Glow plugs and resistor

[Hoka Hey]

Thanks Buck, Larry and Gemini. Actually the resistor was glowing white within a few seconds and now I know why. The black and white wire that is supposed to supply the glowplugs on the downstream side of the resistor was wired to the ignition switch at terminal 19. So the first resistor was recieving 12 volts then straight to ground...burning it up. Also in this configuration, the glow plugs were receiving 12 volts and the resistor was bypassed. Thanks to the genius who cobbled this one together! Now I have the wiring straightened out. And the first time I tried to heat the glow plugs it took a very long time (90 seconds)to get a dull red glow out of the resistor. But, the tractor started right up(25 degrees f). An hour later I tried it again and the resistor would not heat up at all. There was power to the bus bar as my test light indicated (my volt meters' battery died so I didn't use that to test). I did a resistance test on the glow plugs just before trying to start the tractor (new plugs). They seemed fine, I even bench tested one plug to make sure it would heat up.It heated. I assume that I again have a bad plug? Any sure fire way to test the glow plugs without subjecting them to 12 volts? Continuity test or a specific resistance figure? I think I am getting closer to the cure, I hope. Merry Christmas. Erik

 

[mopacman]

might try a 6 volt battery charger. If the glow plug gets warm or hot it is ok. If you have a glow plug burnt open It takes longer for the resistor to glow and the other plugs are getting more voltage than they are rated for so they can be damaged easyer......Larry

 

[gemini5362]

If they are on a busbar all three recieves the same voltage that is available after the amount the resistor dropped. What you are saying is confusing. If the resistor heated up it has to have a current path to ground somewhere. if not through the glow plugs did one end of the resistor go straight to ground ?
Also just to satisfy my curiosity with the switch in the normal run position not in start or not to the left are you getting a voltage on the busbar. I am starting to wonder if the wireing is messed up and you have voltage on the glowplug while you are running the engine. I could see that being a problem.

 

[Hoka Hey]

Larry, I do have a 6v charger and will try that...didn't even dawn on me. Gemini, yes there was a path straight to ground. If you remember my tractor came with a light bulb in the resistor location. When the tractor genius 'refurbed' the tractor he probably didn't want to buy a new resistor. So, he wired the glow plug wire straight to the ingnition switch. This gave him 12v to the glow plugs when he used the switch to heat the plugs. This works when you hold the key all the way to the left and when the starter is receiving power, but not when the ignition switch is in the on position. The ignition switch is theoretically functioning the way it should. But, tractor genius also wanted a glow plug indicator, so he had a hot pigtail left over (the one that is supposed to run to the upsteam side of the resistor) so he stuck that on one side of the light bulb socket. On the other side he needed a ground so he neatly spiced in another wire to the wiring harness(looked pretty good until I dug into the harness a little ways). Voila, a light bulb lights up when you hold the key to heat the plugs. Problem is, when you stick a resistor in there the glow lugs still don't realize a drop in voltage and the resistor quickly burns up. Buyers should be VERY carefull when dealing with this dealer and rebuilder. The dealer told me the it was my problem even though he advertised as an excellent running tractor etc. The tractor never started on its own since the day they delivered it.

 

[Hoka Hey]

I am trying to understand the glow plug system. Now, as I said before, my resistor won't heat up and the same goes for the glow plugs. I pulled each plug and hooked them up to a batt charger on 6v...no heat, then I switched it to 12v...nothing. There is resistance in each plug as the ohm meter says, 60 ohm? I am getting about 12.5v at the bus bar (+/- .5v). Does this mean that all plugs are again bad and the resistor can't heat up because there is no resistance with bad glow plugs? Please help me understand how this system is supposed to work.
It seems that if you have a plug burned closed then all juice goes straight to ground and fries your resistor. But, if you have a plug that burns open there is not enough resistance for the resistor to heat up properly and it will then supply too much voltage to the two good glow plugs, eventually burning them up. Is this how it works? Thanks. Erik

 

[mopacman]

You are thinking right but saying it wrong. Zero ohms is low resistance or a dead short. 60 ohms is high resistance or more to the open side of things. It has been awhile sence I looked it up but I think a glow plug should be 1/2 to 1 1/2 ohm resistance. A resistor will not drop voltage with out a load on it, the more load on it the more voltage it will drop. you can figure this out with ohm's law. If you have 3 glow plugs and 9 volts on your bus bar with 12 volt battery suppling power this is good. If you have one plug go open you will have 10 volts on the buss bar. If you have 2 plugs open you will put 11 volts on the last plug. When the last plug burns open it will show 12 volts on the bus bar. So it looks like your plugs are open again. If one plug burns closed it can fry your resistor. Their are 5 volt plugs available for this engine make sure you are getting the right ones. I hope this helps........Larry

 

[gemini5362]

ll juice goes straight to ground and fries your resistor. But, if you have a plug that burns open there is not enough resistance for the resistor to heat up properly and it will then supply too much voltage to the two good glow plugs, eventually burning them up. Is this how it works? Thanks. Erik

ok Erik the very very first thing i would do in your circumstances is go to an automotive supply store and get an inline fuse holder I would put that inline with the wire coming from my resistor to the busbar. (make absolutely sure that the wire going to the busbar is coming from the resistor. If you have to put in a new wire.) Now then if the previous poster was correct about the glow plugs haveing around an ohm ( I dont have any idea but that probably is a good number) then you need to figure out the current so you know what size fuse to buy. The three glow plugs hooked like you have them are a parralel circuit that is in a series circuit with the resistor. I know you might not know what this means but bear with me for a second. resistances in paralel are always smaller than the largest resistance. If your three glow plugs have a resistance of 1 ohm each then in the parralel circuit they have a resistance of about .33 ohs effectively. Resistances in a series circuit are always larger than the smallest resistance because they add in a series circuit. so if your resistor you bought was a 5 ? ohm resistor then that in series with the glow plugs will give you a circuit that is equivalent to a 5.33 ohm resistance. with Ohms law says that current is equal to the voltage divided by the resistance or 12 divided by 5.33 or 2.25 amps. I would put a three amp fuse in there and see if it blew out when you tried your glow plugs. if it holds then run on it and see what happens. I am sure by now that buying glow plugs is getting expensive and with the fuse you might have to buy a few 20 cent fuses instead of glowplugs.

I almost forgot I agree that your new glow plugs are probably bad now. I would measure the replacements and see what the ohm reading is before I put the new ones in just to have some baseline data.

 

[mopacman]

The resistor is about 1/12 of an ohm. This is to small to check with an ohm meter. At 1/12 of an ohm this would put the amprage at 30 amps or more. The ohm reading will increase when things get hot. This makes it hard to figure out the amp draw on this circuit..Larry

 

[gemini5362]

The resistor is about 1/12 of an ohm. This is to small to check with an ohm meter. At 1/12 of an ohm this would put the amprage at 30 amps or more. The ohm reading will increase when things get hot. This makes it hard to figure out the amp draw on this circuit..Larry

Are you sure about the resistance of the resistorthat small a resistance is not going to drop much voltage across it. I= e/r ( 1/12 ==.08, 12/.08= 150 ) If it is that small there is almost no impact on the current so your current will be what is drawn by the three glow plugs. would you not use a resistor that was in the 3 to 4 ohms range then you would limit the current to the glow plugs at about 3 or 4 amps tops even if one of them opened up ?

 

[mopacman]

Three 1 ohm plugs hooked in parralel is .33 ohm as you said and 9 volts on the buss bar. The glow plugs have to drop the 9 volts. I = 9/.33 which is I = 27.27 amps. This makes the resistor drop 3 volts at 27.27 amps. R=3/27.27 which is R = 0.110 ohms. I did not do all the math the first time just gessed at it. When the plugs and resistor heat up the resistance will go up a little. you have to use more than 3 or 4 amps to have enough watts to heat the air in the cylenders 4 amps X 9 volts = 36 watts. That is 12 watts per plug, not enough heat. 27.27 amps X 9 volts = 245.43 watts of heat. That must be enough to work.........Larry