Ghud, good questions, those dual hybrid 12/24 volt systems are hard to understand for some. Good info below from Jim and Bob, and I will answer your questions and provide more information. If you ever get a chance and Buick n Deere presents another 24 volt Seminar Workshop like the excellent one I had the pleasure to attend TAKE IT IN !!!!!!!!!!!! he as well as Tx Jim and Bob know their 24 Volt stuff???..
1) You posted ?I?d like to replace the old broken cab lights on my 3020. The wires coming from the switch to the rear of the tractor are reading 24v (positive ground).
FYI the source of 24 volts is the series connection of two 12 volt batteries. It?s used to power the two wire 24 volt FRAME ISOLATED starter motor while the series 24 volt battery configuration is charged by the also FRAME ISOLATED 24 volt generator system. UNLIKE a 12 volt tractor, those gennys and starters have no electrical connection to frame, they are TWO WIRE 24 VOLT ISOLATED UNITS
The 24 volts WAS NOT from the factory Positive ground, its FLOATING with respect to frame. The starter motor and genny system are ISOLATED OFF FRAME and the 24 volts is from the + of one battery and ? of the other as 12 + 12 = 24 WELL DUH.
At the series center tie point of the two 12 volt batteries (+ of one and ? of the other) there?s a jumper wire connected to frame ground, therefore, one battery has its POS connected to frame while the other has its NEG connected to frame DUH?. That?s how two 12 volt sources are derived, one 12 Volt battery is POS grounded while the other 12 Volt battery is NEG grounded BUT THE 24 VOLTS IS NOTTTTTTTTTTTTTTT GROUNDED?..
The lights are 12 volt, some are connected/switched via the A & B (Pos/Neg) fed light switch to the NEG grounded battery with others the POS grounded battery. Obviously if two identical 12 volt lights/loads are wired across the 24 volt series battery connection even without that center tie point frame connection each would still work but NOT if one was burned out and open just like those old series Christmas lights.
NOTE if you?re actually reading 24 volts positive ground it?s not wired correctly BECAUSE if correct you would read 12 volts NEG ground or 12 volts POS ground depending if your fed off the A or B circuit, one battery or the other.
NOTE HOWEVER If you?re measuring across the A & B light switch feeds (One Pos other Neg Grounded) orrrrrrrrr somehow across the series connection of the two 12 Volt batteries THAT WOULD INDEED BE 24 VOLTS
NOTE HOWEVER the 12 volt lights are ONLY 12 VOLTS and fed from one or the other 12 volt batteries, one or the other A or B light switch feeds, one or the other of the Pos grounded battery or the Neg grounded battery???...
BUTTTTTTTTTTTT if wired right there WOULD NOT be 24 volts to frame ground
2) My question is - can I install led lights that are rated for 24 volts?
Even if it would work that?s NOTTTTTTTTTTTTT how I would do it.
3) Also, I thought the lighting circuit on this tractor was supposed to be 12v. Does this sound right? YESSSSSSSSSSSSSSSSSSSSSSSSSSS
YES the two lighting circuits are 12 volt. The lights are fed off the A B light switch which has 12 volt feeds from the POS grounded battery as well as the NEG grounded battery.
NOTICE do NOT take my word for it.. This is NOT my design, BUT
IT IS how Official John Deere Service Bulletins indicate,
IT IS how official John Deere Wiring Diagrams indicate,
IT IS how they were wired from the factory,
and as a used tractor dealer, farmer and electrical engineer who used, worked on, and farmed with those tractors I can tell you THEY WORKED AS DEERE BUILT THEM
SOOOOOOOOOOOOOOO if how Deere built them,,,,,,,, And their wiring diagrams,,,,,,,,,,,And service bulletins indicate is wrong or can?t possibly work that way GO FLAME AND TELL THEM NOT MEEEEEEEEEEEEEE LOL
Hope this helps
John T Retired Electrical Engineer