Glow plug resistance changes with ignition on?

[top_prop]

The truck: M1009, purchased from Gov Liquidation ~4.5 yrs ago. REC 12v mod, Glow plug resister removed, 12V fed to GP relay from Civi 12V buss near GP relay. AC 60G glow plugs. Glow plug controller card removed. By pass button placed on Blue wire. Electric fuel pump, with 3/8 inline pre filter, spin on fuel filter mod.

Was running and starting great. My daily driver. Then I had to depart on 3 weeks of temporary duty. When I got back it took about 20-30 seconds of cranking to get it to start. If cold takes 20-30 seconds of starting. When it catches it revs up and makes a big white cloud.

If warm starts immediately like it always has after pushing the GP button for a longer count cold, and a shorter count warm.

Work performed:
-Checked glow plug resistance: all 1 to .9 Ohm (pulled one to check, and it read the same,.9 Ohm and looked OK... brand new AC 60G read .9 ohm on same meter).
-pulled all GP leads, and checked individually: 12.5VDC at each lead.
-Changed both the inline and spin on fuel filter. Both looked OK. Fuel looked clean and bright, no water. Re primed and ran a flow check. Pumped over a quart of fuel in 10 seconds.
-Have clear line on IP: always full of fuel, no air. no bubbles when running
-Freshened all grounds at bus, the passenger alternator (only alternator.. 12v REC), and on the block behind the air filter.
-Rechecked GP resistance, but with the key in run. Got 22OHM's on Passenger side and 30 ohms on drivers. Turned key off... got 1 to .9 OHMs... checked resistance between block and neg terminal of both batteries... .01 ohms....

So what could cause the resistance to change with the key on?

What would you do next to get the truck to fire up like it should cold?

 

[dependable]

Sometimes the glow plug relay gets pitted or fails to make full contact for some reason. Still sounds like it has engaged, but if you have a working volt meter, you might notice it does not draw as much. Relay failure like this can be intermittent, as relay throws, but contact is not consistent. Repeating contact will often get relay to work, until it gets worse.

 

[tim292stro]

AC60G are self regulating. So yes, they change as the temperature changes - it's called PTC or Positive Temperature Coefficient. As the temperature goes up, so does the resistance, meaning it draws more current when cold, and less when hot - in this way they get to a design point where they draw less current that can keep them warm so they cool down to a point where they heat up again (thus the self regulating). This is why AC60G's are not ideal in a ballast resistor 24V system - some plugs heat up faster than others, and kill the stragglers... then it's a cascade failure for the ones left.

This does lead to a hint though - if the glow plugs on one side are lower resistance than the other side, it indicates that they haven't gotten to that self-regulation point. don't just measure the resistance of a dead circuit, measure the voltage from the load to ground and various points to ground. You may have a marginal wire on your driver's side that isn't passing a lot of current. A meter will only put a few milliamps at most across a wire, not enough to detect a bad wire, you need to check it with a big load to see that the voltage drop is from one end to the other (like at the glow-plug relay and at the glow plug, if it's more than a 10% drop - there's a big problem).

Personally I would not have removed the glow plug controller, if it didn't work, I would have fixed it... Getting these trucks to start cold is more than a short 10 second glow, a crank, and driving away. On really cold morning the after-glow can be running for a minute or more.
https://www.youtube.com/watch?v=MTkOl947aTk


After the glow system, the first thing I'd check is the battery health. This is the time of year when weakened batteries show their true colors - just measuring the voltage isn't enough, you need to load test them to see if they can support the starter load. That, and even if your fuel has no water in it and your glow plugs are running right, it might be an air leak in your fuel system causing you to lose a bit of prime.

 

[top_prop]

Thanks for the fast replies!

More info: Glow plug controller went bad... so I bypassed it. And I don't go for a 10second hit on the button unless the engine is warm. And In the winter I do a bit of manual after glow as well. As for the resistance changing the engine and the plugs were cold... its when the key is turned to run that i measure a different resistance than the normal 1 to .9 ohm at the plug.

To rule out the batteries, I put the truck on a charger all night, then put a plug in jump starter on when I cranked... still slow start (20-30seconds of cranking)... next day I fired up my 2500 pick up with a 100amp alternator... connected it to the CUCV with jumpers, let the 2500 idle for a while, then revved it to 2500 rpm... then hit the glow plug button a 55 seconds and started the CUCV from it... still 20-30 second cold start. So I don't think it is the batteries. But I did clean the terminals just to be safe (they are sealed type batteries).


I'll try measuring voltage drop across the glow plug wires next. Thanks for the suggestions... I'm open for whatever you got.

tom

 

[tim292stro]

If it ain't the fire, it's the fuel.

If you say you have a clear line to see fuel flowing that kinda rules out the fuel a lot. White smoke can be bad compression on a cylinder or more, bad injectors (or injector lines, or the IP). Double check the wiring first, then let's have a compression test on the engine. If you aren't getting compression, your compression-combustion based engine won't make the fire.

I also see you're in Virginia, how's the weather been there? Cold enough for any concerns it might be gelling?

 

[top_prop]

Want to close the loop on this... swapped out the glow plug relay... and now all is well. Strange thing is that it tested fine.. must be like "dependable" said.. relay is throwing and contacts are making enough to pass voltage but not enough to allow it to pull any amps through the plugs... new GPR109 relay is working great even on cold mornings!