The "Feedback Switch" is for completing the alternator exciter circuit while the engine is running and is located on the left side of the dash above the "Key Switch". The dash mounted Alternator Indicator Lamp is a number 1815 bulb (3,000 hour) in a Radio Shack number 272-0325 lamp base. With the indicator lamp connected in parallel with a Radio Shack number 271-0135, 100 ohm, 10 watt resistor the + voltage to ground reading on the # 1 terminal on the alternator with the engine not running and the key and feedback switch on is 1.8 volts. Well within the 1 to 2 volt recommended range. The alternator kicks in at 600 RPM.
Note that the alternator terminal # 2 (voltage sensing circuit) is connected to the load side of the ignition switch. The most common practice is to connect this circuit to an unswitched "remote voltage source". The reasoning for this was so the alternator internal regulator could sense the voltage at a remote point where it was most critical that the voltage be maintained at an optimum level. This made sense in the "muscle car" days when boom boxes and other voltage sensitive accessories were common but in an 8N the ignition circuit is by far the most voltage sensitive location. It makes sense to me to sense the voltage at the beginning of the ignition circuit and I am taking it a step farther by connecting to the load side of the ignition switch. By connecting to the load side I will be eliminating one more potential current leaking point.
Note that terminals 6 and 7 are both connected to the unswitched positive side of the battery.
Tape on wire marker numbers are available from local wholesale electrical suppliers.
If I were doing this conversion again I would move the terminal strip to the left edge of the fire wall because there is more space between the metal and the steering gear box behind that location.
The wiring from the starter relay through the ammeter and to the terminal strip and alternator* is two #12 wires in parallel. I paralleled two #12s because I didn't have access to #8 AWG wire.
The ignition coil resistor at this point in time is a 1.7 ohms ( RU504 ) and the coil (NEW HOLLAND E8TF12029BA) primary resistance is 1.6 ohms for a total of 3.3 ohms in the coil primary circuit. This is subject to change. I say this because the impedance of the coil dictates the amount of resistance that is required to be in series with it. Simple, right? --------- wrong. It is more trial and error than it is science because the people that make coils don't think the impedance of their coils is something the consumer needs to know. They keep that information close to their hearts. Most other types of transformers come with their impedance marked on the label but not gasoline engine ignition coils. Impedance, simply put, is the total opposition to the flow of current in a circuit. This includes pure resistance, inductive reactance and capacitive reactance. You can measure the pure resistance in a circuit while it is not energized but this does not tell you the total opposition to the flow of current while the cicuit is energized. Capacitive reactance and inductive reactance kick in when the circuit is energized with an alternating or pulsating current source.
* The alternator is an O'Reilly "ULT 01-0039. For a 79 Chev. truck, C+K 10, 20, 30 P/U, V8-350, 5.7L" .... With that description it should be a Delco 10SI "A" circuit type 100, 205 series IR/EF alternator. This information is relevant when replacing the internal regulator. 10SI internal regulator information.
SEQUENCE OF OPERATION ....... Starting engine: Turn gas on .... Put gear shift in neutral .... Engage clutch .... Set throttle lever at 50% .... Turn key switch on .... Pull out feedback switch .... Pull out choke .... Engage starter button .... Adjust choke.
Stopping engine: Turn key switch off .... Push in feedback switch .... Turn gas off.
Thanks to all the people that have given me access to their research and ideas. In particular I want to thank Ken LaRue, "John the Old Tractor Guy and llamas" and the Ignition Coil Simulation Tool By Bowling & Grippo .
This post is not a "how to" set of instructions on 6 to 12 volt conversions. It is about how I converted my 8N, a conversion that evolved over a period of several months. It is my hope that it may be of use to others that are not afraid to think outside of the lines.
" Far better is it to dare mighty things, to win glorius triumphs, even though checkered by failure... than to rank with those poor spirits who neither enjoy nor suffer much, because they live in a gray twilight that knows not victory nor defeat."
Update ...... 12-12-08 --- This conversion has been in place for a year now. The tractor starts fast, runs good with a load on my 5' brush hog and will idle smooth at 400 RPM for an indefinite period of time. The distributor point gap was set with a dwell meter at 55° and has not been changed. After running for several hours the coil is barely warm to touch.