Massey Ferguson Coil

[Roger37]

Trying to find out the correct ignition coil for my MF-35, S/N 216167, 12Volt Delco distributor with clip style cap. My confusion pertains to coils stating "use with external resistor" or "use without external resistor". What is this resistor and where is it located? All the tractor parts places are of no help so I"ve come here to the experts as I have before! Any input would be appreciated, Thanks!

 

[Bob]

Do you have a coil on there at all now?

If so, read the battery voltage on the ignition switch side of the coil with the switch "ON", points CLOSED.


If you are seeing full battery voltage, you need a 12-Volt coil marked "NO External Resistor Required".

If you are sseing only about 6 or 7 Volts at the coil, there IS a resistor somewhere between the switch and the coil, and you need a coil that is marked "Use WITH external Resistor".

 

[Jerry/MT]

NAPA part No IC-14 SB (~$15) 12 Volt coil has the internal resitance built in and needs no external reistor. Do not buy a 12 volt coil that requires a separate resistor in series with the coil. It's just adds another potential failure mode.

 

[Bob]

However, with an EXTERNALLY-ballasted coil, you can have a "starting bypass" for the resistor, giving a LOT hotter spark when cranking.

 

[David, Surrey UK]

(quoted from post at 03:44:09 08/20/07) However, with an EXTERNALLY-ballasted coil, you can have a "starting bypass" for the resistor, giving a LOT hotter spark when cranking.

Yes, the idea of shorting the resistor is to compensate for the battery's voltage drop when the starter is cranking, particularly in cold weather. The voltage drop is due to the high current of the starter flowing through the battery's internal resistance. The terminal voltage can be 6V or less for a 12V battery.

Bypassing the resistor provides more current to the coil during starting and results in a decent spark at the plugs. During normal running the battery's being charged by the dynamo or alternator, and iits voltage rises up to somewhere around 14V. The series resistor reduces the current to the coil so it doesn't overheat.

David

 

[Jerry/MT]

True, but then you add some more failure modes.

 

[David, Surrey UK]

(quoted from post at 00:52:10 08/21/07) True, but then you add some more failure modes.

True, but everything in life is a compromise! :wink:

As one wise power supply engineer told me, "The only component that can't fail is the one you designed out of your circuit". :lol:

David

 

[Roger37]

Well I'm back...found out it wasn't the coil as I switched it with one from another MF-35. By the way, it was an internal resistor coil as I had 12 volts at the ignition switch side of the coil with points closed and key on. Still don't get any spark from the coil (held the coil wire 1/4" from ground ). Also, when I put my coil on the other tractor it started right up. Someone told me it's the regulator, but I thought that only had to do with the charging system not the primary starting system? It turns over real strong, just has no spark. Ideas?

 

[David, Surrey UK]

(quoted from post at 01:44:38 08/23/07) Someone told me it's the regulator, but I thought that only had to do with the charging system not the primary starting system? It turns over real strong, just has no spark. Ideas?

An electromechanical voltage regulator has two coils; one (the cutout) effectively acts like a diode to prevent the battery discharging through the dynamo when it isn't putting out enough, the other (the voltage regulator) stops the battery voltage from going too high and overcharging.

So it probably isn't that, assuming you have 12V powering everything else on the tractor, e.g. lights.

Since your coil works fine in the other tractor, we can eliminate that. I would look at the low voltage ignition circuit. Check that the points are making and breaking properly (0V across them when closed, and 12V across them when open). Check the condenser (capacitor) which is connected across the points by replacing it or just disconnecting it for a short time, as this is a common cause of failure in ignition systems. As always, check all your ground connections.

David