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John Deere Tractors Discussion Forum

late model 4020 wiring

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Author  [Modern View]

06-15-2020 18:44:32

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Bought my first late model 4020 last fall.
Put a clutch in it this spring.
That went well, but the electrical stuff is not my strong suit.
There does not seem to be any power to the accessory circuit No lights in the gauges, no power at the lighter, fuel gauge does not work, and only half of the 4 headlights light.
Any suggestions, thoughts, ideas would be helpful.

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06-17-2020 19:01:31

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 Re: late model 4020 wiring in reply to farmer4020tom, 06-15-2020 18:44:32  
Ser #224457R

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06-17-2020 18:59:47

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 Re: late model 4020 wiring in reply to farmer4020tom, 06-15-2020 18:44:32  
Making progress. There were 2 wires on the back of the key switch which were not making contact.
One was the power for the accessory circuit.
The other was a ground, I think. LTG-36

Now have instrument lights. Now have oil and electric indicator lights. Air cleaner light still not working.
Cig lighter is disconnected. Terminal is gone, wire still hanging there.
Driving lights are incorrectly wired. Going to have to redo them. Seems that the wire colors are faded, and then someone sprayed green paint all over them. So that will be fun.
Thanks for the help.
Oh, fuel gauge still doesn't work, but it may be the gauge.
Sender was recently replaced, and there is now current, so I'm betting the gauge is bad.

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wore out

06-17-2020 07:00:18

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 Re: late model 4020 wiring in reply to farmer4020tom, 06-15-2020 18:44:32  
Quoting Removed, click Modern View to see

Accessory circuit gets it's power from the "A" terminal on the keyed ignition switch. Looks like it passes through the the upper of 3 circuit breakers, then to the lighter and fuel gauge.

There's a wiring harness connector involved in that circuit.

The light switch uses two separate battery feeds, through the lower 2 circuit breakers to "BAT A" and "BAT B" on the light switch. One of those terminals not being powered would explain half of the lights and the gauge illumination lights not working.

With keyed ignition switch "ON", check for power at it's "A" terminal, then at the group of 3 circuit breakers.

The upper breaker (if no one has switched them around over the years) should have "ACC" power at both sides, the other two breakers should have battery (unswitched) power to both sides to feed the headlight switch and both the "BAT A" and "BAT B" terminals on the light switch should have power.

If you need more help, post back.

Also, there's some serial number breaks in the "late 4020" years as to the wiring. It would help to know the serial number.

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06-16-2020 19:55:10

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 Re: late model 4020 wiring in reply to farmer4020tom, 06-15-2020 18:44:32  
Went and got some loader parts this evening, so didn't have time to do more investigating tonight.

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06-16-2020 19:53:40

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 Re: late model 4020 wiring in reply to farmer4020tom, 06-15-2020 18:44:32  
I have the JD Tech Manual, and wiring diagrams.
There is no power to the accessory circuit.
Trying to figure out where that circuit gets it's power from?
Checking the bulb holder on the tach, no power to it,

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06-16-2020 15:35:17

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 Re: late model 4020 wiring in reply to farmer4020tom, 06-15-2020 18:44:32  
Accessory circuit: you'll have to chase the circuit with a test light or a multimeter. It may help to remove the toolbox sheet metal and the floor pan (under the operator's feet); this will give you access to most of the wiring from the dash back. There's a big connector for the rear harness under the operator's right foot.

No lights in the gauges: There is one bulb which provides light for all the gauges through fiber optic cables. The housing for the bulb almost resembles an octopus under the dash.

No power at the lighter: could be a bad wire to the lighter, or the wiring harness "modified" and the lighter bypassed. Just have to look at it.

Fuel gauge doesn't work: on the Modern View, you can search this forum and find lots of helpful advice addressing non-functioning fuel gauges, including how to ground the sender. You'll also find information covering matching the resistance of the sender to the resistance of the gauge.

Half the headlights light: I would suspect the non-functioning lights have lost the ground path, as they ground through the fender, through the mount, and then into the axle housing. First, I would check to see if the lights are getting voltage to the terminal leading into the light housing (it's fast and easy and tells you quickly whether the issue is a voltage issue vs bad bulb/poor ground.) It could also be a bad light switch, because the two outside lights connect to a different pole on the light switch. Is it both lights on one side, the two inner or outer lights, or none of the above?

Let us know what you find.

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06-16-2020 14:04:12

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 Re: late model 4020 wiring in reply to farmer4020tom, 06-15-2020 18:44:32  
You can find deere service manuals on fleabay at reasonable price or on this site as well. But the jensales copies are not in co'ors and you can find some in colors on fleabay.

After you got your schematic, it is very easy to start from battery, then main circuit breaker by starter, then small ones under dash, then fuse on red wire, the key switch for power. Takes less than 5min usually to find problem. All the second gen have same harness around dash. I have bought cheap 7020 harnesses under dash at salvage yards and put them on 4020 and 4620 that were messed up.

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Bob Bancroft

06-16-2020 04:42:41

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 Re: late model 4020 wiring in reply to farmer4020tom, 06-15-2020 18:44:32  
Assuming that's still the case- JD has been very good about offering service manuals for individual models, right down to the serial numbers. They are very complete. I have found them indispensable for working on my John Deeres. Aftermarket manuals are not complete. With some other companies you have to buy a volume of manuals for many models at once, for several hundred dollars. I hope JD hasn't gone that way.

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Tx Jim

06-16-2020 02:59:18

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 Re: late model 4020 wiring in reply to farmer4020tom, 06-15-2020 18:44:32  
Welcome to YT

I suggest to check wiring bulkhead at RH side of firewall for good connections.

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