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Curtis B.
Curtis B., Technician
Category: Construction and Road Equipment
Satisfied Customers: 1231
Experience:  Technican turned service manager with multiple lines of equipment and rental units.
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JD battery, new alternator and alternator

Customer Question

JD battery, new alternator and alternator diode. not charging at the alternator and only showing 12.2V on display.......12.74V before starting........lights pull the display voltage to 11.7V while running regardless of engine rpm
Submitted: 11 months ago.
Category: Construction and Road Equipment
Expert:  Curtis B. replied 11 months ago.

Hello! Let's see if we can work together and come to a satisfactory conclusion!

Expert:  Curtis B. replied 11 months ago.

The alternator has to have 12 volts to the field coil , to activate the alternator. Is there voltage to the field terminal on the alternator?

Customer: replied 11 months ago.
do you mean with the unit not running?............if you mean while running yes i have 12.2V or so
Expert:  Curtis B. replied 11 months ago.

No, with key on and running, the field terminal has to have battery voltage to energize and start the alternator charge cycle.

Customer: replied 11 months ago.
i have the orig alternator on hand...........the machine is 150 mi away and wont be back to it till next thursday. Any thoughts on a course of action if I:
1- have 12 V at the field term with it running
2- dont have 12V at the F term with it running
its a bosch alternator just wondering which term if the "F" terminal?
thanks i can stay in touch with you if that is possible??
thanks Curtis!
Expert:  Curtis B. replied 11 months ago.

Looking at the alternator, the upper plug or the lower plug has to be the energizing terminal. as the large terminal is the output, which has to read battery voltage, and has to rise when the alternator is energized. Energizing terminal only gets voltage when key is on and and engine starts. Once energized, the output becomes greater than battery voltage, and charges the battery.