Bypass resistor on 12 volt conversion

Mark-Mi · Mar 26, 2015

I did a 12 volt conversion on my Farmall H from the link below a couple years ago but it starts poorly and seems to start best when I let off the starter button. So I ran a wire from from the lug on the starter to the output side of resistor to give the coil full 12 volts at cranking. But adding the wire back feeds power to the starter when I pull out the ignition switch.

I think I need A diode in that wire to the starter, but I dodn't know anything about diodes.
I did add the 1N4001 on the alternator but dodn't if that would be heavy enough in this situation.

I know I could put a toggle switch in the wire but did not want to go that route.

Thanks, Mark
12 volt conversian

Ed Hotaling · Mar 26, 2015

A diode acts like check valve for electricity. The schematic symbol looks something like -->l--. The way the arrow points is the way the current can flow. If it's not doing what you want it to, you have it hooked up backward.

Bob · Mar 26, 2015

That diode is rated at 1 Amp forward current, not enough to power the ignition system.

Trouble is with using a diode, the forward voltage drop across the diode will drop the voltage by .6 Volts, which is counterproductive to what you are trying to do. An alternative would be to use an "ice cube relay" activated by voltage to the starter that would switch full battery voltage to the coil during cranking.

john *.?-!.* cub owner · Mar 26, 2015

You need a diode rated a minimum of 2 amps, since a coil with the points closed, and the engine not turning will draw that much ( it becomes less when coil starts firing), and the band end goes toward the coil to prevent current from feeding back. The older Chevys used a similar setup, but the starter solenoids had a separate terminal that wired to the coil for that purpose.

old · Mar 26, 2015

Why didn't you just jumper across the resistor??

pete 23 · Mar 26, 2015

Just put a diode rated for more than you anticipate coil to draw which is about 4 amperes max. I make up my own concoction for a diode on alternator and ign systems. I have several old diode bridges from alternators laying around so I knock one off of the bridge . Solder (very carefully so as not to overheat diode) a wire to each side of it and then just hook a test light in series to determine direction of current flow. It will work just fine with no noticeable voltage drop through diode. One thing you have to keep in mind is that the engine will fire up without ign switch turned on while starter is energized. It won't stay running when starter is disengaged but it might catch someone off guard.

Mark-Mi · Mar 26, 2015

Jumping across the resister I would need a switch in the wire. I stated I did not want a switch. Also I have the resister hid up under the hood so it looks original.

RUN-N-LATE · Mar 26, 2015

Mark, I just rewired my 140 and had to put a diode in I used 3amp 400 volt whitch was over kill but works fine, I went to Radio shack and they have them 2 to pack for like 2 bucks. Also on the diode there is a little white stip around one end, that goes pointed to alternator, Hope this helps..... Thankd Mark in NC

Charlie M · Mar 26, 2015

The resistor you currently have is only to cut voltage to the coil. If you have a 12 volt coil then you should have no resistor. To have both would cut the voltage too low to fire the plugs enough and would create a starting problem.

Mark-Mi · Mar 26, 2015

Bob, Is there a simple relay you would recommend.

Mark-Mi · Mar 26, 2015

I think some are getting confused because I mentioned the diode I put in at the alternator.

I want a heavier one to put in the coil wire. Thanks, Mark

John M · Mar 26, 2015

Thats exactly what it sounds like to me too, or theres other problems.

Mark-Mi · Mar 26, 2015

No, I have original coil. Original wiring. Have everything under hood to remain stock appearance.

Charlie M · Mar 26, 2015

Sounds like you are set up correctly. Your starter should be getting 12 volts. Are you saying the tractor doesn't crank well or it cranks but doesn't start easily. Its possible for the resistor to go bad but I think if that happens you don't have any spark. Happened on my H last summer. Also how good is your battery. If a battery gets weak it may crank the tractor but doesn't have enough power to crank and give adequate spark.

Mark-Mi · Mar 26, 2015

Battery spins her over like a top. Just trying to figure away to get 12 volts to the coil at start up.

Charlie M · Mar 26, 2015

My H and M are both converted like your tractor and they start faster than I can get my hand off the starter button. Do you have a voltage meter. Easy check to see if you have 6 volts coming out of the resistor going to the coil. If you don't have a volt you may want to just change the resistor. Is your points and everything in good shape? If you have to put 12 volts to the points for much time you'll be burning up the points and you shouldn't have to do that - something's not right.

Bob · Mar 26, 2015

GOOGLE "ice cube relay", there's a million sources.

techinny · Mar 26, 2015

I just converted to 12v also, put a 12v coil on it and ran 12v from my toggle switch to the coil. The guy that I go to for starter/alternator repair gave me a diode to put in line with the excitation wire to the alternator so it would shut off, but I do not know the specs on the diode. The excitation wire comes off the positive side of the coil obviously with the diode in between there and the alternator. Ran a 10ga wire from the big terminal on the alternator to the amp gauge, then a 10ga wire from the amp gauge to the battery side of the starter button. I'm thinking you need to get a 12v coil and get rid of the resistor.

mkirsch · Mar 26, 2015

Rather than bandaid a bandaid with another bandaid, might I suggest that the source of your problem may lie with your "original" starter.

If the starter is in good condition it won't draw excessive current, leaving plenty of power for the coil to make a good spark.

Mark-Mi · Mar 26, 2015

Starter been rebuilt. Spins her like a top as I said below.

I can jumper the resister in cold weather and it starts great.

All I wanted to know was what to get for a diode. Only one or two people answered my question.

Mark-Mi · Mar 26, 2015

John, thanks for replying how much the amp draw is. I will figure it out from here.

rvirgil_KS · Mar 26, 2015

What is the resistance across the resistor? I've seen several resistors with too high an ohm rating.

Fierce Sparrow · Mar 26, 2015

I've done several hundred conversions like this in my lifetime over the past 50 years. "Bob" has the easiest and most dependable approach. By direct wiring from the starter lug to the output side of the resistor, you will achieve 12 full volts for starting. But, once the engine is started, the 6 volts out of the resister will "back feed" to the starter lug causing excess amperage through the resistor. This will prematurely cause resistor failure and leave you stranded. True, a diode will solve the problem, but I've seen diodes fail too. Some burn out, and you will have no current flow in either direction. Sometimes the diode fuses and provides current both ways essentially creating a solid wire, which you have now. An "ice cube relay" will essntially act as the old car starter solenoids did. When you activate the starter, the solenoid closes sending full twelve volts to the output side of the resistor. When you disengage the starter, the solenoid opens and there is no chance of "back feed". Good Luck

Ferris Boy · Mar 26, 2015

(quoted from post at 12:06:32 03/26/15) Battery spins her over like a top. Just trying to figure away to get 12 volts to the coil at start up.

Mark your not using an internaly resisted coil so you more than likely fryed your coil, but you will not be able to get away from a swich because if the points in the distributor are closed and you leave voltage to that you will fry your points. but you will probably need to get an internaly resisted coil or get a new coil with an external resistor. also you need to check the condicion of your points you may have welded them together with all that extra voltage remember this ignition system was designed for 6 volts and if you have more than that you will destroy componets in the system

Mark-Mi · Mar 26, 2015

Well spoken Fierce Sparrow. Lol

Jim Guarino Agri-Services · Mar 26, 2015

What alternator are you using? Did you add a diode to the excitation circuit, or does that alternator have an internal diode?. On alternator conversions using an external diode, we typically add a resistor in addition to the diode in the excitation circuit. If your alternator excitation circuit is connected to the ignition circuit as it usually is, and the excitation circuit to the alternator does not have sufficient resistance, there could be enough of a draw on the ignition circuit to lower the voltage at the coil + to less than 5 volts during cranking. I suggest that you check the voltage at the coil during cranking, and also when running. The other thing to remember, is to make sure that the primary ignition lead from the coil to the distributor is connected to the coil -, to match the - ground system.

From your photo, it appears that you are using the original wiring for the alternator. If so, be aware that the inner core under the cloth braid is rubber. I have seen many examples of the inner rubber core crumbling from age inside the cloth braid, leaving the wire insulated by only the cloth braid.
Agri Services

Jon Hagen · Mar 28, 2015

I did that 40 years ago, doing an ignition conversion on a GB Moline. I used a salvaged horn relay for the bypass relay. Worked good using salvaged coil Resistor and horn relay off a 55 chevy.
Amazing thing is that not only did the tractor start better, but point life about doubled from the old Moline straight 12V setup.

Dave S. · Mar 28, 2015

Remember, if you run a wire from the starter switch to get 12V
for ignition the engine can start without the ignition switch being on.

Bypass resistor on 12 volt conversion

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