Glow Plug Circuit Check

I got an e-mail from a reader. I post this with his permission:

From: Larry
Subject: Kubota L1500
Date: December 30, 2006 6:57:33 AM PST
To: ironcross11-at-earthlink.net

Just read your articles regarding hard starting on some model Kubota tractors. I have a Kubota L1500 (same as the L175) and have always had problems starting the engine. Starter has been rebuilt, new battery installed but starting it in cold weather is impossible. I seem to have good compression and the engine does puff white unburned smoke when cranking the engine. I'm not sure what amperage the glow plug circuit should supply to the glow plugs, so I haven't tried reading it. Do you have some knowledge of the glow plug circuit that you could pass it on to me? I do see the indicator coil on the dash begin to glow when activating the glow plugs but don't know what it goes through after this on it's way to the glow plugs. I would appreciate any info you could give me.
Thanks,
Larry

I replied:

From: Mike
Subject: Re: Kubota L1500
Date: December 30, 2006 6:57:33 AM PST
To: ironcross11-at-earthlink.net

If it's like mine, power should go from the Key switch to the heating switch to the indicator coil to a big coil resistor (under the battery on mine)and then to the glow plugs. It's all a series circuit so if any part isn't working, none of it should work unless you have a short. Here is the schematic from my cheesy reprint manual.

The check for a short is easy. Without disturbing anything else, very carefully disconnect the second glow plug in the circuit from the ground. If it the glow indicator still lights--you have electric shorts!

If it doesn't glow, have an assistant close the glow plug heater switch and then rattle the wiring harness around (in a dimly lit area) and look for sparks or jumps on the ammeter (if so equipped).

Mike

If you still are not sure, reconnect the last glow plug to ground and disconnect the first glow plug from the circuit. Close the heater switch and check voltage from the open lead to the glow plug. It should be pretty close to battery voltage. If it isn't, you are probably shorting to ground somewhere else.

When I heat my glowplugs it nearly pegs the ammeter, but it did that when it was shorting too. So that isn't a reliable diagnostic by itself.

Apologies for the shoddy quality of the scan: the numbered items are:

1 Glow Plug
2 Glow Plug Resistor
3 Glow Plug indicator Coil
4 Starter/Glow Plug Switch
5 Key Switch
6 Voltage Regulator
7 Alternator/Generator
8 Battery
9 Starter

The Glow plug descriptor reads: 15101-65511 Nippon Tokusho Togyo Make
Direct current 1.5V 40 Amp