Question for John T retired electrical engineer

I am working on a '52 B John Deere for a friends cousin. It has the Delco distributor, but the coil was shot. (open between primary terminals) I had a 12 volt coil that I put on. The tractor runs fine with the 12 V coil, but doesn't want to fire up using the starter. Will I need to get a 6 V coil for this tractor? I have been starting it using a flat belt and another tractor, it fires right up using this method, but I am thinking the starter robs too much voltage for the coil to give enough spark to start. This tractor hasn't ran since around 1980.

Are your sure the coil is still getting voltage when cranking?

Lots of older setups had a second wire feeding the coil, that worked only when cranking and also bypassed the resistor.

I would check the voltage on the primary side of the coil while cranking first to find out what is happening. And yes it may just be too low, or NOT be there at all. This will tell you how to proceed.

To throw more variables... the voltage test could tell you the starter is dragging and or the battery is weak or have too small of cables... but proof will be the voltage test at the coil.

Is your ignition terminal switched with the accessory terminal on your keyswitch thus no power while cranking, have seen it before

Low primary voltage during cranking.

Are you using voltage dropping resistor in series with the primary circuit to reduce the voltage at the coil to the proper level? If so, you will need to bypass (short out) the resistor during cranking. Such ignition start circuits were common place in automotive systems during the era.

Alternatively, you will need to replace the 12V coil with a 6V coil.

The above assumes that the tractor still uses a 6V battery.

Dean

Unlike some later models, I doubt that tractor was originally equipped with any ballast resistor and ballast by pass system (hey a guy could add it) which did improve especially cold weather starting. If its a 12 volt tractor and a correct full true 12 volt tractor, a B isn't that hard to crank over, and should start fine with 12 volts and a 12 volt coil. IS THE COIL WIRED AT CORRECT POLARITY??????????? For + Ground as original, coils + wires to distributor.

HOWEVER IF IT DOESNT the cause may be:

1) Starter is dragging battery voltage down too low, which could be caused by a weak battery (have a shop test and load test it) ,,,,,,,,,,,bad loose burned battery or starter or ground cables (Remove, clean and wire brush and reattach each and every battery and starter and ground cable connections),,,,,,,,,,,,too small cables,,,,,,,,,,,,,,,Ground may be to a weak wimpy rusted sheet metal component like a battery box (try a good frame ground connection or even run ground cable down to starter) ,,,,,,,,,,,Starter motor is defective and draws excess current.

2) Defective resistive ignition switch contacts reducing coil voltage. Try hot wiring the coil direct to hot ungrounded battery voltage source and see if it improves.

If you put a voltmeter on the coils input with ign on and points closed, it should read near battery voltage. If its like .5 to 1 or more volts less, ign switch or wiring may be bad/resistive. If you put a voltmeter on the coils input and crank her over it may drop to 10 or 11 volts (depends on engine and compression and temperature and battery and starter condition) BUT IF IT DROP MUCH BELOW 10 VOLTS THE BATTERY MAY BE WEAK OR CABLES OR CONNECTIONS OR GROUNDS ARE BAD. I like at least 14 gauge wire to and from ign switch and down to coil to reduce voltage drop.

I suspect low coil voltage while cranking perhaps from a bad resistive ign switch or a weak battery or too small cables or bad cables or connections. It don't hurt if the points are good and clean and gapped correct and plugs and plug wires are good. How do the plugs look???? If you want to use a 6 volt coil and ballast and ballast by pass that can help, but Id check the above first.

Sure there's more possibilities, but chew on this a while

John T

This tractor is a 6 V system, I'm thinking that it will require the 6 V coil to give enough spark while cranking. Correct?

(quoted from post at 11:25:02 07/21/15) This tractor is a 6 V system, I'm thinking that it will require the 6 V coil to give enough spark while cranking. Correct?

Click to expand...

hinking correctly, as 6v coil will have a lower primary resistance than the 12v coil. Lower resistance = higher current = higher energy.

If the tractor has a 6 volt battery you should use a 6 volt coil to get enough spark.

A ballast resistor is used on a 12 volt system with a 6 volt coil, the resistor drops the coil voltage to 6 volts so it won't overheat.

A ballast resistor with a starting bypass can also be used on a 12 volt system with a 6 volt coil. In normal operation the resistor drops the coil voltage to 6 volts. During starting only the bypass delivers 12 volts to the 6 volt coil for an extra strong spark.

Others, please correct me if I got this wrong, Thanks.

Old dune buggy and chopper wiring trick- run a wire with a one way diode from the starter relay or main starter terminal to the coil. Starter draws down current voltage when used - but the wire to coil will let whatever is at starter go to the coil especially if bypassing resistors and ballasts. The diode prevents coil current from feeding back to starter after engine starts. Your 12 volt coil may have internal resister with a ignition bypass terminal or might be a 'external bypass' 9 volt actual that really needs the 12 volts from battery and can't work right with the 6 to 7.5 volts of 6 volt battery cranking the starter. Maybe find a old Bosch 'Blue' coil that was 6 volt with a extra set of windings to start VWs in Russian winter weather. will need a metric mounting clamp or a layer of beer can metal to shim if replacing a Ford or some Chrysler coils. RN

Had the same problem with a B. I bought a 6v tractor that would not start with the starter. Got a heck of a deal because of it... Got it in the shop and started working on it. Fortunately one of the first things I checked was the coil. It was a 12v coil. Changed it out for 6v and it solved the problem.

Like yours, my B ran fine with a 12v coil. I just would not start with the starter, but it would pull start.

Dale if the tractor is still 6v negative like it was from the factory it needs a 6v coil. No fancy resistor or bypass system needed, just a plain old 6v coil. Power from the key or switch to the - side of the coil. + side of the coil to the distributor and you should be good to go. I have never thought of a b being a good starting tractor anyway.

Thanks to all that replied. I was pretty sure that it needs a 6 Volt coil, but a little advice never hurt anyone, especially me.

CORRECT, if its only a 6 volt tractor YOU NEED A 6, NOT A 12 VOLT COIL. Id still check the cables and connections and grounds and battery, however.

John T

You're welcome, Yep, while a 12 volt tractor can use a 6 volt coil with an external series voltage dropping (12 to 6) ballast resistor, OR a 12 volt coil unballasted, a 6 volt tractors needs a 6 NOT 12 volt coil for best performance, especially starting.

John T