Hello I'm new to this site so hope I'm posting in the correct area.
I have a 92 Hilux with the ADD system and the push button on the 4x4 shift lever to engage. I recently noticed the front driveshaft turns when the truck is in 2WD. I can go under the truck and grab the front driveshaft and stop it from turning with my hands but it does take some force to do this. I have read somewhere that the front driveshaft is supposed to turn in 2WD with 35ft/lbs of torque. Can anyone with some knowledge on this shed some light on this issue.
Unless there is a synchromesh system in the T case to engage 4WD on the fly then it make sense that the front driveshaft would have to turn in 2WD so the gears don't clash when you push the button on the shift lever to engage 4WD at speed.

 

After doing more research it looks like there is a synchro gear system to engage the front propeller shaft at any speed below 80kmph from 2hi to 4hi and back again. Since this is the case there would be no need to have the front shaft turning in 2wd. Is it possible that it is just the viscous drag of the tcase oil causing the front prop shaft to spin? This seems to be a big waste of energy to be spinning all of the front wheel drive components when they are not needed.

 

The reason I am asking this question is because I had a CV boot tear open along a fatigue crack in the rubber from a combination of age and always turning whenever the truck was driving. I have the solid factory axle flanges on the truck that lock the front tires to the front axle shafts permanently. After this happened I decided to install Aisin locking hubs in place of the factory axle locking flanges. The intention was to unlock the hubs when in 2WD and then not have any of the front running gear turning while in 2WD. Immediately after installing the locking hubs I had a noticeable vibration from the drive train around 80kmph that was not there before. I jacked up the truck and put it on jack stands. With the front hubs unlocked and the truck in 4th gear, I ran the speedo up to 80kmph and the vibration was there. I locked the throttle in place and took a look underneath and saw the front driveshaft spinning just as fast as the rear driveshaft. I could however grab it by hand and slowly bring it to a stop but as soon as I let it go it quickly went back up to full speed. Neither of the front axle shafts were turning during this operation.
This means that the axle stub shaft on the ADD side was turning at double speed and the axle shaft on the non ADD side was stationary when the front driveshaft was turning at full speed. When the factory axle locking flanges were installed the non ADD front axle would always be turning at the same speed as the rear axle shafts while the truck was in motion. Since there front drive shaft is turning at the same speed as the rear driveshaft (but not powered) then the ADD stub shaft axle is also turning at the same speed as the non ADD axle. Once I unlocked the wheels from the axle shafts with the addition of the Aisin hubs in the unlocked position it allowed the non ADD axle shaft to not turn when the truck was in motion for the first time in the life of the truck. This forced the short stub shaft on the ADD side to spin twice as fast as it had ever spun in its entire life therefore creating a never before experienced vibration. I would have to speculate that the stub shaft and associated bearings and side gear were not engineered to spin at these high RPMs. I removed the AISIN hubs and do not think it is a good idea to use them on this model of truck that uses the ADD. If however the front driveshaft did not spin in 2WD then I simply would only have the hubs locked in anytime I was in 4WD and would not have this problem. The other hope was to have nothing up front turning when in 2WD and thereby saving wear and tear on all the front running gear including the CV boots etc.
So the question of the hour is , Why is the front driveshaft turning in 2WD? Is this normal due to viscous coupling? Is it intentional on the part of Toyota for some reason? Do I possibly have something wrong inside my Transfercase causing drag on the 2wd/4wd synchro assembly? I'm leaning toward it is totally normal and the truck was never intended to have the front wheels decoupled from the axle shafts, nor is it a good idea to install locking/unlocking hubs on this type of drive system.

 

here are two awesome educational videos of the TCase and ADD system on the Tacoma but almost identical in operation to the pickup



ehjoy

 

problem solved

 

Good sleuthing....