# 4020 Ignition switch replacement



## fixingMY4020 (Aug 3, 2021)

I have a 1967 4020 diesel that sits outside unfortunately and I got the ignition switch corroded inop. So I bought
another switch from a Deere dealer and now am trying to sort out how the pins are connected. The pins are different on the new switch. I have a Service Manual
SM-2039 that has a schematic that seems fairly close to what my wires colors are, but not in all cases, so thats why I 
am posting here, I am looking for knowledge about what wires are presented to this switch, and also which ones need to be connected for the switch to perform as it should.
I am hoping not to rip open all of the wire looms, just any that I must once I know more about which wires the switch must connect.

I guess I'm looking for someone familiar enough with exactly what this switch is supposed to do to ask these questions to.


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## jd110 (Nov 23, 2015)

I am curious about what part number you got. For a 1967, you should have gotten a R39552. Is your serial number between 119,000 and 145,660? Replacement switches are usually identical to originals so I am not sure why yours is different.


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## Tx Jim (Jul 28, 2020)

I agree with what jd110 stated.


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## fixingMY4020 (Aug 3, 2021)

I looked up the serial number, looks like its on a tag visible from behind on the right side, 
T213R 163810R
Guessing the later part would be serial number.
Sometimes I save receipts for purchases like this new switch, but did not this time, so I can't say what
part number it is. Its not printed any place on the switch.
What I could do is call the JD dealership I bought it from and ask what part number they would have given
me for my serial number, and as a long shot ask if they see a record of my purchase on 1/9/21

This gives me more leads to track, thank you.

I still seek to understand what connections (starter/charging/voltage regulator) the switch would be trying to make.

I know the schematic shows power is supplied to the instrument panel and various lights, those are easier to test
to ring out wiring. But how they enable things like the generator and voltage regulator functions, which I think is something an ignition switch with an "on" mode would be doing. I want to know what they intend to connect, not going
to burn it down with guess work,


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## Hacke (Feb 22, 2011)

fixingMY4020 said:


> I looked up the serial number, looks like its on a tag visible from behind on the right side,
> T213R 163810R
> Guessing the later part would be serial number.
> Sometimes I save receipts for purchases like this new switch, but did not this time, so I can't say what
> ...


Ask the dealership how the switch should be connected. They sold you a replacement, it is not up to you to do detective work to find out.


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## Tx Jim (Jul 28, 2020)

Due to you stating you have SM2039 I'm wondering what else do you need to know about the wiring? JD tech manual I have for 3000 series shows wiring colors & locations on switches/accessories.


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## unsquidly (Jul 13, 2021)

Here is the switch that you should have for your tractor...........This is the OEM just not an overpriced John Deere part........I used this same switch a couple of years ago in my 68 model 4020 diesel.....It is a direct replacement wire for wire.......Not hard to replace.....Just remove the old switch and swap one wire at a time to make the new switch match the old one.....



https://www.steinertractor.com/JDS825-OEM-Ignition-Switch-Key-Switch-includes-2-keys-


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## fixingMY4020 (Aug 3, 2021)

Tx Jim said:


> Due to you stating you have SM2039 I'm wondering what else do you need to know about the wiring? JD tech manual I have for 3000 series shows wiring colors & locations on switches/accessories.


I have lost faith in John Deere producing any manual that accurately describes the tractor I have for two reasons
First they made so many ad hoc changes their manuals can't keep up. And second- previous owners took liberty editing various wires like the starter switch and so on. 

The basic question I've had is what connections does the ignition switch on a diesel 4020 actually make. From what 
I've read about how the voltage regulator is designed it will automatically insert the generator into the circuit to charge once the tractor is running and disconnect the generator once the motor stops. So next test I do will be to ignore reconnecting any ignition switch wires and see if it will start and engage charging. Then from there I will use the 6 blade switch I just ordered from SteinerTractor and follow my notes about wires 1 thru 6, I marked where #1 started but didn't think to record CW or CCW. So thank you for your worthy info.


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## Tx Jim (Jul 28, 2020)

Have you studied similar photos of ign switch wiring? Red wire on ign switch leads to generator indicator light where it connects to a black wire which leads to voltage regulator. Six connections on ign switch so I can't envision the wires being very difficult to connect correctly. Batt A (blue wire) & Bat B(brown wire) plus "S" terminal(yellow wire) only leaves 2 connections one being red wire(generator) & 1 connection(black wire) for headlights.

But I've had many hrs/yrs experience diagnosing/repairing JD 24 volt electrical systems!


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