Getting a Grip on Gear Differentials

Question: Would you please explain the advantages and any disadvantages of positive-traction or limited-slip differentials? Are they the same thing? Is it a good option to have on a four-wheel-drive vehicle? How do they operate and are there any special maintenance problems that can result from them?--N.D.

Answer: The function of any differential is to permit the two wheels that provide driving force for a vehicle to turn at different rates of speed, hence the term “differential.”

Any time a car goes into a turn, the outer wheels will have to turn faster than the inner wheels, otherwise the car would hop around the road surface. Just about the only vehicle that doesn’t have some form of differential is a dragster, which uses a “Detroit locker” rear end.

Power goes from the engine to the transmission and then to the differential. The axles come out of the differential and supply power to the drive wheels, whether in the front or the rear of the car.

In most ordinary differentials, the power from the engine is transferred equally to both wheels when traveling in a straight line. In a turn, however, the differential supplies more power to the faster wheel.

While this inexpensive system works fine most of the time, it does cause big problems when a car needs traction. In an ordinary differential, power usually goes to the wheel with the least traction, which is why a car wheel tends to dig itself into snow or mud once you get stuck.

So, auto manufacturers have long offered a special rear end that supplies power to the wheel with the most traction. Ford calls its differential a “limited slip.” Chevy calls its version “positive traction.” Basically, they are the same thing.

All differentials work through a complex system of gears, utilizing pinion and ring gears that drive two smaller pinion and side gears.

In a limited-slip differential, a series of clutches prevents one wheel from spinning without the opposite wheel turning at the same speed. The clutches or cones inside this type of differential work automatically.

Front-wheel drive, which most cars have nowadays, already provides much better traction than rear-wheel drive. So, the need for limited-slip differentials is not as great.

Q: We have a 1983 Chevrolet Caprice Classic in good condition. I noticed last summer that when starting the car, the starter would hesitate and then turn over the engine. It happens when the engine is hot. My mechanic says this is common on the Caprice. Could you tell me how to correct this problem?--E.G.W.

A: An engine is somewhat more difficult to start when it is hot because the internal engine parts are swelled and create a higher compression ratio inside the engine.

The higher compression ratio inside the cylinder puts a greater load on the starter motor. Even so, a properly operating starter motor is supposed to be able to handle this additional load.

First, you might have your engine’s ignition timing checked. It’s possible that the ignition system is firing your spark plugs much too early, which results in a force trying to push the engine backwards. You recognize it as a hesitation.

You should also check that the battery is fully charged and that battery cables are firmly connected and clean. If, after all this, you still have a problem, it’s likely to be the starter motor itself. The brushes or the solenoid inside the starter motor are possibly beginning to wear out.