John Deere 4020 24 volt problem October 22, 2014 12:41AM
Friend asked me to take a look at his tractor complaining that his lights dont work. Took my multimeter and found that the majority of his bulbs were blown, and with the light switch on, 24+ volts are going to the bulbs through his light switch. The light switch has two posts on it for battery "a" and "b" however, one post reads 24volts while the other one reads nothing. I get the same readings on the two fuses mounted a little further down the panel. 24 volts traveling through the upper one and nothing through the lower fuse. Both fuses have continuity through them. The tractor is a late model 4020, and has a small fused link between the common battery cable and the chassis. This fuse blows instantly. Any ideas what might be wrong?
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Re: John Deere 4020 24 volt problem October 22, 2014 12:50AM
All late 4020's were 12 volt, he didn't put the wrong batteries in did he.
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Re: John Deere 4020 24 volt problem October 22, 2014 01:25AM
Could the batteries be mounted in series rather than parallel?
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Re: John Deere 4020 24 volt problem October 22, 2014 02:05AM
He has a 24 volt generator. Puts out roughly 26 volts while running at mid rpm range. It is a positive ground system. Two 12 volt batteries hooked in series. He had a local shop rebuild his generator, and he reinstalled it himself. The ammeter in the dash has a pair of wires that are on seperate posts that head all the way up front to the generator and regulator, but neither of them are hooked up. Also there is a three wire harness specifically for the generator. It has a brown blue and pink. As I recall, he has the brown wire hooked to the bottom terminal on the generator and the pink and blue are up top. Cant remember what one was on what post. I believe he had a small jumper off of the generator up 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?
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Re: John Deere 4020 24 volt problem October 22, 2014 02:49AM
There is a conversion kit available to swap the 24 volt to a 12 volt system. If you are interested I have one available. Call me at 608-553-1528, Don.
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Re: John Deere 4020 24 volt problem October 22, 2014 01:26AM
The older 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 to happen. You need to find a early wiring diagram and check the wiring. Thank God they stopped using it on the later 4020 Diesel Tractors. A Service Manual would have the diagram, the OM will too. Good luck.
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Re: John Deere 4020 24 volt problem October 22, 2014 09:10AM
Look up a wiring diagram.. there's a small ground wire for one of the batteries that is mostly likely missing or disconnected.

I found this on a quick search...
[www.bing.com]



Edited 1 time(s). Last edit at 10/22/2014 09:11AM by JDGnut.
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Re: John Deere 4020 24 volt problem October 22, 2014 11:45AM
Thanks for the help. Yes, I did the whole quick search thing. I did the in depth search thing, etc. I mentioned in an above post that the little wire you are referring to is present and the fuse blows instantly. All of the diagrams are great, but when they are only labeled as colors, and not numbers- it makes it pretty hard. Also- none of the diagrams shown on your search list any components by name. A person who isnt familiar with Deeres wouldnt know if hes looking at a cigarette lighter or the ammeter by the drawing. Know what I mean?
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Re: John Deere 4020 24 volt problem October 22, 2014 01:29PM
the only place you should have 24 volts is at the starter when the 2 12v com together.everything else is 12v pos or neg so you need to find out what is making you have 24v at the light switch
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Re: John Deere 4020 24 volt problem October 23, 2014 01:34AM
your body ground may be on the wrong post...seen it happen a few times...if you have a cross over cable from right to left bat it needs to be grounded ...if on the wrong post you will have 24v..
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Re: John Deere 4020 24 volt problem October 23, 2014 09:35AM
Yup.. I know what you mean.. These are two out of the old TM... but there are no colors listed... and you probably already found them online somewhere.. Somewhere I have the electrical manual for these... have to look for it..
I'm hoping it will have some colors listed..


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Re: John Deere 4020 24 volt problem October 23, 2014 12:34PM
Appreciate the help JDGnut. And everyone who has lended a hand in this so far. Thankyou. Any pictures will help me piece the puzzle together.
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Re: John Deere 4020 24 volt problem October 23, 2014 12:44PM
Sir, The fuse blowing is indicating there is a short to ground on one of the 12 volt lines/circuits. Ground one baterry at a time to determine which line is shorted. Most often the short will be found 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 ground wire fuse to blow. When this happens 12 volt lights get thier ground through the other 12 volt battery sending them a 24 volt supply. A quick way to check starter field for short to ground is to simply diconnect battery cable from side of starter motor and leave wire on same connection connected to cable. If fuse does not blow and lights go back to working problem is in starter. These 24 volt systems are not hard to reair/diagnose or work on for a seasoned JD mechanic. Just got to understand how they work. Good Luck, DW
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Re: John Deere 4020 24 volt problem October 23, 2014 01:11PM
As stated above every thing on the system is 12 volts except the starter. 1 battery is grounded neg. the other is pos. ground
The light switch is 2 switches in 1 half the switch works the neg . ground circit the other half works the pos .ground circit . every thing should read 12 volt if you go from the wire to ground. a digital meter will show if you are on the correct circit.
hope this helps. Dane Pickerell
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Re: John Deere 4020 24 volt problem May 05, 2021 02:28AM
I replaced the light switch in my 4020 diesel, Ser#136410. When I have the light wire hooked to the HD2 terminal it appears to want to cook the small resistor on the top. of the switch. Have I crossed a wire or is something else going on here. It has NOT been converted to a 12V system. Any insight would be appreciated. I have the Service Manual, but no mention of the light switch wiring.
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Re: John Deere 4020 24 volt problem October 23, 2014 01:14PM
if your still having problems call me. I have my own shop and I will step you threw what to check. sounds like a breaker on the starter. just fixed a 24 volt system last week thanks scott 417-262-1466
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Re: John Deere 4020 24 volt problem April 20, 2019 07:06AM
My husband is working on a 4020 JD. He has put on a new wire harness. The problem is that when he tries to hook up the battery the tractor acts like key is turned on and the starter tries to start causing sparks to battery. Any clue???
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Re: John Deere 4020 24 volt problem April 20, 2019 12:12PM
If this is still a true 24 volt system, unplug the neutral start switch and see if quits cranking when batteries are connected?? it does check neutral start switch for short to ground. If it is OK then move to starter button / switch to unplug starter energize circuit / wire to see if problem goes away. Let us know what you learn. DW
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Re: John Deere 4020 24 volt problem April 20, 2019 10:43PM
The 69 4020 starting a tS.N.# 201000 was a 12 volt system. .The earlier 24 volt system could be troublesome and there is a kit to update it to the 12 volt system. and a high % have been converted.
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Re: John Deere 4020 24 volt harness July 03, 2022 10:47AM
I want to know how reconnect a 24 volt harness on a John deere 4020 because this harness is a little different than the original one.
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Re: John Deere 4020 24 volt problem July 04, 2022 04:54AM
Only John Deere could create such a disaster as there goofy electric systems. At least someone along the way had sense enough to correct the disaster.
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