> My question is, he says to check the differential and brakes > this way: jack up one side of the car, block the > other wheels, and run the car through all gears (this is on page 25 of > my "15th" edition), is this possible? Won't the other rear > wheel try to rotate too, with dangerous results? Has anyone > done this test? >

> Mark Janello > mjanello@umich.edu > University of Michigan School of Music

Mark, Short answer. No, the other rear wheel should not try to rotate. If it does, stop the test and don't buy the car. Any car should pass this test, unless it has a limited slip differential, and you know what that is, and want one.

Safety warning. I'd use a stand, not just the jack, when doing this. But you only need to get one rear wheel off the ground to demonstrate how the differential works, which is the point of this test.

Long answer. The purpose of the differential is to allow the two rear wheels to revolve at different speeds as you turn. There's a crafty arangement of gears that link the two rear wheels together, but allow them to turn separately. If you jack up both sides in the back, (engine OFF!) you can turn one rear wheel one way and the other will turn the other way. Straaaange. Inside the transaxle, engine power is passed to the differential. One wheel can go slower than the other if the other goes an equal amount faster- that's why turning one one way makes the other go the other way. The engine makes the differential go, and the two wheels split that power however they need to.

I actually did this with a couple of cars I've owned- once on my Rabbit after changing the transmission, once on the 914 to confirm that the whining sound was associated with 5th gear, and to a lesser extent, 4th.

On the 914, you can't see the fan housing to turn the engine when setting the valves, so I would put it in 5th, jack up one side and turn that wheel with my foot, while watching the distributor rotor. The other wheel wouldn't move, but the engine would, and that's how I moved each cylinder to TDC.

Bill