John Deere 4640 problems Electrical System

John Deere 4640 problems Electrical System Problems

Last year I bought a 1978 John Deere 4640. The tractor works fine and the electronics work fine (e.g radio, lights, gauges EXCEPT for the Voltage Gauge which never worked). The problem is the batteries are losing charge, caveat: I don’t know my electronics.

This is where I am wrong. The tractor includes 2 John Deere Strongbox 5D 6-V batteries. They are connected in series, so I think 12 volts are connected to the starter. But when I check the voltage going to the starter, it’s 24 volts. Does it make sense? How does (2) 6 volt batteries connected in series produce 24 volts on a voltage meter?

While the tractor was running, we checked the voltage coming off the alternator, which shows a voltage of 24+. I have read a few articles about John Deere having 24V systems, but found nothing to do with the 24V JD4640, and in fact the tractor specifications I found show that the JD4640 should have a 12V system (6.6V batteries).

Am I missing something or does it make sense 24 volt (2) 6 volt batteries are dropping?

The failure to charge the batteries is a key factor. If there is an open circuit between the alternator and the electrical system, the voltage may and will be too high. The system is not regulated. Either the wire is disconnected, which detects system voltage, or the alt output is not connected to the batteries. There can only be 12V in the system when it is turned off. When operating, 14.2 to 14.7 are common numbers for the entire battery.

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