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

 

[Quebecguy]

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.

Hi again James,
So you are saying if the current is 20 amperes and the glow plugs are connected in parallel then each glowplug would measure 5 ohms?
.6 = 1/5 + 1/5 + 1/5

 

[Quebecguy]

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

Hi Donnie,
Your words almost make me think that you don't believe glowplugs could be wired in series. I can't show you these tractors but I will try and attach a couple of wiring diagrams from an I&T Shop Service manual.

View attachment 452065View attachment 452066

 

[k0ua]

Hi again James,
So you are saying if the current is 20 amperes and the glow plugs are connected in parallel then each glowplug would measure 5 ohms?
.6 = 1/5 + 1/5 + 1/5

I was saying the maximum current is about 20 amperes for EACH glow plug. x3 for a 3 cylinder diesel, or x4 for a 4 cylinder. That is why the total glow plug current is 60 to 80 amps initially. They do draw somewhat less after a couple of seconds as they heat up, but it is a "ship load" of current to suck out of a cold battery. Each glow plug statically measures less than 1/2 an ohm when cold on my Kioti. I assume others are similar.

 

[k0ua]

Hi Donnie,
Your words almost make me think that you don't believe glowplugs could be wired in series. I can't show you these tractors but I will try and attach a couple of wiring diagrams from an I&T Shop Service manual.

View attachment 452065View attachment 452066

Well sure enough, now we have seen two glow plugs in series. with that left diagram.. I can't make much out of that right hand schematic. That is about the most PiXX Poor version of a schematic I have seen lately. The guy that drew that one, has never worked with electrical schematics much.

So yes I guess you CAN have glow plugs in series.. Why in the heck you would want to do that is beyond me. It is not a good way of doing things for many reason.s

 

[Quebecguy]

I was saying the maximum current is about 20 amperes for EACH glow plug. x3 for a 3 cylinder diesel, or x4 for a 4 cylinder. That is why the total glow plug current is 60 to 80 amps initially. They do draw somewhat less after a couple of seconds as they heat up, but it is a "ship load" of current to suck out of a cold battery. Each glow plug statically measures less than 1/2 an ohm when cold on my Kioti. I assume others are similar.

Hi again James,
Thank you for your prompt reply. Let me see if I understand you correctly: with 3 glowplugs connected in parallel each passing roughly 20 amperes the total current is about 60 amperes. So using Ohm's Law:
12 V = 60 I (1/R + 1/R + 1/R)
or .2 = 1/R + 1/R + 1/R
or .2 = 3/R
or .2R = 3
or R = 15 ohms.
This is much more than your Kioti. What am I missing now?

 

[k0ua]

Hi again James,
Thank you for your prompt reply. Let me see if I understand you correctly: with 3 glowplugs connected in parallel each passing roughly 20 amperes the total current is about 60 amperes. So using Ohm's Law:
12 V = 60 I (1/R + 1/R + 1/R)
or .2 = 1/R + 1/R + 1/R
or .2 = 3/R
or .2R = 3
or R = 15 ohms.
This is much more than your Kioti. What am I missing now?

What is missing is I didn't state the resistors in parallel formula correctly. MY BAD. it is 1/Rtotal= 1/R1+1/R2+1/R3.

So E=IR
Therefore 12=60 (Rtotal) This 60 amp current load is the total current load for all 3 of our glow plugs

12/60 =Rtotal (dividing thru both sides by 60)

.2ohms = Resistance of the total of the 3 glow plugs in Parallel.

So if the total resistance is .2 ohms then each of the 3 glow plugs would be .6 ohms So it would appear that My Kioti with the cold resistance of somewhere around .5 ohms probably draws a bit more than the 60 amps upon initial application of power Keep in mind as the glow plugs heat up the resistance changes.

Also keep in mind it takes one heck of a sophisticated ohm meter to read .4 or .5 or .6 ohms. Not something your average guy is gonna have. They will look pretty much as a dead short.

 

[oberscrounge]

Oberscrounge ,here, I have the other two glow plugs out , Both read infinity-- "Open Circuit", second two are "Isolated" from the plug casing, not like the first one where all surfaces were "Common", -- one has a hole melted in the tip, with a deposit, I believe from the inside , welded at the hole --- see pics , no wonder this thing is "Hard "starting" !!! No fuses, Just the 20A breaker , I will replace it seeing as it is 27 Yrs old already (1988 machine)

 

[oberscrounge]

Can someone tell me why my pics are not posting ? I went thru the browse computer for pics several times, also I cannot find a "Help" tab anywhere on this site to look it up ?? @@@------@@@

 

[k0ua]

Can someone tell me why my pics are not posting ? I went thru the browse computer for pics several times, also I cannot find a "Help" tab anywhere on this site to look it up ?? @@@------@@@

After you browsed and found the filename of the picture did you hit the upload button?

 

[Quebecguy]

What is missing is I didn't state the resistors in parallel formula correctly. MY BAD. it is 1/Rtotal= 1/R1+1/R2+1/R3.

So E=IR
Therefore 12=60 (Rtotal) This 60 amp current load is the total current load for all 3 of our glow plugs

12/60 =Rtotal (dividing thru both sides by 60)

.2ohms = Resistance of the total of the 3 glow plugs in Parallel.

So if the total resistance is .2 ohms then each of the 3 glow plugs would be .6 ohms So it would appear that My Kioti with the cold resistance of somewhere around .5 ohms probably draws a bit more than the 60 amps upon initial application of power Keep in mind as the glow plugs heat up the resistance changes.

Also keep in mind it takes one heck of a sophisticated ohm meter to read .4 or .5 or .6 ohms. Not something your average guy is gonna have. They will look pretty much as a dead short.

Hi again James,
Thanks for another prompt reply. I think you have the calculation right this time. I'm surprised by some of your comments. The ohmmeter I have was the cheapest available when I bought it almost forty years ago and it is fairly easy to see the difference between a half ohm resistance and a short. As for glowplugs connected in series: there were probably some good reasons for doing it that way but I'm getting off topic with that. I would guess there were many tractors wired that way - those two schematics were the first two I looked at and I stopped looking after finding those. At any rate, it seems that oberscrounge has found his problem which is the main thing.