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4020 electrical system

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Lee Volp

04-26-2000 07:36:16

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I'm working on the lighting system and have a few questions. The owners manual says the starter system works on 24 volts and the lights work on 12 volts.

Where and how does the 24 volts get stepped down to 12 volts? Is it as simple as taking the point where the two batteries are series (connected) together and grounding it to the frame?

The tractor I'm working on has had the batteries moved from the origional position and thus the wiring has been changed. The starter system works fine and the starter seems to be totally isolated from the rest of the electrical system. It seems that I have the necessary power to the lights but missing the return path through the frame.

Any help is appreciated.

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Richard E. Draper

10-14-2000 21:43:15

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Re: 4020 electrical system in reply to Lee Volp, 04-26-2000 07:36:16

Since the lights are divided equally between the two 12 volt batteries, is it safe to assume that the lights on the left side of the tractor are run off the left sided battery and vice versa for the right side?

I have a 4020 with a light on each fender, lights on extensions just anterior to the instrument panel, sticking out on each side, a tail light mounted on an extension off the middle of the rear of the tractor, and a flood light on the left fender. Here is the problem:

The left front fender light works great, but the front extension lights and the right front fender light are very dim. The rear lights don't work at all.

Another strange thing, the light that burns the strongest is only pulling about 8 volts. The dim lights are pulling only 4 or 5 volts.

Are there shorts in the three dimly lighted lights?

Any help would be appreciated. Maybe I just need to replace all the wiring harnesses, eh? I would appreciate any help on how to trouble shoot this electrical system. Are there any written resources?

Are front "extension" lights supposed to be the bright lights?

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John T (Dave ??)

04-26-2000 09:23:52

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Re: 4020 electrical system in reply to Lee Volp, 04-26-2000 07:36:16

Lee, Ol Dave there wrote the book on the 24 volt systems so I will be very brief. All you basically have to remember is you have two 12 volt batteries connected in series to make 24 volts to the two wire frame isolated 24 volt starter. The Generator is also two wire non frame connected so it supplies 24 volts across the two battery non frame ends so it charges both batteries in series. And, the common connecting point where the two batteries are tied together, happens to be the iron tractor frame. Therefore, the non frame end of one battery to ground is 12 volts positive, while the the other is 12 volts negative. Since most DC loads like lights are non polarity sensitive, the loads are distributed approx equal between the two different batteries. So, any load you connect between one battery non frame end and frame will work at 12 volts. The only thigns that are 24 volts are the starter and generator and they work off the opposite battery ends where 24 volts appears since thats the two 12 volters in series. Ol John T in Indiana. Dave, a few pages back I asked you to confirm my Respectfully Disagrees response to the other electrical engineer who disagreed with me claiming that the high voltage lead in a coil was connected to the coils case frame and NOT to the common high and low voltage leads at + on the coil like I insist. My response to Coil Polarity then my later post Coil Polarity Revisited, stakes out my claim the the low voltage primary and high voltage secondary leads are connected at + on the coil, with the high voltage other end going out the high voltage top terminal and low volt primary other end going out the coil -. Do you agree with my statement or agree with him the High Voltage lead is connected to the coils case which I respectfully as possible claimed is wrong as rain. John T

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Dave Wickman

04-26-2000 08:51:12

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Re: 4020 electrical system in reply to Lee Volp, 04-26-2000 07:36:16

Don,
This is a post I wrote a while back that should answer your quuestions and then some. It is long but I think you will find reading it worth while.
The 24 volt systems used on John Deere diesel tractors were functionally the same whether it was a two cylinder diesel or a 3020/ 4020 diesel. To fully understand and to be able to work on these systems one first has to know and understand some simple electrical systems. The first is batteries and parallel verses series connections. When two batteries are hooked in parallel the two positives are connected together on the positive side and the two negatives are hooked together for the negative side leaving the voltage output of the two batteries the same. Example two 12's stay 12 volt two six's stay six volt or five 12 volt batteries hooked in parallel are still 12 volt. When batteries are hooked in series, whether it be two batteries or five, the positive post of one battery is hooked to the negative of the next battery and the positive of that battery is hooked to the negative of the next so on and on. Each time a battery is added to the (series) circuit the voltage of that battery is added to the system. Example two twelve volt batteries hooked in series will have a combined voltage of 24 volt or four six volt batteries in series will have 24 volt. The second thing to understand is a circuit. Example of a simple circuit would be a power source (battery), a line (wire) from the battery to the load or function (motor or bulb) and a line (wire) back to the battery. I know this all sounds pretty elementary to people who work with electrical systems but this is where the confusion I feel lies in understanding the 24 volt John Deere systems. On most electrical systems on all brands of tractors and auto's the frame or the chassis of the vehicle is one side of the electrical circuit meaning it is the connection from the electrical function back to the battery. On a 24 volt John Deere system all 24 volt circuits are isolated from the frame chassis. That is why there is two isolated lines (wires) to the charging system and two insulated cables from the batteries to the starter. I like to look at the John Deere 24 volt systems and explain them as two individual systems sharing the same battery source. The one system being 24 volt and the other 12 volt. The starting and charging system is a 24 volt system, completely isolated from the chassis of the tractor. The rest of the tractor is a 12 volt system and uses the frame of the tractor as the ground side of all the 12 volt circuits (lighting). This is where it gets interesting. To get 12 volt from the two 12 volt batteries (hooked in series) a ground wire is put in between the two 12 volt batteries or in the center of the four six volt batteries to the frame of the tractor. This ground wire has no effect on the 24 volt part of the system because the 24 volt systems has no relationship to the frame. What it does do is make one battery and isolated 24 volt line 12 volt positive ground and the other 12 volt battery and isolated 24 volt line 12 volt negative ground. When ever power is taken off (one) of the isolated lines and used for a 12 volt function, that function is grounded to the frame of the tractor. Supplying that function with 12 volt. Depending on which line the power is taken from and sourced back to ground, it will be a 12 volt positive or 12 volt negative ground circuit. When ever the two opposite 12 volt polarity lines are used together (starting and charging) you have 24 volt. Being the batteries are charged in the 24 volt series circuit balancing the 12 volt loads on the individual batteries becomes a very important concern. That is why half the lighting circuit is on one battery and the other half on the other battery. To accomplish this sharing of battery loads the key and light switch have both power lines supplied to them so the loads on theses two lines can be distributed evenly. They are not ever used together for any 24 volt functions, rather individually to balance 12 volt circuits. This is the best explanation I can give of how the system works. Good Luck Don!!, Dave W.

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Dave Wickman

04-26-2000 08:55:08

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Re: Re: 4020 electrical system in reply to Dave Wickman, 04-26-2000 08:51:12

Lee, Sorry for addressing you as Don had previous posting in my head yet. Please excuse me, Dave W.

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