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John Deere Tractors Discussion Forum

4020 24v charging question

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11-15-2023 16:15:56

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Here's the problem. My brother brought his 1968 4020 to me with mouse eaten wires. I replaced the damaged portions. He said his charging light had quit working some time ago. I noticed the light blue wire that I repaired went to the charging light and thought that would fix the problem. Nope. I did a bit of research and found that blue wire went to the gen terminal on the regulator. I ran a jumper from that direct to the resistor and still doesn't work. Can someone explain to me how the charging light works in that it is on when not charging and off when charging? Is there something in the charging system that removes the ground or power to the light when the generator is working? I admit I know just enough to be dangerous around automotive electrics but I have never understood how that light works.

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11-17-2023 14:57:46

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 Re: 4020 24v charging question in reply to perzaklie, 11-15-2023 16:15:56  
Finally got a chance to go to the farm and do more testing.

Between A1 and A2 is nothing.

A1 to ground 24v

A2 to ground 25v

Were you able to find the wiring diagram you spoke of?

Serial no. is above 91,000

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11-16-2023 10:26:45

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 Re: 4020 24v charging question in reply to perzaklie, 11-15-2023 16:15:56  
The SN is above 91000

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wore out

11-16-2023 08:27:57

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 Re: 4020 24v charging question in reply to perzaklie, 11-15-2023 16:15:56  

Third Party Image

Here's a simplified version of the charging system.

A-1 is the negative side of the generator's 24 Volt output, A - 2 is the (+) side.
With it charging there should be 28 - 29 Volts measured between A-1 and A-2.

Is the serial number above or below 91,000, I can post the complete diagram once I know that.

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11-16-2023 07:09:26

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 Re: 4020 24v charging question in reply to perzaklie, 11-15-2023 16:15:56  
Thank you so very much! I am embarrassed to admit that I spent 30+ years as director of fleet maintenance for a trucking company and never learned how a charging light worked! Just never crossed my mind. Oh, well. Now I know!

Ok, so he brought the tractor to me because of the light. I have not trouble shot the charging system. He's out on the farm and when he gets back, I'll ask. I did pull the resistor out and meter it with two different quality DVOM's and it was erratic. So I looked up a new one. $76.00 at the dealer! I went through my stash and found some 20ohm and 10ohm resistors. I have put two 20ohm and one 10ohm in series to produce 53ohm. They are 5% resistors. I'll give that a try. One question. What is the proper way to test this 24v generator? In all my years, I never encountered a 24v system. This one has two post on the top and one on the bottom. I traced one wire to the GEN of the regulator and one wire to the FIELD of the regulator and the bottom to a post on the top of the regulator.

If you have a wiring diagram I would appreciate having it. Again, many, many thanks!

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wore out

11-15-2023 21:07:57

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 Re: 4020 24v charging question in reply to perzaklie, 11-15-2023 16:15:56  

To be clear, is the charging system working and charging the batteries in the 28 to 29 Volt range?

If the warning light is the only issue, have you verified the bulb is good and the resistor measures around 50 Ohms? (The purpose of the resistor is to allow the use on a common 12 Volt bulb in a 24 Volt circuit.)
The way the system works is that the lamp is wired across the cutout contacts, if the charging system is working the cutout contacts are closed there's very little voltage drop across them, and the lamp doesn't light.

If the generator isn't charging there's a voltage differential across the cutout contacts that causes the indicator lamp to light.

I hope this helps, if you need more info or a wiring diagram, post back.

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