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 | nicktc29d
Join Date: Dec 2006 Posts: 4 Iowa 
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2006-12-26 138354
I am having starting problems with my 2001 TC29D (with 1300 hours), Can anyone tell me what the resistance of a good glow plug is? My main 30 amp fuse blows when I turn the ignition switch to run the glow plugs. It blows right away . It has about 1300 hours on it and has nsever gave me problems like this before. Each glow plug is reading about .8 ohmes of resistances. I have removed the relay from the glow plug circuit and it does not blow the fuse and will try and start but does not.I have replaced the fuel filter and bled the line and it still does not start. I can use starting fluid and it starts right up. So that tells me that it is not a fuel problem. Any help would be appreciated. Thanks
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| jd110_1963
Join Date: Feb 2002 Posts: 87 westminster, md
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2006-12-26 138355
How are you testing the glow plug resistance? The glow plugs need to be disconnected to be tested properly. the wire going from plug to plug has to come off. I like to actually remove them from the head to test them and visually inspect them. You can also hook up some test leads to 12 volts to see if they glow red within a few seconds, just don't leave it hooked up too long or you can burn it out if it can't dissipate the heat to the cylinder head. ....
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 | greg_g
Join Date: Jan 2004 Posts: 1816 Western Kentucky 
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2006-12-26 138356
I know a little about glow plugs, but almost nothing about the TC29D. So forgive me, but I would have figured the Boomers had glow plug relays. If true, isn't that relay supposed to protect the 30A fuse?
Relay notwithstanding, how you arrived at that 0.8 ohms number is definitely important. Is that the total resistance of the entire glow plug circuit as installed (still on the tractor? Or did you test each glow plug individually? On the tractor or off? Proper test procedure would be to test them individually, after being removed from the tractor, cleaned, and brought to room temp (70F) before measuring resistance.
The other critical factor is the DMM itself. Because glow plugs operate at low voltage and low resistance, the most accurate test results come from a low voltage/low resistance DMM.
//greg//
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| Gearhead
Join Date: Jul 2005 Posts: 75 SEVEN mILE OH
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2006-12-26 138357
If you unhook the main wire to the glow plugs at the head, does the fuse still blow? Can't remember what ohms should be but it should be higher i think. I checked the spec on a kubota glow plug the other day and it was 1.5 1.6 ohms. A reading of .8 ohms is almost straight to ground. It could be possible that all are bad but doesn't happen offten.
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| nicktc29d
Join Date: Dec 2006 Posts: 4 Iowa
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2006-12-27 138372
Thanks to everyone that has helped. I removed the glow plug and then took the measurement. If I do remove the feed wire from the bussbar that is connected to all the glow plugs then the fuse does not blow. I found out that I have some relation that workes for an NH dealer. He told me that each glow plug should measure 4 to 5 ohms.
For the purpose of rounding off we will use 4.5 ohms per glow plug. 4.5 ohms per plug, three of them in parallel would bring the total resistence down to about 1.45 ohms. usining E=IxR that would equate to about 8.27 amps of draw. Now if you use my actual reading of each glow plug which is approximatly .8 ohem per plug that figures to a total of about .266 ohms of resistance.Use the formula of E= IxR which e = volts, I = amps, r= resistance. 12volts/.266 ohms = 45.11 amps. To figure out what you need for total resistance disconnect the power wire and read total resistance. 12v /1.45 ohms = 8.2 amps. I hope this helps anyone else. Thanks for all the help ....
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| greg_g
Join Date: Jan 2004 Posts: 1816 Western Kentucky
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2006-12-27 138373
Except Nick, resistance in parallel is calculated like this:
R(parallel) = R1 × R2 x R3 divided by R1 + R2 + R3.
As opposed to
R(series) = R1 + R2 + R3
And measuring them on the engine - as a single circuit - is only a gross results test, good only to determine if the entire circuit is operating within the limits of the fuse.
I still maintain that you should physically remove each glow and bench test them individually - if you want accurate results that is. Obtain room temperature resistance values from clean glow plugs, then stick THOSE numbers into the appropriate formula. Just as in compression testing - where you want to see all cylinders come within a few PSI of each other - in glow plug testing you ALSO want to see all plugs measure within maybe half an ohm of each other.
And I know glow plugs come in various heat ranges, but I'd be very surprised if a 4.5 ohm glow plug will get hot enough to start an engine. They're almost always under 2 ohms, more typically 1.5 and less.
//greg// ....
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| nicktc29d
Join Date: Dec 2006 Posts: 4 Iowa
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2006-12-28 138374
Resistance in parallel is calculated as 1/RT=1/r1+1/r2+1/r3.....And I did take each glow plug out and measure them seperate. Thanks for all your help. ....
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| greg_g
Join Date: Jan 2004 Posts: 1816 Western Kentucky
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2006-12-28 138376
No Nick, it's not
//greg// ....
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| nicktc29d
Join Date: Dec 2006 Posts: 4 Iowa
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2006-12-28 138385
Greg, I looked it up on the internet and in my old elctronic books and they both say that my method is correct. Anyway, Greg I do appreciate your help with this. Because I did order one new glow plug and I was told wrong the new glow plug reads 1.2 ohms not between 4 and 5 ohms. It looks like I need to look deeper into this problem Now I did check only one glow plug that was out of the engine and it did read .8 ohms. I will take all them out tonight and read each one seperatly out of the head of the engine. I will let you know what I find. Thanks for all your help. ....
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| greg_g
Join Date: Jan 2004 Posts: 1816 Western Kentucky
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2006-12-28 138386
Nick, look at your formula again - where you said "Resistance in parallel is calculated as 1/RT=1/r1+1/r2+1/r3". Your error is in trying to solve for the inverse of R(Total). Solve for R(Total) only. If you had, you'd have come up with the same answer as I did with my version: R(parallel) = R1 × R2 x R3 divided by R1 + R2 + R3. My calculation is just another way to express RT=1/r1+1/r2+1/r3 without all those obviously confusing inverse numbers.
//greg// ....
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 | oneace
Join Date: Mar 2004 Posts: 1490 south central pa
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2007-01-24 139162
What did you find out with your glow plugs? ....
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| whiterose
Join Date: Dec 2015 Posts: 2 Redmond,OR
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2015-12-30 194268
All these volts/resistances and current draws!
In the end a glow plug is very similar to any other.
What is important is Watts. To get say 80 watts they could make a glow plug draw 10 amps at 8 volts. Or 40 amps at 2 volts.
The product of the volts x the amps is the watts, That's watt its all about really!
From the watts you can get the resistance. There is a formula but seeing we have both volts and amps- use them.
R = V/I ( I used for Amps). So the resistance is 8/10=0.8 ohms in case one and 2/40= 0.05.in the second case.
One thing about using a 'cold' measurement when trying to read resistance is that as that resistance gets hotter and hotter the resistance gets higher and higher. So its basically a waste of time doing this.
Measure the current and then the voltage across the glow plug- work out its resistance then compare with the reading your multimeter gives you. You'll find a big discrepancy. But taking the current through the load ( the glow plug) and the voltage across it will gave an accurate Wattage and the resistance will be good two. ....
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 | Murf
Join Date: Dec 1999 Posts: 7249 Toronto Area, Ontario, Canada 
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2016-01-08 194304
I apologize for being a little late to this party.
Anytime you need to check a 12 V circuit for either continuity or roughly the resistance, by far the best way to do it especially if you suspect a dead short, is to use a 12 V sealed beam light bulb.
I use a rubber 12 V tractor work lamp with a 55 W halogen bulb in it. I mounted it on a large magnet using the quarter-inch mounting bracket supplied with the light. A couple of feet of wire for each of the ground wire and a positive terminating in alligator clips completes the test set.
Something like a glow plug all you need to do is contact the 12 V positive wire to the positive battery terminal and then touch the connector of each individual glow plug with the negative wire of the light, you can quickly judged by the color of the light how low the resistance of each glow plug is.
The beauty of this system is that you are using the light bald as a fusible link and since it is a resistive element you cannot blow it because it can never take more current than the lightbulb filament requires.
Best of luck.
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