m1008 glow plug/wait light issue

Hello all. Bought a 1986 M1008 CUCV a while back. Looking for some advice cause im at a standstill. When I first got the truck it would start up fine with no issues. However after the engine was started, the glow plug "wait" light would flicker while driving occasionally. Drove the truck many times like this. It still ran normal and started normal with the 'wait" light activating to cycle the plugs before start up but then flicker while driving (as said on occasions).

After a few weeks of driving it, parked it one night and the next day I went to start and no "wait" light would illuminate when turning the key to cycle the plugs. The engine would crank but not start. Tried several times and kept having the same issue. I had to get the truck moved out of a spot one day and was reduced to using starting fluid spray (I know not good but it had to be moved). It cranked and ran fine. Turned it off and then turned it back on a while later still warm. Still no "wait" light.

Long story short borrowed a buddy's glow plug card and still same issue no wait light and will not turn over. On the glow plug relay with no key in the ignition, i have 24 volts at the top terminal (resistor) and 0 volts on the other 3 terminals.

With the ignition turned to the on position i have 24 volts at the top terminal. Second terminal down from top, 11 volts. Third terminal down from top, 11 volts, and the bottom terminal 0 volts. Bought a new relay and same readings with same issue.

I checked all the fuses as well.....all good.

Also, dunno if it matters. On the relay, the second terminal down from top i have the pink wire connected and the third terminal down I have the bluish wire connected. This is how it was connected when the truck was working. I have seen pictures on this website where the wires are switched and also in the wiring diagram......does this matter? cause the truck was working? In addition i switched them to see if it would fix the problem....no.

Done rambling .... Any input would be appreciated! Thanks!

Did you check the glow plugs themselves? I am not sure if they are all bad if that would cause the light not to come on.

ghastlyghoul454 said:

Long story short borrowed a buddy's glow plug card and still same issue no wait light and will not turn over. On the glow plug relay with no key in the ignition, i have 24 volts at the top terminal (resistor) and 0 volts on the other 3 terminals.

Click to expand...

That's all good.

ghastlyghoul454 said:

With the ignition turned to the on position i have 24 volts at the top terminal. Second terminal down from top, 11 volts. Third terminal down from top, 11 volts, and the bottom terminal 0 volts. Bought a new relay and same readings with same issue.

Click to expand...

That's not good. You should see 0 volts at that third (blue wire) terminal. If a working controller card did not fix your problem, it's probably the ground for the controller card, or the blue wire itself.

Do this (with a helper): Turn the key to the RUN position (so the GPs should engage) and connect a jumper between the third (blue wire) terminal and ground. Make sure it's really a good ground.

You should hear a solid clunk from the relay, and your GPs should get hot. You should now see 12v top and bottom of the relay.

IF you do, then you know it's either a ground issue with the controller card, OR, it's a continuity problem with the blue wire.

ghastlyghoul454 said:

Also, dunno if it matters. On the relay, the second terminal down from top i have the pink wire connected and the third terminal down I have the bluish wire connected. This is how it was connected when the truck was working.

Click to expand...

This is correct.

ghastlyghoul454 said:

I have seen pictures on this website where the wires are switched and also in the wiring diagram......does this matter?

Click to expand...

Not really. But yours is correct by the book. I suggest you leave it that way.

Have you checked to see if your glow plug temperature sensor switch is operational? It could be stuck in the closed position and fooling your glow plug controller that the engine is warm. I think if you just unplug it, the glow plug controller will think its a cold engine and cycle your glow plug system. Mine acted up but in the opposite way and would cycle the glow plugs with a warm engine. Worth a try.

I think dougco1 is on the right track.

Disconnect the temperature sending unit that goes to the GP card and see if the Wait light comes on and the card closes the GP relay when you try to start the engine. By disconnecting the sensor, you give the card a high resistance on the sensor input to simulate a cold start condition.

Check the wires going to and right at the top of the sending unit. If they are shorted together or the yellow wire is shorted to ground, it is possible to have the issues you are describing. Some previous discussions here have found chaffed and compromised wiring causing the same issues you are experiencing.

Lastly, when the engine is cold, (less than 70 deg F) measure the resistance of the GP temp sensor, depending on the temperature it should be somewhere between 2500 to 7500 ohms. Disconnect the wires at the sensor and then get the resistance right at the top of the sensor and note the temp of the sending unit if possible.

The GP sensor is not a switch as such, but it is a variable resistor that changes value with temp. When the temp sensor is is cold, the resistance is high, as the sensor gets hotter the resistance gets lower.

Thank you all for the input. I tried connecting a jumper wire on the relay and it worked. This week I will disconnect the temp sender to what happens with that.

There ya go. My bad - I forgot about the temp sensor, but now we know the GP relay and resistor bank is all okay. Now to find out why the controller is not providing the correct ground. Temp sensor, continuity problem with the wiring, or bad ground are all possibilities.

I am at this point with a truck I am working on. Everything is wired stock but I can not get the relay to clunk in. I did a visual of the glow plugs. They are all good. The wait light comes on but the relay does not close. I swapped the relay same results. I started with just Gen 1 light I found the oil pressure sending unit disconnected. Plug it back in and the oil light works. The horn blew constantly and I snipped the diode out. The Gen 2 light would not light. I grounded the brown wire and it lit. I swapped out alternators and the light works now. I think if I can get the glow plugs to work I will be home free. I hot wired the glow plugs and the truck started right up. Still no clunk of the relay. Does not engage. Tried another card I had around no joy. I was going to pull it out of the M1028 I am driving but it is the only thing I have that runs right now. Any ideas? Merry Christmas.

Sir Richard, have you tried grounding the relay manually? If yes, and it works, check continuity from the relay to the controller card. Also check for a good ground source for the controller card. It can't provide ground if it doesn't have a good ground.

Might be a partial ground, too.

I have it mounted on the firewall. I had these HD relays on several CUCV's in the past. The metal body of the relay is mounted in fresh drilled holes with new screws. Anything else I can check? Also the volt meter just pegs itself when I turn the key on. I guess I must get the fluke out. Does pegging the gauge indicate a bad gauge?

Try grounding the light blue wire and see if it closes the relay, if it does, measure the voltage on the GP side of the relay to see if it is getting the proper voltage.

Then check to see if the light blue wire has continuity from the relay to the GP card connector, if so then you may have 2 bad cards.

I can do that I think. I also have new cards with fancy lights I never used. I could always put one of them in if you think it would be safe to do so? Well?

What is this on the injection pump? could it disable the glow plug relay? Should I leave it hooked up or disconnect it?

cucvrus said:

I can do that I think. I also have new cards with fancy lights I never used. I could always put one of them in if you think it would be safe to do so? Well?

Click to expand...

If you blow it up, Ill fix it.

What is that thing on top of the injection pump cover and why is the advance wire hooked up to it? Does anyone know?

Have you ever seen one peg the volt meter? I wonder if that could have been from the bad Gen 2.

On the original issue, lately I fixed a few issues by cleaning the main ground that goes to the dash components, it's in the cab right above the parking brake. Before I cleaned I was getting odd issues like my gas gauge being off and parking brake light not turning on, as well as the wait light never turning on or closing the relay.

Cleaned the ground and all is back to normal.

Worth a shot.

That would be too easy. I have the cluster out and everything working on the cluster. I replaced the star washer and bolt on the ground at the E brake. Still no joy. I removed the glow plug relay from a running CUCV that operates perfectly installed it and the relay will NOT engage. I have 2 relays coming. I will get thru this in good time. Set back and watch. I must learn the way of the Jedi. Happy New Year.

The ground by the PARKING* brake is a major one, but just because you have cleaned it up, don't assume that your controller card has a good ground. You are showing weird behavior (well, not you, I mean the truck ), and weird electrical behavior is usually a ground problem. Those electrons get lost and find their way through various unintended paths to where they want to go. Hilarity ensues.

You need to measure from the actual controller card socket ground connector to the true electrical ground of the vehicle; the - post of the front battery. You also need to measure resistance on the blue wire to the correct connector on the controller card socket.

Also examine that socket carefully. It's possible the connector for the ground circuit is toast.





* (there is NO "emergency" brake on a modern vehicle. Hasn't been since.... about the 20s or 30s, IIRC.)