4 Runner 4wd Inop, what am I missing

Hi, I cant wrap my brain around this system. I have a 2000 4Runner on the bay. It had mil p0171 & ABS light on w complaint of no 4wd operation. We repaired the abs and cleared the mil for p0171 in case it inhibited 4wd, no change. We pulled a Diagram ( attached ) 4 the 4wd and did some tests, The fuses are good and have power the relay has power at pin 1 & 6 as expected. in 2wd the VSV for 2wd is energized and works. this is where it gets weird for me, pin 2 has 12 V all the time from the ecu, we unplugged all switches still 12v so I think the ecu has a transistor which sends sense voltage until certain switches change position and pull it down.
It looks like closing D3 detection switch will cause the relay to power the 4 wd VSV for 4wd operation. the switch is good (tried a new one and tested both new and old )
If I jumper ground to either side of D3 detection switch it works. That black white wire has 12v even disconnected. How, why, where is it coming from? We added a ground to that wire attached to the transfer case and it still does not operate. I know that does not make sense.

So what am I missing on this diagram? what is the grey rectangle at the bottom just after j9 connector? quite a bit about this system has me scratching me head like the shorting bar.

To me it looks like pin 2 (of the relay) has 12v not from ECU, it has it through the relay coil. So when ECU provides ground to pin 2 the relay switches and the 4wd vsv gets its power. And when it switches to 4wd the sensing switch closes and thus providing constant ground to the relay and the light cluster lamp. On the other hand i can't figure out how it is back to 2 wd then. Triangle is a ground point.
Which "balck-white" wire has 12v constant? If it is the white-black one (going to d3-d4-d5) then it can get 12v from D5 when in park or neutral. So if you provide ground to the D3 pin 1 does it work as intended? This can mean you don't have good ground to the D3.

Welcome to the forums! Thanks for providing the diagram, too, love it.

Like borntoroll, I'm very interested in the 12V constant on black/white wire (or is it actually white/black?). It's voltage from D3-4-5 that's looking for a ground, and can't find one. When you jumpered ground to both sides of D3, where was the other end of the jumper connected? Was it attached to the transfer case, too? I'm asking because I believe you're looking at an open ground, and adding a ground SHOULD have worked.

I think it'd be worth adding a ground to pin 1 of D3 again, but this time run some jumpers over to battery negative. This way, you know you're getting a solid connection. If everything now works, then you can go chasing the open ground.

EDIT: Just saw the iATN thread . Glad to see they're thinking the same thing!

the bw wire at pin 1 of d3 switch when disconnected has power with key on. will not light a test light
It works if i back probe pin 1 on d3 and apply a ground to it but not if i use a test light to ground. I scabbed a wire to ground 6 inches from pin 1 and it does not work! Someone suggested that the wire my be broken at the connector which is gonna make me kick myself.

Thanks Tyler. This one is humbling to say the least. I wish i was able to get the logic for the system it would answer some questions for sure. Without it and with our mutual friend Scanner Danner's help I was able to nail that it is a control side problem and "be the switch" but there are some issues with this diagram that have me guessing and thats never good.

Johng17050 wrote: the bw wire at pin 1 of d3 switch when disconnected has power with key on. will not light a test light
It works if i back probe pin 1 on d3 and apply a ground to it but not if i use a test light to ground. I scabbed a wire to ground 6 inches from pin 1 and it does not work! Someone suggested that the wire my be broken at the connector which is gonna make me kick myself.

Oooooh yeah, I like the broken wire suggestion! Or, more like a partially broken wire/high resistance/green corrosion. This would explain how there could be 12V at the white/black wire, BUT it won't light a test light connected to B+ or B-. The load causes a voltage drop, which would also explain why the additional ground 6" from pin 1 doesn't change anything.

Thanks Tyler. This one is humbling to say the least. I wish i was able to get the logic for the system it would answer some questions for sure. Without it and with our mutual friend Scanner Danner's help I was able to nail that it is a control side problem and "be the switch" but there are some issues with this diagram that have me guessing and thats never good.

No problem, I'm eager to hear what you find! It's always unsettling when you see computers involved in a circuit, but don't know their role I haven't looked in Mitchell on this 4WD system, but it sounds like there's very little theory/operation available. For what it's worth, I still think the ECM isn't providing any power/ground to this circuit, but merely watching voltage. Could be wrong

To e it looks like the 'AWD' output of the ECU should provide a ground which would trigger the A18 relay and it also provides a ground source to the D3 switch (4wd position detect) and A10 the A10 (ADD position switch). If the ADD switch is triggered then you have a ground source for the 4WD instrument cluster lamp and also a ground source fed to the ABS ECU (likely to turn of ABS when in 4WD).

If the AWD output of the ECU remains at 12V then you may have a bad ECU or an input to the ECU to tell it that it should switch to 4WD mode is not being made. Switches D3 and D4 would seem to tell the ECU that the transfer case is in a 4WD mode.