John Deere 4020 24 volt problem

John Deere 4020 24 volt problem Problems

A friend asked me to take a look at his tractor John Deere 4020 complaining that his lights are not working. I took my multimeter and found that most of his bulbs were burned out, and with the light switch on, 24 + volts go to the bulbs through his light switch. The light switch has two posts on it for the battery “a” and “b”, however one post reads 24 volts, while the other reads nothing. I get the same readings on the two fuses mounted slightly lower on the panel. 24 volts travels through the upper one and nothing through the lower fuse. Both fuses have continuity across them.

The tractor is a late model John Deere 4020, and has a small fuse between the common battery wire and the chassis. This fuse blows instantly. Any idea what can be wrong?
It has a 24 volt generator. It emits approximately 26 volts when it works at medium speed. It is a positive ground system. Two 12 volt batteries connected in series. He had a local shop rebuild his generator, and he reinstalled it himself. The ammeter on the dash has a pair of wires that are on separate poles that run all the way to the front of the generator and regulator, but neither of them are connected.

There is also a three wire harness specifically for the generator. It has a blue and pink brown. As I recall, it has the brown wire hooked to the bottom terminal of the generator and the pink and blue are on top. I don’t remember which one was on which post. I think he had a little jumper from the generator to the middle post on his regulator as well. Does this sound close to correct? I thought the early 4020 systems were a four wire harness.

John Deere 4020 24 volt system

Older John Deere 4020 diesel tractors had a strange 12 volt system. It was 12+, 12-. The generator has a negative and positive output to the corresponding battery. If someone changed a wire or a switch it could cause bad things. You need to find a first wiring diagram and check the wiring. Thank goodness they stopped using it on later 4020 diesel tractors. A Service Manual would have the diagram, the OM too. Good luck.

Thanks for the help. Yes, I did the quick search thing. I did the deep search thing, etc. I mentioned in a previous post that the little wire you refer to is present and the fuse blows instantly. All diagrams are great, but when they’re only labeled as colors, and not numbers – it makes it pretty difficult. Also, none of the diagrams displayed in your search list the components by name. A person unfamiliar with Deeres would not know if he is looking at a cigarette lighter or ammeter from the drawing. You know what I mean?

A blown fuse is indicating a short to ground in one of the 12 volt lines / circuits. Ground one battery at a time to determine which line is shorted. Most of the time the short is in the starter motor. The starter and generator are NOT grounded to the frame. If a field winding in the starter, starter solenoid, or field winding in the generator is shorted to frame ground, it will cause the ground wire fuse to blow. When this happens, the 12-volt lights get their ground through the other 12-volt battery that sends them a 24-volt supply.

A quick way to check the starter field for short to ground is to simply diconnect the battery cable from the starter side and leave the cable in the same connection connected to the cable. If the fuse doesn’t blow and the lights are working again, the problem is with the starter motor. These 24 volt systems are not difficult for an experienced JD mechanic to repair / diagnose or work with. You just have to understand how they work.

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