Electronic hubs always locked in?

02-06-2012, 09:59 PM
I have a 2007 Tacoma 4x4 and until recently everything was fine. I even got almost 75000 miles out of the stock tires. About a month after the new tires the battery died so I replaced it. Since then the front seems to have a slight vibration and a slight noise. Kinda sounds like when you hear a 4x4 with big mudder tires on pavement. Both the vibration and sound are so slight that others cannot hear or feel it, I just notice having driven it for over 5 years, mostly highway.
When rotating my tires I noticed the front "axles" (sorry, don't know what the proper name is) turn with the tires. With the wheel off I tried holding the "axle" while turning the disk brake but it was as if the hub was locked in. A neighbor who seemed to think he'd heard of it before asked if I'd changed out the batter or had to disconnect it which of course I had. He said that maybe the electronic hubs were not unlocking. The tires still spin freely as the transfer case is not engaged but both of the axles coming from it spin with the tires.
I'm pretty sure this is not normal and wonder if anyone has had or heard of this and what the fix is? Is there something that needs to be reset?
And I'm guessing about the battery part, just that it seems to have started right around that time.

Brian R.
02-07-2012, 01:23 AM
I may be mistaken, since I don't have your vehicle in front of me or one to refer to, but in general for Tacomas and 4Runners, the wheels are always locked to the axles so what you are seeing is normal. The engagment of the 4WD occurs in the differential and the transfer case, not outboard at the wheels. This is called the ADD system.

The ADD system (if that is what your vehicle has) uses manifold vacuum to lock and unlock the axles to the propellor (drive) shaft which runs from the front differential to the transfer case. There would be a vacuum actuator on the differential with rubber vacuum lines running to it from a control module in the engine compartment.

In 2WD mode, the propellor shaft is disconnected from the transfer case and from the front differential. When you shift into 4WD mode, the transfer case locks the front propellor shaft to the transmission output and the ADD actuator gets vacuum from the intake manifold and locks the front axles to the front propellor shaft.

If this is your set up, in 2WD, you should be able to go underneath the truck and turn the front propellor shaft by hand, easily. If something is locking your front axles to your propellor shaft, the propellor shaft won't turn without turning the wheels.

02-07-2012, 03:58 PM
Ok. Thank you for the fast response. I wasn't sure about it and the front end just seems different. The last 4x4 had manual hubs and I didn't know if the push button style actually locked the hubs in or they were always locked.
You answered my question and I thank you again.

03-07-2012, 09:05 PM
Brian is correct, Toyota has been using the ADD system since 1990 or so.
No hubs at all, manual or automatic. Drive flanges have the wheels permanently "locked".
My Isuzu Trooper has a similar system, as do Jeeps.

03-07-2012, 10:02 PM
Thanks fourwd1. Unfortunately this hasn't gotten better and possibly worse. It's a constant hum that goes with the speed of the truck and not the gears. When I turn to the right it lessens so I don't know to suspect a bearing or the mini axles from each wheel. I know that's not the proper term. I think it's something like cv joint or something but hopefully you know what I'm talking about.
I'm going to put it on jack stands this weekend to see if I can at least figure what is going out.
Low on money and having to move for a job has kept me from taking it in. Hopefully I'm not hurting something more that will cost even more but don't have too much choice right now.

Add your comment to this topic!