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Discussion Starter #1
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! :good2:
 

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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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Discussion Starter #4
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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Did these tractors have a neutral switch in the start circuit? I did not see one in your diagram :unknown:
 

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Discussion Starter #7
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?
 

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Discussion Starter #9
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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Did these tractors have a neutral switch in the start circuit? I did not see one in your diagram :unknown:
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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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.
Having both positive posts of battery attached to starter solenoid will give more cranking amps to starter. Only reason JD added a starter relay was to increase amps on yellow starter solenoid exciter wire. If ones tractor starts good then relay isn't required or needed.
 

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Yes. 1967 diesel synchro. I bought a kit. Instructions seem fairly easy if my original harness was in good shape. Turns out I don’t have one. Everything has been ripped out. Instructions say brown and blue wire should go from alternator to starter. Question is: do those wire go directly to starter or is there anything in between? Circuit breakers etc. this is what I’m working with so far.
732855
732856
 

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Discussion Starter #16
They go directly from the alternator to the starter, nothing in between. The reason why they were both there originally was to serve both halves of the 24 volt system, meaning the two separate 12v circuits on the tractor. The brown and blue wire went to the voltage regulator which was connected to the generator. You basically are just extending those two to connect to the new alternator. It makes a little more sense to see how they were used when looking at the original 24v wire diagram.

I used the same diagram you posted as a reference when redoing ours. I found it much easier to just rip it all out and start from scratch vs, trying to figure out whats what with the original rats nest that had broken/bare wires every where. I bought all new wiring of the different colors/gauge sizes I needed, a box of heat shrink crimp connectors, some braided wire loom, and some weatherpak connectors. Made it all myself based off the wire diagram. With all new wiring you do not need that extra starting solenoid. It's all nice neat and tidy now and everything turned out great in the end.
 

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Your explanation makes a lot of sense now. I ended up buying a whole new wiring harness cause ALL my wires were shot. Waiting for it to arrive.
Good to hear your project worked out. Thanks for the reply.
 
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