, new harness, new KDS, new tsu.... still get the one flash wait light and no voltage drop indicating that glow plugs are cycling.
I was wondering since quite a few people seemed to have been having the same issue, if there turned out to be a resolution? After testing the plugs, my next guess will be an engine harness, as I still haven't replaced that. All the grounding points are solid, ground off all the paint and ensured fresh contacts etc..
edit: hmmm just read about a fuse located behind the instruments that is potentially tied to the KDS...