M1008 Glow Plug Intermittent Issue Any Ideas?

Hi all- I have a 1986 M1008 with 89,966 miles on it. Never had any wiring/starting issues with it until now. Original 24V wiring has not been messed with. In October 2013 with exactly 80,000 miles on it I totally restored the truck- all new sheet metal, SM465/NP205 conversion, injection & water pumps, put a 1973 nose on it- spared no expense, did everything. Put almost 10,000 miles on it since then, has never seen the rain. Now I am experiencing a glow plug issue. Noticed that glow plugs would not activate every once in awhile when the key was turned on. Wait light did not illuminate and truck would not start. By cycling key on/off several times, Wait light would eventually illuminate and truck would start. It got worse, (more key cycling required) and happened more frequently, so I decided to do something about it. Put new card in it along with new ST-85 relay- no difference. Discovered bad temp sensor behind driver head. (New sensor had continuity at dead cold and old sensor was off the charts). Reinstalled original glow plug card, but left new relay. Installed new sensor and Wait light illuminated on the first try and truck fired right up. Was really happy. Waited for good weather and took the truck out today. Started truck with no problem first thing in the morning, about 5 miles from home while driving 50 mph Wait light briefly flickered. This has done this previously and I figured my original problem was still there. Truck sat inside the garage at my work all day and sure enough come 4:00- truck would not start and Wait light never came on. Had to cycle key twice and light finally came on. Anybody have this issue that could save me some time and point me in the right direction? Also, dont know if this is related but....there are 3 M1008s in the family and this truck has never cycled the glow plugs like the others do where the voltmeter dropped and you could hear the relay clicking on and off. Never thought much of it because the truck has never let me down once since purchasing it in 2009- until now. Sorry for the long post, but wanted to give as much info as possible to try and sort this out. Any info is greatly appreciated- thanks for your time!

What temp sensor did you put in? The WT3000 is the proper sensor. It has high resistance when cold and a lower resistance when warm.

Unplug the temp sensor and see if the Wait light comes on.

I used the original GM sensor, GM #10045847. This is the same sensor I used from 37,773 miles to 80,000. I will try to unplug the sensor tomorrow morning when I am positive it is cold and see if the light comes on.

This number also crosses to NAPA #TS4052 which I see also crosses to WT3000.

If you unplug it, it will simulate a cold engine.

I will try first thing in the morning- thanks for your help. I am positive the one was bad I removed, don’t know why the replacement worked one time and now I am back to square one.

AFC1 said:

I will try first thing in the morning- thanks for your help. I am positive the one was bad I removed, don’t know why the replacement worked one time and now I am back to square one.

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This morning I turned the key on with the temp sensor still hooked up- Wait light never came on. Tried it only once. Disconnected sensor harness and tried it again. With harness removed, Wait light still did not come on. This is going to drive me crazy. I was thinking perhaps a ground issue? Why every once in awhile the light would flicker going down the road is what gets me?

AFC1 said:

This morning I turned the key on with the temp sensor still hooked up- Wait light never came on. Tried it only once. Disconnected sensor harness and tried it again. With harness removed, Wait light still did not come on. This is going to drive me crazy. I was thinking perhaps a ground issue? Why every once in awhile the light would flicker going down the road is what gets me?

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It sounds like you may need to trace the wire going to the sensor to see if it has rubbed through on something and getting a intermittent ground.

porkysplace said:

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It sounds like you may need to trace the wire going to the sensor to see if it has rubbed through on something and getting a intermittent ground.

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Yeah, I will have to trace them. Hope I am focusing on the right area and it is not something in the column. I truly felt it was a loose wire/connection somewhere until I found the bad sensor. Got all excited over nothing, have to start over. Usually it was fine overnight, truck would start on the first try.

It may be a good idea to clean the connector contacts in the card holder/connector as well.

This is absolutely insane. Haven’t been able to touch the truck in awhile. After truck has been sitting for 11 days, went out to test the temp sensor wires. Unplugged, no power at the wires- yet the WAIT light illuminated. Didn’t do that on 10/2 when I checked it first thing in the morning. This truck was not disturbed in anyway, even the hood was never closed this whole time- can’t see how any wires or connections could have been jiggled. Just crazy....

Why leave the hood up for 11 days?
Just curious.

Any resolution on this issue? I am currently tackling the same problem. I found 2 areas in the black & yellow wires to the temp sensor where the insulation had worn through. Replaced wires and issue is still present. I'll have to trace the wires completely today.