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How to replace an alternator and drive belt

Over the past several years, the reliability of vehicle charging systems has certainly improved. However, if you own an older vehicle, sooner or later a warning light on your dashboard is going to tell you that the alternator has failed.

Alternators seldom experience a total failure, but the relatively low cost of completely re-conditioned units has virtually eliminated any need for the do-it-yourselfer to try to repair an alternator – they are simply replaced.

Starting in the early 1960s, alternators began to replace generators as the standard means of charging your vehicle's battery. Like generators, alternators are belt driven from the crankshaft pulley of the engine. Today, two basic types of drive belts are used: The V-belt and the serpentine (ribbed) belt. The V-belt belt design predominated until the late 1980s, but today most vehicles use a ribbed, serpentine belt. Since serpentine-belt-driven alternators vary substantially in how the belt is held in "tension," replacement techniques vary quite a bit. This DIY article will concentrate only on the older, V-belt-driven systems.

When replacing the alternator, you will need to decide between purchasing a new alternator from your vehicle's dealership or buying a rebuilt unit from an auto parts store. The cost difference is substantial, so weigh the pluses and minuses before you buy.

Park your vehicle inside or outside in a well-lighted, well-ventilated, level area. Locate the alternator under the hood in the engine compartment. Here are some hints:

  1. The alternator is always belt-driven off the front of the engine, even on transverse-engine, front-wheel-drive vehicles.
  2. The alternator has electrical wires running to it.
  3. Most alternators are made of aluminum.
  4. Most alternators have a small, built-in fan visible behind the belt pulley.

Check the owner's manual for more car maintenance information. In many cases, you'll find a diagram of the engine compartment with the alternator identified.

Once you have located your vehicle's alternator, carefully note how it is attached to the engine. There should be two points where the alternator is bolted to the engine (one that allows the alternator to swivel or swing to vary the tension on the V-belt, the other, usually a curved bracket with a long slotted hole in it, that limits the amount of swivel and locks the alternator in place).

Make sure you can reach these bolts with either socket or combination wrenches. In many cases you must use two wrenches – one on each side of the bolt – to loosen it. If the bolts/nuts appear to be corroded, test your ability to free them.

If you cannot locate the alternator, reach the attachment bolts or free the nuts, stop and let a professional technician handle the job.

Note: This DIY task benefits from having a second vehicle and/or another set of hands. More on this later.

Tools required
Basic hand tools are needed for this job:

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  • Combination wrenches
  • Socket wrenches
  • Vice grips
  • A large, flat-bladed screwdriver

Other than a new or rebuilt alternator, the only other material required is a new V-belt, masking tape (or similar) and a marker pen. You may also need some fine-grit sandpaper if the nuts and washers on the alternator are corroded.

Since you have to loosen the V-belt when you replace an alternator, unless the drive belt is relatively new, it makes good sense to replace it. The alternator and V-belt can be purchased at your car's dealership or, for most cars, at auto parts stores. When you purchase the parts, you will need to know the year, make, model, engine displacement, and even information about options on your vehicle, such as power steering and air conditioning. All this information is needed since V-belt length varies depending upon what engine accessory is being driven.

Here's where it's beneficial to have another running vehicle or a friend's car. Ideally, you should remove the old alternator and V-belt from your vehicle and take them to a parts counter. In this way, you can ensure that the replacements match perfectly.

The job
Park your car (inside or outside) with plenty of space to work around each side of the engine compartment.

Step 1: Disconnect the negative terminal of the vehicle's battery. If you don't do this, some of the wires that run to the back of the alternator can short out if they touch the car body or engine. Disconnecting the battery will protect you and the electrical circuits of the vehicle.

Step 2: Loosen the two bolts that hold the alternator in place. If these bolts have nuts on the opposite side, you may have to use two wrenches on one or both of these bolts. Do not remove the bolts yet.

Installing new v-belt on the engine pulleys

Step 3: Once you have loosened the bolts, try to rotate the alternator to loosen the slack in the V-belt. Rotate it toward the direction that the V-belt runs. Just move the alternator enough to remove the V-belt from the alternator pulley.

Step 4: If you are replacing the V-belt at this time, take the drive belt off the rest of the pulleys on the engine. This may take some patience and involve a couple of scratched knuckles. You may have to work the belt over the radiator fan and shroud.

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Step 5: Remove the electrical wires that are attached to the back of the alternator. Sometimes they are held in place by plugs with wire retainers; some are held on with nuts and washers. Using the masking tape and marker pen, mark the location of each wire that you remove so that you can install them in the same place on the replacement alternator. Make a diagram of the wire locations on the back of the alternator, and write down the color of the wire or plug that goes to each location. Check that the replacement alternator has the same type and location of fittings for the wires. You may have to use the old nuts and washers on the replacement unit. If so, clean them and even give them a quick wipe with some fine sandpaper.

Step 6: Finish loosening the bolts that hold the alternator in place and remove these bolts. Remove the alternator and set it aside.

Step 7: Take the replacement alternator and, if not already done, feed the bolts into place within the alternator housing. Position the alternator so that you can start to feed the bolts into the holes on the two engine brackets. Loosely place the nuts and washers on the back of the bolts. Make sure that you position the loosely held alternator so that it won't swing down and hit your knuckles!

Step 8: Install the new V-belt on the engine pulleys first (and on any other engine accessory that it drives) and finally, on the alternator pulley. If the new V-belt is not the right size, you can temporarily re-use the old belt. You can visit the parts counter again to get the correct size, and install a new V-belt at a later time.

Step 9: Now you need to position the alternator in the correct place and begin to get the right tension V-belt before you tighten the bolts that hold the alternator in place. This can be frustrating, particularly if you are working alone, but there is a way to simplify the job. First, hand-tighten the bolt/nut that holds the alternator to the engine – you still need to rotate the alternator to the correct position on the slotted bracket. Then rotate the alternator until it is holding the V-belt in place with no obvious slack. Now, take a pair of vise grips and adjust the jaws so that they can tighten on the slotted bracket. Position the vise grips on the slotted bracket just behind the "ear" on the alternator housing. In other words, let the vise grips be your extra set of hands holding the alternator against the V-belt in the proper position. In effect, you are using the vise grips as a "stop" on the slotted bracket. 

Step 10: By just moving the vise grips slightly you can now add the right tension to the V-belt. First, as noted above, make sure that there is no discernible slack in the V-belt. Then pull the alternator about 1/2 to 3/4 inch further in order to pull the V-belt tighter (or, stated another way, apply about 20 pounds of force against the belt). With the alternator now held in this new position, reposition the vise grips on the slotted bracket to hold the alternator in place. At this point, you can take your time and tighten all the bolts that hold the alternator in place.

Step 11: Remove the vise grips, and replace all of the electrical wires on the back of the alternator. Use your wiring diagram as a reference.

Step 12: Re-connect the battery negative terminal.

Step 13: Taking all appropriate safety precautions, start the engine. First, ensure that the ignition warning light in the instrument panel goes out after the engine has been started. Then listen to the belt. If you hear any belt squeal, the drive belt is too loose. Shut off the engine, and try to further tighten the alternator on the slotted bracket. Be careful not to over-tension the drive belt. Too much pressure on the engine accessories can ruin the bearings of the water pump or alternator in a relatively short time.

Clean and return all of your wrenches and other tools. If you bought a rebuilt alternator, return the old alternator to the parts counter to get a "core" refund. That's it!

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