I never had a chance to work on a 24V 2 cyl, but worked on many of the new generation 4 and 6 cyl tractors. It was a cheap way to make a 12V / 24V combination system, but it did not work very well, especially as it aged.
As the starter wore a bit, it's innards got a coating of conductive brush dust, and those internals were electrically hot to chassis ground 24 hours a day.
That would put a constant slow drain on whichever battery that was connected to the starter case terminal next to the engine block. If the tractor sat unused for a few days, that battery would be partially discharged while the other one was still at full charge. When you started the tractor, the charging system would immediatly start recharging the low battery with the full one hooked in series with it. This lead to short battery life, as one was constantly having it's fully charged guts boiled to death, while the other one was always low and being ruined by sulphation. Having to replace both batteries every 1-2 years was normal, more often every year.
In our dryland prairie state, where wheat was king, the most cost effective way to a low cost, high capacity wheat harvest was using a pull type, PTO driven combine. With those combines being PTO driven, they had no engine or self contained electrical system, they pulled electrical power from the tractor.
Harvest was when the electrical thing got really ugly for the 24V JD new generation diesel tractors. The most popular combines of the day were the IH 914 and the JD 6601 windrow machines. Both needed aprox 15-20 amps at 12V to power the bank of work lights and the big magnetic clutches that started / stopped the feeder and unload systems. The electrical supply for those combines had to be tied to one battery of the 24V JD. You could not wire a SPDT switch to to alternate this 12V electrical load from one battery to the other as the harvest day wore on, because the magnetic clutches on the combine had a diode wired across the leads to absorb that big voltage spike generated when you switched off power to that clutch. switching to that other battery with it's opposite ground wiring would cause a dead short across the diode that would instantly blow the fuse. ( atleast you hoped it blew the fuse instead of the diode.)
With the average day of the combine pulling power from that battery all day and several hours into the night with all the lights on, you could be nearly certain of one thing the next morning, the combine had sucked one battery dead and the 4020 would not start. The most common crutch if the tractor / combine had to stay in the field, was to lock the header in the raised position and park the tractor / combine where you could pull start it with another tractor the next day. After a month of this, the one battery was overcharged to death and the other was terminally near dead and sulphated. New batteries were usually required to get the tractor reliable starting for fall tillage work.
When JD finally got over that PITA 24V system and switched to a chassis ground 12V system in 69 ? All those problems went away.
I would estimate that 95% of the pre 69 new generaton diesel's have been converted to 12V and are far better for it. 12V loads are no longer a problem and batteries average a half dozen years life. About the only 24V tractors left in the area, are the ones used strictly as a loader tractor in winter, and do start a bit better.
The most sucessfull, remove the starter yearly for a cleaning to remove brush dust and /or a rebuild. You need to keep the generator and starter free of brush dust to eliminate the parasitic deain on that one battery, and never ever let that fiber thrust washer at the commutator end of the starter wear out. If that washer wears too thin, the commutator bars touch the starter end cap and put a dead short on one battery. The moment this washer wears too thin is when the little jumper wire DOES go poof !! Most have a fuse or automatic reset breaker inline to protect that wire. When that wire burns off, you lose headlights, oil light and fuel guage.
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Today's Featured Article - The Day Mom Drove the 8N - by Brian Browning. My Dad was wanting to put in a garden but couldn't operate the 8N and handle the old horse drawn plow he had found and rigged up to use with the tractor. Well, he decided to go get Mom out of the house and have her drive the tractor while he walked behind the plow. You got to understand that while my Mom is a hard worker who will always help whenever she can... she had never operated farm machinery before that day. Dad got her out there, explained how the clutch was the same as in our o
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