4020 Rewire Help
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    4020 Rewire Help

    Howdy, I've been a long time reader on here but haven't actually posted anything I think.
    Anyways, we have a '66 4020 Diesel that I feel the need to do some rewiring on. I was in the process of doing a coolant flush and fix a leak and when I tried to go start it to move, it wouldn't start. I did some looking around under the dash and found out that the wiring situation is in pretty p**s poor shape with cracked and exposed wires every where. The original harness are gone, none of guages are hooked up, no lights, basically just a homemade starting circuit and that is it. The gauges and lights have never worked so long as we've owned the tractor so they aren't really a concern to me, although I am replacing the coolant temp gauge with the current work and I might try and convince my old man to get a new tach as well. As much as I would love to just order all new harness and have a fresh system, that is not in the budget. What I would like to do is wire up a new starting circuit and do it correctly.

    About the tractor, I believe at some point it was changed over to a 12 volt system. There is an alternator instead of generator, and the battery cables are hooked up so that both positives run to the starter, and both negatives are grounded individually to the trans case. The alternator has a one stud and then a small plug with two wires coming out. I do have the service manual and have a fairly decent understanding of how the original 24 system works but then again it doesn't quite help me with the tractor being switched to 12v.

    Could someone help me out with a good wiring diagram to accomplish this? One of the biggest things I don't know to do this is the correct wire gauge sizes to use and for what.

    With the current no start problem I am having, I believe it is the key switch. I have ordered a new key switch and starting push button switch. I know the push button still works because it'll spark when grounded to the dash, and batteries are fine because I jumped the starter and it started up fine. But after seeing the condition of the wiring I want to redo it.

    Thanks for any help!
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    1010's Avatar
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    Wire Harness, Tractor Parts, Harness Repair, Alternator Conversion, Braided Wire

    John Deere Wiring Harness - Steiner Tractor Parts

    I guess you plan on staying with the 12v. Just make sure the folks you buy from know you have upgraded. Ebay has them also.

    Here is a JD parts John Deere Parts Catalog showing a diagram, make sure that's you're serial number, or look for yours on the 4020 JD parts using your serial number.
    Last edited by 1010; 12-10-2018 at 04:08 PM.

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    Technically about the only thing that needs to be changed on wiring harness when changing from 24 volt to 12 volt is relocating brown wire on starter solenoid to the large top post together with blue wire. This is assuming batteries are attached correctly to give 12 volts & alternator is connected correctly. IIRC fuel gauge &/or sender need to be replaced for gauge to operate correctly.
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    Last edited by Tx Jim; 12-11-2018 at 07:38 AM.

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    Right, and that would work except I don't have a brown wire. And I want to rip out what is there and start new. Would the attached diagram be an acceptable way to wire up a starting circuit? Using a #10 wire to go from the alternator to the batt post on the starter, and then #14 wires to go to everything else? I tried to use the factory colors for the wiring in the diagram.

    Another thing, the tractor does not have a starting relay, is that needed? I just see it mentioned a lot when I was searching the forums on how to do this.
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    Yes, that wiring will work for starting and charging circuit. If you want to replace the entire harness, I would recommend brillman.com . They make quality harnesses that are same as factory and they make them already set up for 12 volts or original 24 volt if you prefer. Their prices are very good also.

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    Zebrafive's Avatar
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    Did these tractors have a neutral switch in the start circuit? I did not see one in your diagram
    J
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    Yes I believe they did, there is one in the service manual. Pretty sure it has never been hooked up for as long as I've been around. I'll look into ordering one of those as well.

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    Why hook both batteries directly to the starter? I had my right battery positive hooked up to the positive terminal on the left battery. Which is more likely to turn the engine offer faster?
    John
    '68 John Deere 3020 Gas, '51 Farmall Super C, '56 Loboy, '57 F-Cub, Woods59/Flail Mowers, Front and Rear Grading Blades, Cultivators, Planters, Harrows

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    I don't think it would make much difference with both hooked to starter vs one and having the other daisy chained. Shouldn't matter at all unless your cables were in bad shape.

    Anybody know if it is worth it to install a starting relay? The tractor has never had one and has always worked so not sure if it matters.

    Thanks for the feedback.

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    Quote Originally Posted by Zebrafive View Post
    Did these tractors have a neutral switch in the start circuit? I did not see one in your diagram
    All JD 4020's came from factory with neutral start switch, Syncro-range trans had 1 NS switch & Powershift trans had 2 NS switches

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