4020 24v charging question

[perzaklie]

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.
Thanks,
William

 

[wore out]

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.

 

[perzaklie]

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!

 

[wore out]

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.

 

[perzaklie]

The SN is above 91000

 

[perzaklie]

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

 

[perzaklie]

Ok, it's now August 2024 and still no charging light.
I tested the output of the generator at the starter at 1200 RPM and the voltage is 29 volts.
There is a resistor on a pigtail from the lamp socket inline to a gray wire.
I checked the resistor and it is zero Ohms.
I am guessing this gray wire is the wire from A1 terminal of the generator.
I made up a 10 watt, 50 Ohm resistor and put it inline. Still no light.
I checked the voltage on the wire from the switch to the lamp with switch on and it is 24.93 volts.
I checked the gray wire that is connected inline to the resistor then to the lamp and it is zero volts.
I ran a jumper wire from A1 terminal and it measures 24.92 volts.
Are these the correct voltage readings? I was thinking they should be in the 12 volt range.
I started the tractor and checked the voltage from the switch and it reads 8.9 volts, engine running 1200 RPM!
I checked the voltage on the gray wire, engine running and it reads negative 4 volts, engine running 1200 RPM!
We polarized the generator, which is new and also replaced the regulator with a new OEM part.
Can anyone help us with this?

 

[JD Dave]

You are throwing a lot of symptoms at us so I suggest you get a schematic (tech manual) of the 24 volt system and make sure everything is hooked up properly. Also, a tech manual will give you test procedures, step by step. Twenty nine volts is too high in the system. Check the regulator. For no charging light, - did you check the light bulb?

 

[Jim.ME]

less than $10.00 will get a download copy of the JD service bulletin SB268 for the JD 24 volt system, from the Green Magazine.

 

[perzaklie]

Thanks all. Did I check the light bulb? That was the very first and most obvious thing to check.
Buy a copy of the service bulletin? Almost exactly what my granddaughter suggested. "Pawpaw, can't you get a book to tell you?" "Sure darling, but there are lots of helpful folks on various forums eager to offer their expertise, so I'll give them a try. We might find a quick and simple solution and get this thing back to work". "Farmers are always wanting to help each other, you know like we do around here".

So thanks all, I'll figure it out myself.

 

[Tx Jim]

no text

 

[wore out]

"Twenty nine volts is too high in the system."

No, it's NOT, it would be normal.

 

[wore out]

Thanks all. Did I check the light bulb? That was the very first and most obvious thing to check.
Buy a copy of the service bulletin? Almost exactly what my granddaughter suggested. "Pawpaw, can't you get a book to tell you?" "Sure darling, but there are lots of helpful folks on various forums eager to offer their expertise, so I'll give them a try. We might find a quick and simple solution and get this thing back to work". "Farmers are always wanting to help each other, you know like we do around here".

So thanks all, I'll figure it out myself.

Sorry, I didn't notice that you had posted back with a serial number.

From your voltage readings, it seems you are missing the ground from the midpoint of the batteries to the chassis.

This should include a 20 Amp slo-blo fuse or be a length of fusible link wire of approximately the same rating (shown as "Ground" next to the batteries).

With the midpoint ground jumper in place, looking at the key switch connections, the brown wire (2) should be +12 Volts (nominal) with respect to the chassis and the blue wire (1) should be at -12 Volts (nominal) with respect to the chassis.

With the key switch "on" the pink "idiot light" wire (9), (which becomes gray at the ignition switch connector) should be at -12 Volts(nominal with respect to the chassis.

The other pink "idiot light" wire (10) should show the came voltage as/continuity to the :GEN" terminal on the voltage regulator.

Once all the above check out the "idiot lamp" should work!