Kubota B1700 D905 diesel glow plug problem - Driving me nuts!

[joestewart]

OK, this tractor has the three cylinder Kubota D905 diesel engine.

It became difficult to start. We isolated the problem to the glow plugs. I checked all three glow plugs by removing them and hooking them directly to a 12v battery. One of them was bad so I bought a new one.

Installed the glow plugs back in tractor, but still having difficulty getting it started. After a lot of cranking and cycling of the glow plug circuit, we got the motor started and it runs great (once it is started, that is!).

Let motor cool back down. The next day:

I then disconnected the wire to the glow plugs and ran a wire directly from the positive terminal of the battery to the glow plug bus bar. When I touch the wire to the glow plug, I noticed that there is minimal arc (I would have expected a large arc). Held it on there for thirty seconds: Still no start.

I then began to think that the motor was poorly grounded. I ran a wire directly from the negative side of the battery and grounded it to the lifting eye on the cylinder head. Touched the wire from the positive terminal of battery to glow plugs: minimal arc and no start.

I then removed all the glow plugs and tested them again (by hooking them directly to a 12v battery: they heat up instantly - in about three seconds - burning my fingers!). OK, so then I took a wire wheel and cleaned all the threads on the glow plugs, the contact surfaces, the bus bar and all my wire connections. Hooked everything back up: Still minimal arc and no start! I cannot feel any heat on the top of the glow plug when I'm doing this, so I'm convinced that the tips are not heating up.

This seems like a bad ground (but the starter works great, arguing against this), or not enough current being delivered (but I ran a wire directly from the pos side of battery - can't see why these glow plugs will not heat up!)

OK, so I'm nearing the end of my rope with this tractor. I've searched the archives. Does anyone have any suggestions for me?

Thanks in advance!

 

[horsepuller]

Your troubleshooting so far has shown that the three glow plugs work ok. You probably have too high resistance somewhere in the glow plug circuit. This is a good situation for voltage drop testing. Take your digital volt meter (buy a cheap one, if you don't have one) and start measuring voltage drops across various parts of the circuit.

With the glow plug bus bar directly connected to the battery, with the meter on a low voltage range, probe across each element of the circuit, starting at the negative battery terminal. If the circuit was working properly, you would find almost all the voltage being dropped across the glow plug bus bar to engine ground at the head. In the case here, where the glow plugs are getting inadequate voltage (probably), you should be able to isolate one section of the circuit where a significant voltage drop occurs, where it should not be. If you add up all the voltage drops around the circuit the sum will equal the battery terminal voltage.

Given your troubleshooting so far, I am suspicious about the engine head ground. I would find a convenient spot on the engine head, and grind or file it down to bare metal to assure a good ground spot for testing. Good luck, low voltage, high current circuits, are sometimes difficult to troubleshoot.

 

[joestewart]

Great. Thanks for the excellent suggestion(s). I have a volt meter and I'm probably going out there tomorrow (the tractor is across town). Will print out your response and test accordingly. Thank you again, horsepuller!