Must shut off truck to use 4wd.

To set the mood:

2007, 4.0 auto
I use or at the very least engage 4wd once a month.

Recently I've been randomly having to shut the truck off and restart it to engage 4x4 a 2nd time if I disengaged after using.

For instance I engage 4wd, perform whatever task then disengage 4wd without any issues. Then minutes or even an hour later if I have not shut the truck off and try to engage 4wd I get nothing, no light blinking or anything. This is the first bit of research I've done, it's getting dark out and the mosquitoes are too rambunctious.

Anyone have any experience with this issue or something similar? I was under the truck today performing my regular inspections and greasing zerks. I didn't notice anything out of the ordinary, no loose\hanging wires, but I wasn't looking into this issue since it hasn't happened in months.

 

So nothing happens when you move the switch, lights, beeping, or anything?

What about if you turn it to L4?

 

About a month or 2 ago I had a local shop replace my rack & pinion while I was offshore. This is the first time I've had anyone work on my truck, not saying this has anything to do with this current issue.

I've also owned this truck since new.

 

Correct. Once I've used 4wd and disengaged it, if I go to re-engage there is nothing.
If I shut the truck off and restart it 4wd works flawlessly.

 

Very interesting, kind of suggests an issue with the 4wd ECU being you have nothing happen but I'd still want to check the voltages at the 4wd ECU particularly the main Power (IG) (F12 Pin 5 (Violet)) & Ground (GND) (F12 Pin 6 (White/Black)) and 4wd switch signal voltages (2-4) (F12 Pin 8 (Grey)) & (LO) (F12 Pin 15 (Blue)).

Everything on testing the 4wd system can be found in the thread below.
https://www.tacomaworld.com/threads/4wd-system-operation-help-thread.742420/

 

Greatly appreciated.
It's an odd issue. I do have an FJ transfer case in the shed, but I'm not ready for that yet.

 

First off thank you for this amazing write up!
I was so excited to have a starting point with baseline info that I didn't follow the above post. I instead was running down the list from the linked thread verifying voltage. Which when working is 100% correct.
Once I disengaged 4wd numbers are spastic, no 4x4 indicator light or beeping but I can feel the actuator engage. With the indicator not indicating my voltage is 16.5 in all positions 2H,4H & 4L at the 2L pin.

At this time I stopped everything and wanted to start over following this thread.
So I returned to 2H, following the normal procedure, listening to know when I was in 2wd. Shut the truck off and then turned the ignition to ACC. 4wd indicator wouldn't work. Tried this a few times then actually started the truck, when that didn't work I went and unplugged the F12A connector and plugged it back in while the truck was off. Started the truck and everything works properly for the next several tries both in the driveway and driving while going in\out of 4wd. That plug was not noticably loose by any means, rather difficult to unplug honestly. I have the glove box still opened. The weather here has turned to sideways rain and my carport isn't providing sufficient coverage.

I'm sure something is on it's way out so although it's working properly I'll need to find this gremlin the next time it shows it's face. Admittedly I'm no pro at trouble shooting things of this nature but thanks to this write up I'm not just winging it.

 

Rain slacked off, so here are my results.
Vehicle status: Ignition on 2H\4H
Ignition pin 5 (violet)
2H- 12.2v..........4H- 12.19v

Ground pin 6 (white\blk)
2H- 1.4mv........4H- 1.8mv

4wd sign 2-4 pin 8 (grey)
2H- 1.8mv........4H- .67v

LO pin 15 (blue)
2H- 12v..........4H- .67v........4L- 1.6mv

 

Where were you seeing 16.5V?

Could be a cracked solder joint on the board, wouldn't hurt to pull the 4wd ECU, take it apart, and inspect it.

Those are correct, was this when it was working or not working?

 

16.5 was seen on TL1 with indicator malfunctioning.

Those numbers were pulled while everything was working properly.
Everything has been functioning correctly ever since my initial post today.

 

Definitely suggests something going wonky in the module to have an input voltage of 12v and a bias voltage output of 16.5v.

Being it started behaving after you unplugged the connectors I'd bet it's a cracked solder joint.

 

I'll pull the ECU tomorrow to see what I can find.
I can't thank you enough for your willingness to share your knowledge.

 

We rarely see the ECUs fail but there have been a few cases of it, I still suspect a solder joint being that it fixed itself after messing with it.

I just got tired of watching people struggling with diagnosing the 4wd system and replacing parts unnecessarily so I went on a mission to reverse engineer it, figure out how everything worked, and a better way to diagnose it.

It's certainly became more of a success than I ever expected it to.

 

Alright I've pulled it, but I'm unable to get the circuit board out to examine it. Delicate plastic for sure.

 

There should be a latch on each side where the connector is to take the cover off, then the board should slide out.

 

I removed the cover but the circuit board doesn't come out. I was pulling with a set of medical pliers, tried prying with a small flat head. She's got a scase of the don't wants.
I'm quite famous for breaking plastics so I'm cautious.

 

Broke my jaw many years ago and these were given to me to use in case of an emergency to remove the wires binding my jaw.
They're very handy.

 

Interesting, it should come right out with the cover removed.

 

Ok I reinstalled it. Left the cover off and used 2 sets of those pliers and managed to get it out.

 

I don't really see anything amiss but I can't see the pins well enough to tell if any solder is cracked.