Glow plug troubleshooting

[oberscrounge]

New to this Forum, 1st diesel engine this year, L455 New Holland Skid Steer W/ 3 Cylinder Kubota D1402---Kinda a generic Question for all Diesels,
If one or more of my glow plugs reads "0" ohms--- ( I believe shorted) did that blow out a Fuse somewhere ? it has been hard starting lately

 

[davesl708]

Hi. Welcome.

If the glow plug reads 0 ohms then the coil is burnt out. Zero equals open connection. It should not have blown a fuse. If a fuse is blown you most likely have another problem. A blown fuse would disable all glow plugs and make it hard to start.

 

[Quebecguy]

Hi daves1708,
On my ohmmeter zero ohms equals a short, infinity equals an open.

 

[yankeerider]

Two years ago the 3 cylinder Yanmar in my 755 became hard to start. It seemed like only one cylinder was firing when I cranked it in below 25-30 degF conditions. I thought I had 2 bad glow plugs. I checked the resistance of each and one was open ( very high resistance) and the other two measured in the specified range. Being cheap, I only replaced the bad one. Tractor has started great ever since.

 

[k0ua]

Depending on the resolution of your ohm meter, a glow plug should read NEAR 0 ohms. Zero ohms is a dead short, and obviously the glow plug is not a dead short, but it is darn close. on average a glow plug will draw close to 20 amps of current at near 12 volts. Keep in mind the resistance of the plug will change dynamically as it heats up, just like any heating element. So lets explore Ohm's law. E=IR where E is Electromotive force measured in Volts. I stands for current which is measure in amperes (amps) and R is the resistance measured in ohms. So with simple 9th grade algebra we can plug in the values we know and solve for the values we don't know or want to do a "sanity check" on.

So the voltage we will call 12 volts (that is the E) the current we know from manufactures specs at 20 Amperes (that is the I) and the resistance of the glow plug we will solve for in the following equation

12=20R so lets divide through both sided of the equation by 20 we come up with 12 divided by 20 =R so 12 divided by 20 is .6 so R=.6 ohms

So your glow plug is about .6 ohms or just over 1/2 of an ohm. Again depending on the resolution of your ohm meter, it will measure near zero.

I hope this helps.

 

[oberscrounge]

Thanks to everyone for the Quick responses--- I was out in the Driving sleet yesterday in southern Wisconsin, trying to pull these Glow plugs, ( Machine temporarily completely outside) ---Only got the center one out before Mother Nature won the Battle--- The reason I thought about the fuse is that this plug is TOTALLY "Commoned Out" --- meaning every surface of it reads 0 ohms to all other surfaces-- Being a 40 year IBEW retired Electrician this means to me that 12VDC is going thru this plug DIRECTLY to Ground-- Being a "Shorted Circuit" -- heading outside Now to pull the other two and do some circuit testing and Ohm Out the other two plugs once out of the Head .. Thanks again, will post results---
oberscrounge

 

[George2615]

The reason I thought about the fuse is that this plug is TOTALLY "Commoned Out" --- meaning every surface of it reads 0 ohms to all other surfaces-- Being a 40 year IBEW retired Electrician this means to me that 12VDC is going thru this plug DIRECTLY to Ground-- Being a "Shorted Circuit" -- heading outside Now to pull the other two and do some circuit testing and Ohm Out the other two plugs once out of the Head .. Thanks again, will post results---
oberscrounge

You are correct that the first plug you pulled is bad. By now I'm sure you've checked the others. If they are also shorted they're bad too. If you have resistance they may be good. You can check the fuse to see if it's blown but probably faster to turn the key on and see if you have DC power to the wire that feeds the plugs.

 

[Quebecguy]

Depending on the resolution of your ohm meter, a glow plug should read NEAR 0 ohms. Zero ohms is a dead short, and obviously the glow plug is not a dead short, but it is darn close. on average a glow plug will draw close to 20 amps of current at near 12 volts. Keep in mind the resistance of the plug will change dynamically as it heats up, just like any heating element. So lets explore Ohm's law. E=IR where E is Electromotive force measured in Volts. I stands for current which is measure in amperes (amps) and R is the resistance measured in ohms. So with simple 9th grade algebra we can plug in the values we know and solve for the values we don't know or want to do a "sanity check" on.

So the voltage we will call 12 volts (that is the E) the current we know from manufactures specs at 20 Amperes (that is the I) and the resistance of the glow plug we will solve for in the following equation

12=20R so lets divide through both sided of the equation by 20 we come up with 12 divided by 20 =R so 12 divided by 20 is .6 so R=.6 ohms

So your glow plug is about .6 ohms or just over 1/2 of an ohm. Again depending on the resolution of your ohm meter, it will measure near zero.

I hope this helps.

Hi James,
I don't know about Oberscrounge's machine but the wiring diagrams I have for some old International Harvester tractors show the glow plugs connected in series. So in your example wouldn't the .6 ohms be the total resistance of the circuit which would mean each individual glowplug has a resistance of .2 ohms?

 

[k0ua]

Hi James,
I don't know about Oberscrounge's machine but the wiring diagrams I have for some old International Harvester tractors show the glow plugs connected in series. So in your example wouldn't the .6 ohms be the total resistance of the circuit which would mean each individual glowplug has a resistance of .2 ohms?

All the glow plug diagrams I have seen have the glow plugs connected in parallel. So the formula for connecting resister in parallel is 1/r1+ 1/r2+1/r3

My Kioti manual says the glow plugs in the Kioti are .43 ohm each. I suspect others would be near the same.

 

[DonnieP]

Hi,
I have to agree with James on all counts. Never seen glow plugs wired in series. If they were to be wired in series, they would have to be isolated from ground, and then if any one failed none would work. Wired in parallel a failure doesn't kill all them. It make it harder to trouble shoot though. If you can remove the jumper from all of them, you don't have to remove them from the engine to test them. Remove the jumper, and check each to ground. They are like light bulbs, if open then bad, shorted is good. Most meters don't show the half ohm and once again, OPEN IS BAD, SHORTED IS GOOD! You don't need to replace all when one fails, especially if you can find an open one with out too much difficultly.
Donnie