Engine don't want to turn over?

The alternator drive belt ( often called the fan belt) is not tight enough because it is worn or stretched or incorrectly adjusted . This makes it slip on the pulleys . It happens when there is a heavy load on the belt, such as when the alternator is trying to recharge the battery after starting the engine , or when the Power Steering pump is working hard as you turn the steering wheel with the car moving slowly. This fault is more common on diesel vehicles since the heavy duty batteries place a greater load on the alternator.