Our '93 6.5 TD ... the glow plug light comes on one day and it fires right up, then another day we go out and the light won't come on no matter what and we have to short out the hot terminal and the glow plug terminal to start it. Also the coolant sensor in the crossover fried about 6 months ago, and our local parts counter couldn't figure out what sensor it is so we haven't replaced it yet. This problem with the glow plugs just started though so I don't think that sensor is the problem (maybe it is). We tried on a day it wouldn't start to jumper the wires for that switch but it didn't help.

http://www.dieselplace.com/forum/showthread.php?t=157085&highlight=wts+grounds

We have a few 6.5's and grds. are usually the problem. This one I have checked all grounds and even ohm'd them all to make sure it is a good connection.
IT is intermittent somedays it works somedays it doesn't all grounds have been checked and double checked. When we bought this truck the dealer had put a new controller in it supposedly for this reason. That was about two years ago.
Before posting, I tried searching and searching the posts for someone with an intermittent problem but didn't see any.

My experience in intermittent problems is with a bad conection somewhere in the wireing. And yes it is hard to track down. I had a bad wire so that when I went to start nothing would happen. Finally found the bad conection under the dash that was not letting the starter know that it was supposed to go.

Check the fuseible link. Some time they burn and the insulation holds the link together and then relaxes and brakes the circiut.

I feel your ign switch has or is going bad. Especially since it happens then it dont. Heck your truck is a 93(best year), has it ever been replaced!

Have you tested the real GP? instead of all sensors, etc.

GP has a lifetime of about 100K or slightly more than that.

I have just replaced mine, it is starting to swell especially if it is the original.

Get the Bosch Duraterm or AC 60G.

The only temp sensor is in the controller itself on a 93. Maybe this will help.

http://www.dieselplace.com/forum/showpost.php?p=2271406&postcount=9

The only temp sensor is in the controller itself on a 93. Maybe this will help.

http://www.dieselplace.com/forum/showpost.php?p=2271406&postcount=9

Thanks, that is what I was looking for. I have a schematic for '89 which is the 6.2 but it doesn't give good detail on that
I can go out tomorrow and check voltages to the controller and hopefully find the problem. I was trying to find the schematic so I could see if it was calling for it to turn on and maybe the contacts in the controller are bad.

Just to go over it again, it cranks every time, just the glow plugs don't come on every time. The glow plugs have approx. 3,000 miles on them along with the glow plug controller, but I have a feeling the controller may be taking a $#!^ again... Hopefully will trace it down tomorrow with the schematic that Midniteplowboyy has found for us

Just replaced all my glow plugs and controller. Went to fire my truck up and no "wait to start" light. That means no glow plugs. So I went on here and people say look at the grounds. Well all my grounds were good, so I put my old controller on, sure enought "wait to start". So my brand new controller is crap.

Just to go over it again, it cranks every time, just the glow plugs don't come on every time. The glow plugs have approx. 3,000 miles on them along with the glow plug controller, but I have a feeling the controller may be taking a $#!^ again... Hopefully will trace it down tomorrow with the schematic that Midniteplowboyy has found for us

Which GP did you put in there? there was a certain brand GP that gives up easy. Hint: it is not Bosch or AC 60G.

Just replaced all my glow plugs and controller. Went to fire my truck up and no "wait to start" light. That means no glow plugs. So I went on here and people say look at the grounds. Well all my grounds were good, so I put my old controller on, sure enought "wait to start". So my brand new controller is crap.

If you take out the negative of the battery during changing at any time, the WTS won't lit until 2nd crank. Or may be the controller is bad new controller. New does not mean it will work all the time, should be but not necessarily. There is still outliers outside the 6 sigma.

I put the new controller on and the truck had trouble starting. So after sitting over night I went out and tried it and the truck wouldn't even start, just a bunch of smoke. Thats when I put the old controller on. It cycled the glow plugs and fired right up. The new relay would click but wasn't putting power to the glow plugs.

Which GP did you put in there? there was a certain brand GP that gives up easy. Hint: it is not Bosch or AC 60G.

I think they are stock GM's... It was done just before we bought the truck. They put in a new GM engine at the dealer and at the same time new glow plugs, GP controller, and a bunch of other stuff. The glow plugs all work perfect. When the wait to start light doesn't come on we go out and short out the terminals on the GP controller and it instantly fires up no missing runs great.
Unfortunately it is raining out and I can't get it into the garage at the moment because we are pulling the engine on a backhoe in the garage to rebuild. So tomorrow after the rain stops we will check the controller or put a switch in to make it have a manual override.

Lucky for us... When they replaced the engine in this truck, the stupid thing they couldn't figure out... was the clogged turbo return that wouldn't allow the oil to return to the pan forcing it thru the turbo. We bought it and drove it from the lot about 5 miles, smoking so bad we couldn't see behind it. Pulled over and checked the oil and it was at the bottom of the stick in 5 miles. It was kind of fun driving it home, we used about 8 quarts to get it home. This was over a year ago...

Hey Read Your Threads And For What Its Worth I Scratched My Head For 2 Months With The Same Thing On My 96 Trace And Replaced Nothing Then I Notice It Only Done It On Cold Days I Pulled The Pcm From My Other Truck And It Ran Fine Put The Pcm From Truck 1 Into Truck 2 And Again No Glowplug Or Start Would Crank But Not Fire Went To Salvage Yard Bought Used Pcm And No More Problems Hope This Helps