glow plugs

[DirtHauler]

I'm gonna go out on a limb here, but I had a similar voltage problem with my daughters car that the dealer tried several times to fix. I finally found and fixed it. The low voltage problem turned out to be caused by a bad ground cable connection to the body and from the body to the engine. I removed each cable/wire and cleaned the connection points of the factory paint to bare metal and reconnected, there's been no "wierd" electrical problems since. Just an area you could check.

 

[KiotiJohn]

</font><font color="blue" class="small">( The low voltage problem turned out to be caused by a bad ground cable connection to the body and from the body to the engine. )</font>
That's certainly a viable suggestion. Just a partial/intermittent ground could cause problems. J

 

[cdt80]

Sure is hard to diagnose via the post isn't it? /forums/images/graemlins/grin.gif
I am just curious, did you make sure to unhook each glow plug from each other when you tested the glow plugs? Otherwise, you will still show a ground if only one is good.
The only reason I am suspicious is the fact you are leaving the glow plugs on for 30+ seconds. In everything else I have been around 30 seconds will eventually burn out glow plugs.
I agree that battery size (Cold Cranking Amps) is important (and I am not discounting this theory) but unless you put in a way undersized battery I slightly question this as being the problem. However an oversized battery may mask the problem by sending more juice to where it is needed. I would think the juice drained from the battery to run the glow plugs would be very noticeable in a slow crank after. You have not specifically mentioned this as a problem. If it is then I would definitely go with a bigger battery.
I hate admitting defeat and letting somebody else step in. You have done a good job so far diagnosing you just need to eliminate possibles. Check your wiring with an ohmmeter if you are not sure, especially your ground. Make sure your battery is adequate. Make sure the glow plugs are receiving everything your battery is capable of putting out. Make sure your glow plugs are for sure working. Take them out and CAREFULLY bench test if necessary. Don't give up, part of the fun of owning a tractor is knowing you can maintain it. The satisfaction you will get when you fix the problem will make you more proud of your tractor than ever.

 

[BGL990]

30 seconds won't necessarily burn out the plugs - it depends on what kind of plugs they are. The engine in my Mom's GMC truck has specified a few different plugs over the years, the latest being a positive temperature coefficient plug. This means that as the plug heats up, it's resistance also goes up, reducing the current it draws. At some point it reaches an equilibrium where it won't get any hotter. Apparently these plugs are virtually burnout proof. Before I changed all the plugs recently, I powered one up using a bench power supply. After 10 seconds (the factory glow relay timing), it was just dull red at the very tip. After 30 seconds it was bright orange almost all the way down to the threaded part, which is how it should be according to some pictures and other info I've found on the web. At this point it had stabilized and wasn't getting any hotter. From what I've read it could stay like this all day without deforming or burning out.

As an aside, because the glow relay in the truck was actually designed for 6V plugs with shorter cycle times (another long story ...), when I replaced the plugs with these ones, I also rigged up a manual push button on the dash to engage them. Now we just push the button for 20-30 seconds before starting and there is no problem no matter how cold it gets.