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Farmall & IHC Tractors Discussion Forum
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field circuit resistance on Farall B

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dads b

03-30-2008 20:55:34




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Can any of you guys tell me where I could find a field circuit resistor for my 6 volt B? It goes in the switch box and causes a reduction of the generator output when the lights are off so the battery is not overcharged and damage is done to the generator. If I can't find one, besides always running the lights, what should I do? Also, What is the correct type and amperage of the glass fuse in the switch box? Mine is burnt out. I am just rebuilding this tractor and had the generator repaired and tested. The damage to the fuse and resistor was from before the restoration started.

Thanks!

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Gauger

03-31-2008 10:05:35




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 Re: field circuit resistance on Farall B in reply to dads b, 03-30-2008 20:55:34  
Are you looking for the one in the cutout relay or the one for the light switch. The hole centers are different. I have a perfect one for the cutout relay



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dads b

03-31-2008 16:34:07




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 Re: field circuit resistance on Farall B in reply to Gauger, 03-31-2008 10:05:35  
Gauger,

I am not sure what you mean by the cutout relay but if you look at the Pic Bob added, you can see the field resistor that I need. I also need the dimmer resistance coiled wire if you happen to have a good one of those.

Thanks,

Jeff



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Gauger

03-31-2008 18:04:05




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 Re: field circuit resistance on Farall B in reply to dads b, 03-31-2008 16:34:07  
The cutout relay is basically a solenoid type switch which was used on the early tractors before voltage regulators. It's function is to disconnect the generator output to the battery when the engine is not running to prevent battery discharge. In the absence of a voltage regulator, the method used to control the generator output was to restrict the ground leg of the field coil by means of the field resistor. Without lights, the generator "F" terminal is either wired directly to ground (under the mounting screw of the cutout relay) which provides the high output of about 10 amps, or it is wired to the "F" terminal of the cutout relay which in turn runs through a 2.8 ohm resistor on it's way to ground. This is the low output, around 4 amps. With lights, they simply moved the field resistor to the switch where the operator can select the output without moving any wires. The resistor has the same rating whether it is located in the cutout relay or the light switch but the physical size differs between the two. I have one from the relay which won't fit the switch because it is slightly too long. Sorry for the long answer. Geoff

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dads b

04-01-2008 20:43:52




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 Re: field circuit resistance on Farall B in reply to Gauger, 03-31-2008 18:04:05  
Geoff,

Thanks for the long answer. I always learn something from you guys. The one I need is on the light switch so it sounds like the one you have will not fit. Thanks anyway.

Jeff



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ScottyHOMEy

03-31-2008 05:56:45




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 Re: field circuit resistance on Farall B in reply to dads b, 03-30-2008 20:55:34  
The fuse is a 10A.

How far shot is your field resistor? The one on my BN wasn't working. With it in place, I had no continuity across the hollow rivets that anchor it. I cleaned it up with electrical cleaner and a SOFT brush. (The core of it may look like mica, which would be brittle, but it's actually a flexible, fiber strip.) Looking, poking and testing some more, I got good continuity when checking between the very ends of the winding, but still none when checking with the probes on the terminals, as if the windings weren't connecting well to the rivets. A little more cleaning and a touch of solder at each end, where the winding goes under the rivet and voila, a functioning resistor.

Did I mention the smoke? Despite all the cleaning, the first time that resistor heated up, I thought I was going to smoke everything in the box! It was fine, though. The smoke subsided when I switched to high charge, and came back on L when I switched the resistor into the circuit. Letting it cool in between, I ran it maybe three cycles of five minutes each before it burned off the residual crud. I did pull the face of the panel afterward and used some low pressure air to blow away any dust or ash, and it's been working fine since.

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dads b

03-31-2008 16:31:10




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 Re: field circuit resistance on Farall B in reply to ScottyHOMEy, 03-31-2008 05:56:45  
Scotty,

Thanks for all the info. Unfortunately, my resistor is burned through completely. I'm glad you were able to resurrect yours.

Jeff



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dads b

03-31-2008 16:28:37




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 Re: field circuit resistance on Farall B in reply to ScottyHOMEy, 03-31-2008 05:56:45  
Bob, Thanks for the pic. I see that I also need a coiled wire resistor for the light dimming system. I appreciate your suggestion about the V/R but this is just a parade tractor and it is WAY over budget already. If I can't find an original resistor, could I go to Radio Shack and get a 4 ohm resistor and put in that location? If so, could I do the same for the lights? What ohm? Sorry for so many questions but you guys are my only good resource.
Thanks,

Jeff

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Bob M

03-31-2008 05:10:37




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 Re: field circuit resistance on Farmall B in reply to dads b, 03-30-2008 20:55:34  
third party image

Here's a backside view of a switch as on your B. The field resistor is the wirewound unit at the bottom. Seems to me the correct resistance is 3 or 4 ohms - however I'm only going from memory here...

However if it were mine I'd replace the cutout relay with a voltage regulator. The V/R will do a much better job of maintaining the battery, and it will eliminate the need for the field resistor.

I believe the fuse is an SFE 10 (glass tube type, 10 amp). However later letter series and Supers used an SFE 20 (20 amp) fuse - that's what I recommend.

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