4wd switch wiring / activation

I am wondering how the 4wd switch works. I'm looking at moving it. I've looked for wiring diagrams and no such luck

It has three wires, blue, gray, and white. In my mind it makes sense to have it as a spdt switch, with it open in 2wd, closed one direction in 4wd, and closed the second for 4wd low.

but as I open the switch up it has resistors and what not on the wiring in th switch. So I Am a little confused.

 

Why don't you just extend the wires and use the original switch?

 

Seeing the feasibility of using an aftermarket switch

 

Looks like there are two diodes within the OEM switch. A multimeter in diode check mode can verify this.

 

How about this "switch" haha

 

So if I'm readying this right
2wd: grey wire grounded, blue open
4hi: grey grounded, blue grounded
4lo: grey open, blue grounded

 

Yup. Only addition is if you measure the voltage at the grey and blue wires in 4hi they will be ~0.7Vdc above ground due to the forward voltage drop across the diodes.

 

Do you think the 0.7vdc matters?

I just wrote out a way using a spdt locking switch, and two spdt relays.

 

It may be an order of operation. Normal is 2WD, gray low. You then go 4WD which is both low and then 4WD Low is blue low. It might not go into 4WD Low straight from 2WD High. There must be an internal pull-up in the ECU so the default line state is high.

Being a diode drop above ground won't matter, it's a digital input. Google logic levels if you want an explanation.

The basic reason is a digital switch recognizes low as a voltage from, as an example TTL logic, zero up to 0.8V and a high from 2.0V up to 5V. If your input voltage meets these levels the switch recognizes it. The problem lies in the indeterminate region between low and high, e.g. 0.8 to 2.0, which the logic could either pick neither or both or worse oscillate between the two.

 

So going off my drawing, it should accomplish the logic for it going in that order to achieve 4lo. Having to go between 2-4h-4l

Closed = to ground

Switch position: 1(Relay 1 triggered): gray closed/blue open

Switch position: 2(no relays triggered):
gray closed/blue closed

Switch position: 3(relay 2 triggered):
Gray open/blue closed

 

Exactly, whether it's switch or relay contacts, the ECU inputs are indifferent as long as you follow (we assume) the order. I think what you've sketched is probably just fine. I'd probably put some small resistance in series with each signal line to current limit (perhaps 10 ohms) just in case there's none in the ECU.

 

Updated drawing, just need the blue on relay one and the gray wire on relay two

 

I should have added the diodes are there to isolate the grey and blue wires from each other in the factory switch. As mentioned above 0.7Vdc should be considered a logic "0". You'll need to make sure your circuit can't get into the "open" and "open" states on the blue and grey wires since the ECU may not know what to set the 4wd system to since it expects at least one signal to be logic "0" (ground). I'll look over your circuit later today.

 

I think you center position (4WD High) needs both relays triggered.

I'd actually like to know what it does with high/high. Would cool if that might be a neutral or 2WD Low or something interesting. Since the order that the transfer case actuator has to move is critical I wouldn't want to try it on my truck though...

 

When cycling power the computer is fine with powering up in the 2 or 4h position but yells at you for 4l, unless you power cycle with the truck in neutral then it will actually go straight to 4lo.

 

Also I should be able to accomplish the same thing is a spdt normally closed switch

 

That's the best switch. So much want.

 

It seems like this should work if the switch is a SPDT 3 position locking on-off-on switch. The switch off position would be for 4hi.

 

Correct, three position locking, closed-off-closed

I actually called idec and found some part numbers for a contact block that is normally closed. So it'll be open-on-open, so I won't have to use relays, simplifies it a lot.

 

More to come