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Need Resistor for 12v Conversion - Farmall H

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Horse-George

02-03-2008 21:46:08

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My Farmall H came with an AC Delco (chevrolet) alternator. It always worked well until about 6 months ago when it quit charging. I took the alternator to an auto shop who tested it and said it was working fine. It turned out the problem was the resistor, which was molded into the plug that plugs into the alternator. I was able to buy a new plug from the auto parts store, but they did not know what to use for a resistor. A local mechanic told me to install a standard dome or tail-light bulb in the wire, to replace the resistor. It worked fine, but I have been thru 3 bulbs in 6 months. Not from burning out, but because of the location next to the engine block, they bang around and break. I was just replacing a spark plug today, and the ratchet hit the bulb, and once again a broken bulb.

I need to go back to a resistor, these bulbs are not working. Can someone please tell me what I need for a resistor. What OHM, and what WATT. I am familiar with electronic parts, so dont hesitate to get technical.

Is there something specifically made for this application? Is this something I can get at NAPA, or another auto parts store? Or maybe Radio Shack? I mostly just need to know the resistance and the wattage. Also, are they a carbon resistor or a wirewound (or don"t it matter)?

Thanks

Horse-George

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Jossette

02-07-2008 16:32:25

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Re: Need Resistor for 12v Conversion - Farmall H in reply to Horse-George, 02-03-2008 21:46:08

i use chrysler resistors. cheaper and do good

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horse-george

02-10-2008 15:04:37

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Re: Need Resistor for 12v Conversion - Farmall H in reply to Jossette, 02-07-2008 16:32:25

Thanks for the info and the picture.
Just curious, why do you have 2 resistors in series?

As far as guy that said the points last longer when a resistor is used on the coil, I am curious why? I don"t have one, and have not changed my points in 5 years. Just curious.

George

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Jossette

02-10-2008 15:18:03

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Re: Need Resistor for 12v Conversion - Farmall H in reply to horse-george, 02-10-2008 15:04:37

I put 2 in series because the points do not need a lot of volts. This set-up only puts 3.5 volts to the points and the alternator will never backfeed and drain the battery. note that they are 2 different resistors

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mkirsch

02-04-2008 14:33:44

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Re: Need Resistor for 12v Conversion - Farmall H in reply to tractorsam, 02-03-2008 21:46:08

What I did was replace the fuse in the electrical box on my H with an LED from Advance Auto Parts. It was mounted in a chrome enclosure that fit right through the hole, and looks right at home there.

Now I have an "idiot light" for my alternator. The light stays on until I rev up the engine enough to excite the alternator, then it goes out. If we turn the idle down to where it's barely loping along, the idiot light flickers.

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Horse-George

02-04-2008 17:07:18

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Re: Need Resistor for 12v Conversion - Farmall H in reply to mkirsch, 02-04-2008 14:33:44

I had thought of putting a light (12V incandescent bulb) on the control panel. But would have to drill a hole. The fuse on mine controls the lights, and if by chance they shorted, I"d prefer it be there. Guess I never thought about a LED.

Thanks

Horse-George

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Bob

02-04-2008 15:18:58

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Re: Need Resistor for 12v Conversion - Farmall H in reply to mkirsch, 02-04-2008 14:33:44

Several guys who hang around these boards have tried using an LED "idiot light", and it didn't pass enough current to excite the alternator at startup.

You are either LUCKY, or your alternator has a "self-exciting regulator" (one-wire alternator).

In cases where someone wants to use an LED "idiot light", they have used a 10 Ohm, 10 Watt resistor ACCROSS the LED pilot lamp unit to pass enough current to "excite" the alternator at startup.

Just posting this to help avoid someone being FRUSTRATED when their alternator won't excite!

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Brownie450

02-04-2008 11:27:28

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Re: Need Resistor for 12v Conversion - Farmall H in reply to Horse-George, 02-03-2008 21:46:08

Bob--nothing wrong with the diode thing. I Probably shouldn't have brought up the resistor info. Sorry for stepping on your toes.

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Bob

02-04-2008 14:12:38

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Re: Need Resistor for 12v Conversion - Farmall H in reply to Brownie450, 02-04-2008 11:27:28

Sorry for making you FEEL like you were "stepping on my toes". That was NOT the point of my post.

I've had such DARNED GOOD LUCK over many years with the diodes, as you can tell I'm sort of hooked on that method!

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Bob

02-04-2008 10:10:23

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Re: Need Resistor for 12v Conversion - Farmall H in reply to Horse-George, 02-03-2008 21:46:08

As George has already figured out in the other thread, there's NOTHING WRONG with using a diode having sufficent ratings.

A diode will not get HOT like the resistor option does, and can be tucked into the wiring harness with no worry about it getting hot and melting wiring if the key is accidently left on, or there's a failure in the charging system wiring or the alternator.

WHY are you guys so hooked on using a resistor, when a diode has positive advantages?

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John T

02-04-2008 07:22:46

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Re: Need Resistor for 12v Conversion - Farmall H in reply to Horse-George, 02-03-2008 21:46:08

George, the resistor you need is for the alternators excitation circuit and as Jim noted IS NOT the same as any ignition coil ballast..... From some GM literature I read once I highly recommend a current limiting safety resistor in the alternators excitation circuit versus no resistance at all and I use a 10 ohm..... While the power rating in watt isnt requited to be as big, if you buy like a 5 to 10 watt 10 ohm resistor they have bigger easier to wire leads n are more heavy n rugged then a smaller watts power rated unit.

The 10 ohms 5 or 10 watt resistior wires in series after the voltage source (ignition switch usually) and before the alternators lil No. 1 side excitation terminal.

An idiot light is probably even way more resistance then a 10 ohm current limiting safety resistor I use n the alternator still receives sufficient excitaion. I dont usually even use a diode since the 10 ohms usually prevents ignition run on which the diode can be used to prevent.

Ive seen em used with NO resistor or diode n they still worked fine but as a conservative I prefer at least somne degree of current limiting protection in the excitation circuit be it a 10 ohm or an idiot light or a diode which ahs a 0.6 voltage drop across it.

Get a 10 ohm 10 watt resistoir from Rat Shack is my advice.

John T

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Brownie450

02-04-2008 06:19:31

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Re: Need Resistor for 12v Conversion - Farmall H in reply to Horse-George, 02-03-2008 21:46:08

I get my resistors from MCM Electronics at Centerville, OH. Use a 25 watt wirewound 10 ohm--10%. Stock #48-10. Comes 5 per bag for $4. Of course, shipping & handling are going to kill you. Looking on e-bay the other day--saw wirewounds 10 ohm/75 watt 10% for less than $1---. I usually mount the resistor on the wiring harness or a convenient flat bracket near the alternator using a pair of plastic/ nylon cable ties. Back side of the alt is preferred mounting site to keep away from fan air movement. Use soldered connections for good/ dependable connections. Have done probably 20 to 30 of these conversions, & this seems to be the best route. Use strain relief when wiring to the resistor.

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Bob M

02-04-2008 05:07:53

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Re: Need Resistor for 12v Conversion - Farmall H in reply to Horse-George, 02-03-2008 21:46:08

Autozone #AL795 or NAPA #13ICR are both good numbers. (A few days ago I paid $6.99 for an AL795 at Autozone for one of my tractors...)

I don"t know offhand the resistance/wattage. However resistance should be approximately equal to the resistance measured across the coil primary . And I"d guess the resistor is rated for about 25 watts or so.

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Janicholson

02-04-2008 06:47:55

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Re: Need Resistor for 12v Conversion - Farmall H in reply to Bob M, 02-04-2008 05:07:53

With respect, he is in reference to the proper resistor for the exciter circuit, not the coil. A diode is a good answer with 3 amp 1000piv rating as indicated in another post. The band end of the diode should be on the end attached to the alternator end of the wire. JimN

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Bob M

02-04-2008 06:59:37

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Re: Need Resistor for 12v Conversion - Farmall H in reply to Janicholson, 02-04-2008 06:47:55

Oops... Thanks for the correction Jim!

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r4etired

02-04-2008 04:24:03

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Re: Need Resistor for 12v Conversion - Farmall H in reply to Horse-George, 02-03-2008 21:46:08

I always use a block resister going to the coil otherwise points don't last long. Later tractors had an additional terminal on the switch that would allow 12volt start then run thru the resistor,

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