2010 V6 Highlander: Is my 4WD broken?

[MtAiryEd]

So I have a non-hybrid, 2010 v6 Highlander. I bought this for getting through some light snow and also to not have issues at a boat ramp. I pull a 17'-6" bassboat that grosses out (trailer/boat) at about 1200 lbs.

I was at a boat ramp today. The ramp is concrete. Due to high water the last couple of days, there was about an inch of frozen mud covering about 17' of the ramp, from the water's edge. I backed the trailer in the water so that the rear vehicle wheels were on the frozen mud, and the front vehicle wheels were on dry pavement. I wanted to test the situation before launching the boat, so I attempted to drive forward, pulling the boat away from the water. The rear tires started spinning and I was not moving forward. As a matter of fact, I started to roll backwards a couple inches. It seemed like the front tires were doing nothing to help move the vehicle forward. We had to put some dirt/mulch under the rear tires in order to get the SUV/trailer/boat away from the water. Before putting the debris under the tires I tried the "ETC Snow" button. I tired putting the shift into the "S" position. Nothing helped.

I called the dealer and the salesman told me "that can happen". Is this true? Should I get this checked out or am I stuck with a vehicle that seems to not do something basic like get me out of a situation like this?

Thanks!

[TrailDust]

Next time disable the VSC/TRAC system and you should be able to move up the ramp. One of the main problems is computer dethrottles the engine under conditions similar to what you described, and you can't make any progress. I've been on hard-packed snow and solid ice before on an 8% grade and became stuck. Using the Snow button didn't help at all, but turning off VSC/TRAC allowed me to move confidently over the same ground with about 15% slippage of the tires. There's a past thread here where I describe this in some detail and have pictures too. When I find it I'll post up the link. BTW, take a look at your owners manual, which describes how to turn off VSC/TRAC as well as what conditions are suggested to do so.

EDIT: Read reply #30 in this thread, which is the one I referred to above. You might also want to search this forum and use "VSC/TRAC" and also "ramp" as search terms for individual searches (our search feature doesn't do well with long strings of words). They should turn up other threads including past threads about boat ramps.

[katekebo]

Yes, what happened is perfectly normal. The HL has a central differential that distributes torque 50/50 between front and real wheels. This means that if one axle is allowed to spin with essentially no resistance (like the rear wheels on frozen mud), there is very little torque going to the other axle.
Some all-wheel drive cars have a system that applies brakes to the spinning wheel to create additional resistance and transfer more torque to the sheels with traction. Not sure if HL has it. Anyway, this system is usually effective if only one wheel is spinning, but turns pretty ineffective if two or three wheels have no or little traction. Other cars have a torque-biased central differential that sends more torque to the wheels that are turning slower (or not turning at all). This system was first implemented in Audi Quattro. It is better than a simple differential, but still requires at least some level of traction on all four wheels - if one or two wheels are spinning completely freely there will be no torque delivered to the other wheels either.
The HL 4WD system isn't really a true off-road type 4-wheel drive system. It is intended to provide additional traction on slippery roads, but it's not comparable to a true 4x4 system with torque-biased or lockable central differential (like the one you can find in a Toyota Land Cruiser, 4Runner or FJ Cruiser). Remember, HL is a "crossover" (i.e. a car disguised as an SUV), not a real truck-based SUV.
A good example of an excellent four wheel drive system is like the one implemented in Toyota Land Cruiser and FJ Cruiser (manual transmission). It is a full-time four wheel drive with a torque-biased central differential. Under normal conditions, the torque is split 70/30 between rear and front wheels. If one axles losses traction, up to 90% of the torque is delivered to the axle that has good traction. There are no electronics involved and the response is instantaneous. If this is not enough to get you moving, the electronics kick in and apply brakes to the wheel(s) that are spinning faster, creating additional resistance and directing more torque to the wheels with traction. But if the electronics are not "smart" enough to get you going, you can mechanically lock the central differential (and the rear one) so that 100% of the torque can be delivered to a single rear wheel or to the front axle. Stock Land and FJ Cruisers don't have lockable front differential which would create the ultimate traction option (100% torque delivered to ANY wheel), but it can be added as aftermarket option. In practice it's rarely needed unless you engage in extreme rock crawling / mudding. I think 4Runner has similar system, but I'm not sure.
The downside of a "real" four wheel drive system is that everything in the drivetrain must be oversized to handle the possibility of all torque/power going to a single wheel. I have both the HL and FJ, and although the FJ is lighter and has less horsepower than the HL, all drivetrain components (differential, drive shaft, etc.) are at least 50% bigger (and heavier).

[MtAiryEd]

Thank you for taking the time to provide a very informative response.