Tighten a Drive Belt

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.


Tightening the Drive Belt with the Alternator

  1. 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, then slide it off the terminal post. You will not need to disconnect the positive cable as well.[1]
    • 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.
  2. 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.[2]
    • 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.
  3. 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, then pull back on the wood, increasing the tension in the drive belt as the alternator slides within the bracket loop.[2]
    • 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.
  4. 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.[2]
    • 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.
  5. 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.[2]
    • 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.
  6. 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.[1]
    • Make sure the ground cable is attached securely to the negative terminal as it will cause the vehicle to die if it comes loose.

Replacing the Drive Belt

  1. Loosen the alternator bolts or the tensioner pulley. A loose drive belt may be the result of stretching over time. If your drive belt has stretched, it should be replaced. Depending on the vehicle, there are a number of ways you may be able to loosen the tension on the drive belt. A tensioner pulley can be loosened by inserting a breaker bar or socket wrench into the hole in the center of the pulley and twisting it clockwise. In other vehicles, you will need to loosen the alternator bolts and either tip it forward or slide it toward the motor within its bracket to loosen the belt.[1]
    • Check the service manual for your vehicle to determine which way you can reduce the tension on your drive belt.
    • If your belt has been damage or is showing signs of excessive wear, it should be replaced instead of simply tightened.
  2. Slide the old belt off of the pulleys. With the tension off of the belt, slide it off the top pulley. This will produce enough slack to remove it from all of the remaining ones. If your vehicle does not provide a diagram of how the drive belt is wound through the pulleys, take a picture of it before removing the belt to ensure you can place the new one on in the same fashion.[1]
    • The drive belt likely snakes through a number of pulleys in order to power multiple accessories within your car.
    • You may also be able to find a diagram of the drive belt’s route on the auto maker’s website.
  3. Compare the replacement belt to the new one. Before installing a new drive belt, compare it to the old one to make sure you’ve purchased the right belt. While an old belt may stretch a bit, they should be fairly close in length and equal in width. Make sure the new belt matches the old so it will install properly.[1]
    • The wrong belt could fail to power the vehicle’s accessories or even cause internal damage to the engine.
    • If you feel you have purchased the wrong belt, return it to the auto parts store and get the proper belt before moving forward.
  4. Run the new belt through all of the necessary pulleys. Follow the diagram or picture you took to run the new drive belt back through and around all of the pulleys the old one did. If your vehicle comes equipped with a tensioner pulley, turn it clockwise again to bring it closer to the other pulleys as you slide the belt over it last.[3]
    • Check the belt against the diagram or picture again once you’ve finished running it to ensure it looks right.
    • You will need to maintain pressure on the tensioner pulley while getting the belt over it, so this step may be easier with a friend to help.
  5. Increase tension on the new belt. Once the belt has been run through all the necessary pulleys, either release the tensioner or use a piece of wood to pry the alternator away from the motor to increase the tension in the belt. With the belt tight, bolt the alternator in place to maintain tension.[3]
    • Use a ruler to check to make sure the belt is under the proper amount of tension.
    • Place the ruler along the belt and try to pull the middle of it away from the ruler. If it moves more than a half inch, it’s not tight enough.

Replacing the Tensioner Pulley

  1. Loosen the tension on the belt. If the belt is intact and your vehicle comes equipped with a tensioner pulley, you may not be able to tighten the belt without replacing the tensioner. Most tensioners are installed using only one bolt. Start by inserting a half inch drive breaker bar or ratchet into the hole in the center of the pulley. Twist the breaker bar clockwise to reduce the tension in the belt.[1]
    • This may not be particularly difficult if your tensioner has broken.
    • You may also consider replacing the drive belt while doing this, as it will have to come off anyway.
  2. Slide the belt off of the pulleys. Slide the belt off of the tensioner pulley first, then release the tensioner. You may choose to leave the belt where it is to avoid having to run it back through all of the pulleys, but you may also want to remove it if it is in the way of where you will be working.[1]
    • Take note of how the belt is run through all of the pulleys if your vehicle and owner’s manual does not have a diagram to follow.
    • If you do not have a diagram, try taking a picture of the belt with your phone to use as a guide.
  3. Remove the bolt holding the tensioner into place. There will be one bolt holding the tensioner in place that passes through the tensioner into the engine block or a bracket attached to it. Use a hand or socket wrench to remove the bolt. Note the position of the locking pin sticking out of the back of the tensioner as you remove it.[1]
    • The locking pin provides the tensioner with leverage to exert force onto the belt.
    • You will need to re-use the bolt you remove from the old tensioner when installing the new one.
  4. Insert the new tensioner, aligning the locking pin on the back. Slide the new tensioner into place, paying attention to slide the locking pin into the engine block in the same fashion the old one was when you removed it. Insert the bolt and tighten it using the same wrench you used to remove the old one.[1]
    • Make sure to make it tight, as there will be a good amount of force on the tensioner when the vehicle is running.
    • The tensioner will likely not seat properly without the pin inserted in the right place, so if you are having trouble getting it in, check to ensure the pin is sliding into the hole in the block or bracket.
  5. Run the new drive belt through all of the necessary pulleys. Run the drive belt back through and around all of the pulleys. Use a diagram or the picture you took of them to ensure you run the belt properly. Slide a breaker bar or half inch drive ratchet into the hole in the center of the new tensioner pulley and turn it clockwise to lower it as you slip the drive belt over the tensioner pulley last. The new tensioner should hold the belt at the proper level of tension once you release it.[1]
    • Test the tension of the belt by placing a ruler alongside it and pulling the belt away from it. If the belt will come further than a half inch away from the ruler, it is still not tight enough.

Diagnosing Issues with Your Drive Belt

  1. Check the tension of the drive belt. If you can see any visible sag in your drive belt, it is far too loose, but tension issues are not always visible. Pinch the belt between your thumb and index fingers and try to wiggle it from top to bottom. If the belt moves more than a half of an inch or so, it is too loose and needs to be tightened or replaced.[1]
    • You can purchase belt tensioner tools that will measure the tension of the drive belt and let you know if it is too loose at most auto parts stores.
    • If the belt seems loose, it likely requires tightening or replacing.
  2. Watch your voltage gauge. A loose drive belt cannot power the accessories as well as it is supposed to. The alternator in your vehicle is powered by the drive belt, so if you notice dimming interior lights or your voltage gauge fluctuating, there may be an issue with the drive belt.[4]
    • Dimming interior lights and issues with your electrical gauge can both also mean an issue with your alternator.
    • If the lights get brighter when you accelerate, the problem may be the alternator rather than the belt.
  3. Take note of overheating issues. Another common issue caused by a loose drive belt is overheating. The drive belt powers the water pump in your vehicle that runs coolant throughout the engine. If the belt is too loose, it may not be able to sufficiently power the pump, resulting in higher operating temperatures and even overheating.[2]
    • Engine heat issues may also be caused by bad coolant, a leak in your coolant system, or a faulty water pump.
    • Be careful driving a vehicle with coolant issues, as overheating an engine can cause significant internal damage.
  4. Check the voltage reaching the battery. If your vehicle does not come equipped with a voltage gauge, you can check the voltage reaching the battery using a voltmeter. Voltmeters can be purchased at most auto parts or large retail stores and can tell you how many volts are traveling through an electrical system. Touch the black sensor coming from the voltmeter to the negative terminal on the battery and the other to the positive, then check to see how many volts are flowing through the system. If the alternator is generating enough voltage, it should read between 13.8 and 15.3 volts.[5]
    • The black lead from the voltmeter connects to the negative terminal on the battery. You can find the negative terminal by looking for the letters “NEG” or the negative symbol that looks like (-).
    • The red lead from the voltmeter connects to the positive terminal, which can be identified by the letters “POS” or the positive symbol that looks like (+).
  5. Look for signs of wear or damage on the drive belt. Even if you haven’t had electrical or coolant issues, your drive belt may still need to be replaced if there are signs of extensive wear or damage anywhere on the belt. Look for cracks that develop along the ribs of the belt, chunks missing, or frayed fabric on the top back of the belt.[1]
    • Even under optimal conditions, drive belts will eventually wear out and begin to stretch.
    • Look for any indications that something has been rubbing on the drive belt and make sure to address it before installing a new one.
  6. Check the tensioner pulley. Many modern vehicles come equipped with a tensioner pulley that places tension on the belt. Use your vehicle’s service manual to locate the tensioner pulley and try to move it around with your hand. The tensioner is designed to withstand more force that you should be able to easily place on it with your bare hands, so if it moves, it has likely gone bad.[1]
    • If you can see the tensioner pulley with the motor running, start it and watch for any movement in the pulley.
    • The tensioner pulley should not move under the pressure of the belt when in motion.

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Sources and Citations