4x4 engagement problem

Seems like a strange coincidence to me, the 4WD ECU doesn't connect to the OBD system. Check the voltages at the 4x4 ECM pins in the same mode both with and without your tech stream plugged in to see if something is causing a voltage perturbation possibly.

 

The only thing I can think of is there's a problem with the sensor ground circuit, most scan tools short sensor ground and chassis ground together at the DLC when they are plugged in.
I think I would put a voltmeter across pins 4 and 5 of the DLC with the key on, if it reads more than 100mv you have a problem in the sensor ground circuit. It's just odd that it's only affecting the 4wd system and not other things as there's alot of stuff on that ground.

 

I’ve got readings between 500 and 1900mv when bridging those pins. You’re onto something!

 

Now the fun part of finding the broken/corroded wire, I think I'd first go directly to that ground point on the cylinder head and check there.

 

I was hopeful when I found the ground as it was a bit crusty but no dice. One positive takeaway is I know how to separate interlocking ring terminals now. Sanded everything clean but the same problem persists.

how many suggestions are you willing to spoon feed me?

 

Did you give all the wires a tug test to make sure none were broken internally?

 

Nothing much to contribute here except to say what a great thread this is. I may never have this problem but if I did, the diagnosis is all right here. OP: You're closing in on the solution here: don't give up! Frikkin' grounds!!! I've had bad grounds that would cause dimly turned on tail lights when you stepped on the brake... even with the main light switch OFF! Crazy. My 2014 has a fair number of grounding points that look really sketchy: especially under the hood. Only SEVEN years old! My 94 year old Dad was looking over the truck when I first got it and one of the first things he said was: "Clean up all of these grounds." The whole summary you posted: "plug this into that, remove USB but keep it plugged into the OBD...reboot the laptop: " FIXED! ". (Being facetious here of course...) I didn't understand it, but it was fascinating.

 

proper grounding is more important now than ever on cars given the amount of low voltage micro chips and sensors that are needed to run these machines reliably.

 

When does the blinking 4x4 light come on?
Even in 2WD?
Is it both the 4WD and the 4Lo lights?

 

For sure, and there's multiple ground points for various things throughout the vehicle. Vehicles that live in the areas where the roads are salted in the winter tend to have alot of problems with external ground wires corroding away when they get to be a few years old.
One broken or corroded wire can cause alot of headaches and misdiagnoses, that's why its always a good idea to check circuit integrity before replacing parts.
The video in the first link of my sig is a good example (was a shorted wire instead of an open but same principle applies).

 

So true. My F-I-L has had Toyota's for years and with only a few exceptions here and there they have been rock-solid reliable. He busted my chops for years for buying domestic. But the frame rot issues and oil return passage issues on his V6 Camery really threw him for loop. I showed him the various frame and engine bay ground points and his response was: "Your father is right.".

God Bless the Greatest Generation.

 

yes both 4hi and lo lights flashing at all times in 2wd, no mechanical engagement possible

 

I pulled on the 4 chassis grounds in the stack, and all the wires coming into the t-Case actuator, but they’re all wrapped in loom with multiple branches so I feel I’d just be pulling on the electric tape inside binding them all together, unless it happened to be broken a few millimeters away from the ring terminals or amp connectors

 

Most broken wires occur within a few inches of the terminals, rubbed spots and critters are 2 other common causes of open circuits. Those grounds also go through the junction box behind the glovebox just to the left of the Engine Control Module, you could unplug those connectors and look for corrosion there too.

 

Well I’m running again without my
OBD port closing the ground. I never could figure out where where the wiring break was; just installed an overlay to add another ground to my 4wd limit switch. This has solved the problem but that means everything else on the EE ground could have an issue. No other problems for the moment except my vehicle speed sensor code P0500. My speedometer and rest of cluster works tho?

 

That code can be hard to diagnose.
The good news is, you are half way there to diagnosing since the speedometers is working.

See here for a lot of good info. Mare sure to read post #13 & #15 for a good explanation of flow.
https://www.tacomaworld.com/threads/speed-sensor.702360/

 

So these component systems do not share the same grounds so the issues should be entirely unrelated? Basically listening to myself talk. Except that both wire harnesses pass behind the dash and that’s probably where the short exists in both cases?