Wow! I just reread this thread and relearned some things. A BIG thanks to Thai, KLD, and others for helping to clarify these electronic doo-dads we have on our FJ's.
Just a quick review of what was originally posted plus notes from KLD, Thai, CruiserLarry, & Others.
: Traction control. All FJ's have this, both 2WD & 4WD. Works automatically when ignition is started (no push button needed). Works by applying brakes and controls (decreases) engine throttle to give added traction to varying wheels when system detects wheel slippage/spinning ("think spinning your wheels on ice"). Works all the time in 2WD and in 2H & 4H (AT) or HL (MT). This system basically helps to maintain traction in slippery conditions such as wet/icy for forward movement by applying brakes and controlling engine throttle to regain/maintain traction as a safety feature.
TRAC is disabled in 4LO. Again, you don't want this system to control engine power when you are climbing obstacles.
: More aggressive form of TRAC & for 4WD OFF ROAD use only. Does the same thing as TRAC except it doesn't cut/control engine throttle. Instead, it transfers torque from slipping/spinning wheel(s) to non slipping wheel. This gives added traction & forward momentum to overcome Off-road obstacles. This can only be engaged while in 4LO (AT) or LL (MT) and by pressing the "ATRAC" switch.
Also, only 4WD models with the Rear Diff Locker have ATRAC capability. Even then, you need the $50 ATRAC switch installed to use this system if you didn't purchase either Upgrade Pkgs with your FJ (UP or UR).
*AT: to go into 4Lo, stop, put into Nuetral, then shift into 4Lo (short knob).
MT: refer to manual, sorry I have the AT.
: found on 2WD models. Similar to ATRAC for 4WD. Must press the Auto LSD button/switch to activate it.
(vehicle stability control) : integrates ABS, TRACtion control, engine control, etc to prevent the vehicle from skidding when cornering. Prevents oversteering or understeering & will save your ass if your car loses control (Thai). Detects angular momentum and tries to control direction of skid. VSC is automatically activated when the FJ reachs 9mph or 15km/h.
*VSC automatically turns itself off (VSC off light on dash comes on) when 4H/HL or 4Lo/LL is activated (AT/MT) or when the Rear Diff is locked. VSC is for preventing rollovers at high speeds thus not used while wheelin' in 4Lo/4H.
Rear Diff Locker
: Not all 4WD models come with this. Switch/button activated. You must be in 4Lo or LL. It locks the rear axle so that it acts as one rod. If one wheel turns, so must the other so that not all power/torque is lost to the spinning wheel. Diff b/t Rear Locker and ATRAC is that with
Rear Diff Locker, both rear wheels spin in unison and steering tends to be rather difficult (so be careful near small ledges, ruts, drop offs). ATRAC, rear wheels are not locked and steering is not comprimised so you have more control of the vehicle. Apparently one works better than the other for certain situations while wheelin or you can do the Atrac-Locker Hack
and have the best of Both worlds.
So at relatively high speeds TRAC will slow you down & aid in traction control in slippery situations, while VSC will give your more control during turns, skids, and saving your ass (roll over prevention).
While ATRAC and Rear Diff Locker is specifically for use in 4LO. While either of these are engaged, the TRAC and VSC is turned off. This makes sense, b/c they are safety systems to slow you down when you're going to fast, while ATRAC and RDL will aid to move you forward when you're going too slow or stuck.
: Not necessary to be in 4H when it's raining or lightly snowing in the city, since this will deactivate VSC. VSC is what will save your ass at high speeds (per Thai). If you stay in 2H, you have TRAC and VSC to make necessary corrections if you slip & slide. If you do choose to put it in 4H for these situations, at least you still have TRAC working for you. But again, Thai says TRAC is turned off after 40+ mph or so. Many here feel you shouldn't be going fast (>40mph) while in 4H. 4H is probably more appropriate if you encounter thick/compacted snow or icey roads as FJJack mentions. You can quickly find out which is better with a test this winter. Try 2H and 4H and make up your own mind.
4H and 4L should generally be reserved for off roading use. 4H for light trails (gravel/dirt roads) and 4Lo for mod-large rutts/rocks. Obviously if a meteorite hits your town and all hell breaks loose then by all means put her in 4H/4Lo as needed.
Again, A great big thanks to KLD, Thai, CruiserLarry, RoverGGM, Alex, and countless others for their technical expertise on this forum.