My polaris 280 keeps running on the front wheel

Try moving the float closest to backup valve slightly. This can help it stay flat on bottom. Mine was riding on its back wheel and kept climbing too high and getting tail stuck in skimmer...and now it is riding smooth across pool floor now.

Make sure that your thrust jets are properly positioned. You will also want to check for any obstructions in the wheels or axles that would restrict movement. Often with these automatic pool cleaners, one of these two problems is causing restricted movement in the wheels.

When my polaris wheels stopped spinning it was the water jet which spins the turbine was clogged. The in-line filter had failed and some small debris had passed through to clog the jet. Easy fix to take off the top cover then take off the turbine cover and examine /clear the hose (clamped on by zip ties) and the plastic jet.

It's probably a damaged drive gear. Polaris was at one time giving out free repair kits for this problem because of the poor design of the original.

Back wheel does not drive. What can it be?

If the cleaner is going too fast then it is likely that there is too much water flow at the drive turbine. Is there a pressure relief valve in the water supply line (at the edge of the pool)? If so then change the tension on the spring to allow less water to get to the cleaner's drive turbine.
The speed of the Polaris is probably the cause of the front wheel lifting as well, however if (after adjusting the water pressure) the front wheel still won't stay on the pool bottom, then perhaps adjust the little jet at the back of the casing. Mine points a little "down" towards the pool floor and this tends to raise the bac k wheel (or alternatively keeps the front wheel in contact with the pool bottom or wall).

By the way, if you want to see how the cleaner works, then search for eHow and lookm at the repair/overhaul of the Polaris 180............. the mechanism is similar.
Good luck.

I battled with the track falling off for over a season. The pool guys said they fixed it. Tried the previous solution to adjust middle wheel but no luck. My solution was to remove the left wheel and the tank (to get to the screws). The wheel mount allows for minor adjustment but it is also reversible. Turn the mount around and this will extend the wheel base much more. Mine ran for several hours after doing this and so far the track stays on.