FFA Project/Super M Wiring problem

I'm an Ag. teacher in Tennessee, and my students recently finished a Super M restoration--they did 98% of the work, and did a great job. In redoing the wiring harness, we followed a plan from this site that referenced a SM 12-volt conversion. After doing everything (or so I think)by the schematic and directions, the tractor does start and run, but won't shut off. The only difference between this tractor and the schematic is the use of a three position switch instead of the "push-pull" switch to shut it off. The tractor will not start with the switch in the off position, but it will continue to run once started with the switch off. Where the switch is marked "coil" the coil wires are attached, and where the switch is marked "AMM" the wire coming off the ammeter is attached. The third position marked "AC" is empty. I ran a continuity test on the switch, it check outs as being open in the off position, and showing a closed circuit in the on position (.2 ohm is what the multimeter read as I remember) Any ideas? Thanks in advance for your time and help, my kids and I appreciate it.

Bryan, a couple things. You say coil WIRES to the switch, there should only be ONE wire on the switches IGN (to coil) output and that wires to the coil input (NOT to distributor) terminal. The BAT input side of the switch gets hot batetry voltage from the ammeters Load (NOT battery/starter) terminal.

Next in cases where a GM 10 SI 3 wire alternator is used and its fed its excitation voltage (to alternators lil No 1 side terminal) from the coils input (or switches to coil IGN output) THE TRACTOR CAN STILL RUN WITH IGN OFF CUZ THE ALTERNATOR IS BACKFEEDING THE COIL.

The way I prevent that (run on even with IGN off) is by placing a 10 ohm current limiting safety resistor in series in the wire/circuit from the coil or switch to the alternators lil side No 1 terminal. Many people instead use a diode there which also works cuz it, as a one way electric current check valve, allows current into the alternator to excite it BUT not out the alternator to backfeed the coil. Its has to be wired correct polarity remember, - cathode end towards alternator (for Neg ground) .

PS if you used a GM 10 SI alternator or most any other alternator for that matter they require Negative ground remember. Also if a tractor was Pos ground and you change to neg ground the ammeter leads need reversed and the coil wires also reversed.

John T

It sounds to me that you have magneto ignition which needs a ground to kill it which I think was in the push in position. CENTAUR

I just restored an M for My high school Senior project and was told by my ag teacher that Chevron Delo has a competition for a scholarship for FFA tractor restorations. You can enter the group restoration or individual restoration competitions. You can enter after you have done the resto. I think it is due in September and it is for $2500 I beleive so you should definatly look into it. Its worth a shot. Any questions just shoot me a post.

Bryan - Congrat's to your students (and yourself!) on completing the restoration!

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If you used a Delco 10SI alternator for the 12 volt conversion John T hit it on the head: You need a warning light, resistor or diode in the circuit between the ign switch and the alternator #1. Without it the alternator will "backfeed" sufficient current to the coil after the switch is opened to cause it to fire and thus the engine to continue to run with the switch "off".

Punch below for a diagram.
Super M alternator wiring

the "ac" terminal is more than likely for accessories. it is a seperate circuit from the ignition terminal. you can probably eliminate the need for a diode, resistor, or warning lamp by connecting that small wire (that would be for the warning light,etc) from the alternator to the "ac" terminal.

karl f