Glow plug/starter wiring question on Diesel Buck

[Steve777]

I was having some starting issues (slow or no cranking) during this recent cold snap (0F or so), and pulled out my VOM to see what was going on electrically. The starter seemed fine, it would crank at full speed if I jumpered 12v to the start terminal on the solenoid; would have hoped it was OK as it was just rebuilt last spring. Glow plugs get voltage when the key is turned to the GP position as expected, and they seem to be working in that the voltage drops indicating a good current draw. One thing which did surprise me is that the when I looked at the battery voltage when the key is turned to the start position, it drops just like when the GPs were on, even the times that the starter wasn't running??? That seemed odd, so I did some additional testing and sure enough, when they key is in the start position, the start wire on the solenoid gets battery voltage and so do the GPs.

I suspect my problem is mostly due to my battery getting old and in the cold weather it can't supply enough amps to run the GPs and the starter at the same time. So a new battery is in order here.

But I was wondering, are the GPs supposed to come on when the starter is energized? Seems unlikely to me, why put an additional big load on the battery when you are trying to crank the engine?

This is a fairly new ignition switch (a couple of years old now), so it seems odd that it might be failing so soon. But I am curious does anyone know if their Buck runs the GPs with the starter?

Thanks

 

[Ilikeurtractor]

Yes, the glow plugs should have power when the starter is cranking. It does help the engine start a lot easier especially as compression degrades over time. The glow plugs will have the highest temperature during starting and you don't want that to go away while cranking. Some vehicles also use "after glow" where the glow plugs remain on after the engine starts to aid in more complete combustion and better running for a minute or two. One of the wiring diagrams for either the Satoh or Iseki tractors (I can't remember which right now) do not show the glow plugs wired this way but they should be. In fact, most glow plugs are only designed for 10 volts so when your are applying power from the switch only to the glow plugs it goes through the glow indicator which drops the voltage about 2V. When you are operating the starter, system voltage will normally drop to around 10 volts because of the starter load and the glow indicator is bypassed in this case maintaining the voltage of the glow plugs at 10 volts.

 

[Steve777]

Thanks. OK then sounds like the switch and wiring are working as expected, and it is just a weak battery.

 

[Ilikeurtractor]

Check the battery voltage when you're cranking. A weak battery will drop significantly below 10V. If it is supplying 10V while cranking, it probably isn't the battery. Another way to think about this is the starter and glow plugs will draw X amps at 10V (glow plugs will take around 30A - the starter probably 3-350A). If the battery cannot supply X amps, then it will not be able to supply 10V because of the volt-amp relationship (Ohm's law).