Delco alternators were used on US tractors. They used both external regulator type (10DN) and internal regulator style (10SI) depending upon when the tractor was built. You can tell the difference by looking at the plug with the two small wires. If the plug attaches to the alternator from the rear it is a 10DN series with external regulator. If the plug goes in from the side of the alternator it has an internal regulator and is a 10SI series alternator.
To check a 10DN series alternator there should be battery voltage to the large terminal at all times. The F terminal should have voltage to it with the switch on. Since the power is coming from the switch through a light bulb and/or resistor this will be less than battery voltage. The R terminal should have voltage when the alternator is charging. It is used to close a relay in the voltage regulator which bypasses the dash light and resistor. To bypass the regulator to check for maximum output unplug the field (F) terminal and apply battery voltage.
To check a 10SI series alternator there should be battery voltage at the large terminal and the #2 terminal at all times. With the ignition switch on there should be voltage at the #1 terminal. This voltage should be about 1/2 half battery voltage or more. To bypass the internal regulator there is a tab just inside the D shaped hole in the back of the alternator. Shorting this tab to the case will put the alternator into maximum charge.
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