I've don't some fairly extensive electronic investigation into keeping the 4WD active above 25mph. I tried supplying the electro-coupler for the rear drive with power independent of the 4WD ECU. Altho it worked it generated error codes immediately. Since I didn't want to blow out the ECU I ended the project. What I did leave in place was the LED attached to the electro-coupler. With it I can see when the 4WD is engaging or partially engaging.
The LED comes on every time I start off from a stop and stays on up to maybe 10mph depending on how much throttle I'm using.
With 4WD Lock on it stays on up to 25mph but goes off anytime I brake with any authority. That's so it doesn't interfere with the ABS function.
IMO Toyota set the 25mph limit because the whole system is fairly light duty and the forces at higher speeds are too high.
I did some testing on a dirt road to see if 4WD engages above 25mph. It does but with my V6 the car takes off so fast that w/o someone riding to watch the LED it gets pretty hairy to drive and watch the road, the speedometer and the LED!
Dr. Dyno. I don't know if you've provided details of this project before, although I have seen you reference it in other posts.
I am interested in the LED which shows when the the AWD is engaged.
Will you please provide a link, or a diagram?