The drive belt, or serpentine belt as it is often called, transfers power from the crank pulley to each of the accessories in your engine. In some vehicles, the drive belt also serves as a timing belt, keeping the movement of the cylinders in sync with the valves and other components of the cylinder head. A loose drive belt can compromise your vehicle’s ability to power accessories like power steering, air conditioning or your alternator, or could possibly even cause internal damage to the motor. Different vehicles have different methods of maintaining proper tension on the drive belt, but most can be adjusted at home with simple hand tools.
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Tightening the Drive Belt with the Alternator
Disconnect the battery. Before you get started, disconnect the battery to ensure the engine cannot be started while you’re working under the hood. As a precaution, always disconnect the battery before putting your hands in the engine bay of your vehicle. Use a hand or socket wrench to loosen the nut holding the ground cable on the negative terminal of the battery, and then slide it off the terminal post. You will not need to disconnect the positive cable as well.
Locate the negative terminal by following the black ground cable, or looking for the negative symbol (-) or the letters NEG.
Tuck the cable along the side of the battery to ensure it can’t accidentally come into contact with the negative terminal.
Loosen the bolts attaching the alternator to the bracket. Many alternators are bolted to a bracket with a long slotted metal strap that allows you to adjust the angle of the alternator in relation to the engine. By loosening the bolts, you will be able to adjust the alternator’s angle to increase or decrease tension on the drive belt.
Loosen the two bolts going through the slot on the alternator bracket, but do not remove them.
Make sure the bolts are loose enough to allow the alternator to slide within the bracket.
Use a piece of wood as a pry bar to increase the tension on the belt. With the alternator bolts loosened, you will need to place a good deal of pressure on the alternator in order to make sure the drive belt is tight enough. The easiest way to do this is to use a piece of wood as a pry bar. Using wood reduces the likelihood of placing too much force on the alternator as one might with a crowbar, but one can be used carefully. The handle of a hammer tends to work well in most circumstances. Slide the piece of wood between the alternator and the engine, and then pull back on the wood, increasing the tension in the drive belt as the alternator slides within the bracket loop.
Place enough force on the pry bar to pull the drive belt until it is tight over the alternator pulley.
Choose the area of the engine you pry against carefully to ensure you do not crush any coolant lines or electrical connections.
Tighten the alternator bolts in the new location. With the alternator pried back and the drive belt tight, use your free hand to tighten the alternator bolts in their new position in the slot on the bracket. Once the bolts are tight enough, you should be able to release the wood you were using to pry the alternator away from the motor, leaving the alternator in its new position.
Make sure the bolts are tight before releasing the pry bar; otherwise the tension of the belt will pull the alternator back to its previous position.
This step may be easier if you enlist a friend to help you.
Check the belt tension. With the alternator in its new position, applying increased tension to the drive belt, check to make sure the belt is tight enough to run the accessories properly. An easy way to test the tension of the drive belt is to hold a ruler along a stretch of the belt, then use your thumb and index finger to try to pull the midpoint of the belt away from the ruler. If you can pull the belt more than a half inch from the ruler, it is not tight enough.
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You can also purchase belt tension testers at many auto parts stores that will tell you whether or not the belt is tight enough.
If the belt is not tight enough, loosen the alternator bolts and try again.
Reconnect the battery. Once the belt has enough tension, you can reconnect the battery using the same hand or socket wrench you used to disconnect it. With the battery reconnected, the vehicle is ready to drive.
Make sure the ground cable is attached securely to the negative terminal as it will cause the vehicle to die if it comes loose.