Posted by JohnD [173.245.56.71] on Sunday, June 26, 2016 at 20:33:15 :
In Reply to: Also... posted by Greg Coffin [172.68.35.20] on Saturday, June 25, 2016 at 17:03:53 :
Greg,
Have you ever had an engine that wouldn't start until the instant you stopped cranking the starter? It is caused by low voltage at the the coil caused by the high amperage draw of the starter. It's ironic, but the coil won't work at the very instant it is needed the most to start the engine!
Coils run on 9 volts, not 12. all coils need a resistor to drop the operating voltage from 12 to 9. The resistors are internal to the coil, or external to the coil. Avoid internal resistors on these old trucks because they prohibit you from bypassing the resistor during starting.
Our starters aren't gear reduction starters like modern cars use. Our starters draw a LOT of amps, and the 12 volt system can drop to 10 volts or so when cranking the starter, especially on cold mornings. The 10 volts is BEFORE the resistor, so AFTER the resistor you may only have 6 to 7 volts at the coil PLUS terminal. This is not enough to spark the spark plugs. Terminal 2 allows you to bypass the external resistor temporarily during cranking. This puts 10 volts into the coil instead of 6 or 7 as explained above.
Terminal 2 usually comes from the second small terminal on the back of a starter solenoid. It is only live during cranking, and its sole purpose is to send battery voltage directly to the coil PLUS terminal during cranking. It bypasses the external resistor, but everything restores to normal after cranking.
Our old trucks don't have starter solenoids, so you need to run a small 14 gauge or so wire from the foot pedal start switch directly to the 12 volt PLUS terminal on the coil.
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