Instructions for Replacing an Alternator

Battery, Electrical and Lighting Systems

By Rodney (Mechanics)

If the voltage or charging system warning light is ON and your battery is either low or completely dead, likely, your alternator is not functioning properly. To confirm this, you can conduct a quick voltage assessment at the battery terminals while the engine is idle. If the charging system produces no voltage, the alternator is probably defective, rather than other potential issues such as corroded wiring connections, loose or corroded battery cables, or a fault in the charging control module or PCM. Thus, replacing the alternator is the recommended course of action.

On most vehicles, replacing the alternator is a straightforward task that does not require specialized skills. You can complete the task using basic hand wrenches or sockets to loosen the alternator mounting bolts.

Instructions For Replacing An Alternator

Step-by-Step Guide for Replacing an Alternator

To replace the alternator, follow these steps.

1. Disconnect The Battery.

Disconnect The Battery

If you own a 2004 or newer vehicle, refer to the linked article on Battery Disconnect Precautions. Consider using a battery memory saver if your vehicle’s modules would be impacted before disconnecting the car battery.

Disconnecting the battery is necessary to avoid any potential short circuits or grounds arising if you unintentionally touch a battery-positive wire at the alternator against any metal surface. This sudden voltage surge could harm your vehicle’s sensitive electronics or wiring. It’s important to note that there is no risk of electric shock from 12 volts.

2. Remove The Alternator Drive Belt.

Remove The Alternator Drive Belt

You can find the belt tensioner for newer vehicles that use a serpentine belt and automatic belt tensioner and rotate it with a socket and long handle. The direction of rotation will depend on the tensioner’s mounting and belt routing. If the tensioner is difficult to access, you may need to do it underneath the vehicle or use a specialized long-handle serpentine belt tensioner tool.

Note: Before removing a serpentine belt, search for a decal under the hood that displays the belt’s routing. If no such decal is available, draw a diagram or take a digital photo of the belt’s routing to refer to later when reinstalling it. Some belt routings can be complex and difficult to follow, so it’s crucial to understand the correct routing BEFORE removing the belt.

In case your vehicle has a serpentine belt for the alternator without an automatic tensioner, follow the same removal process used for a V-belt. This involves loosening the alternator pivot bolt and adjustment bolt, rotating the alternator inward to slacken the belt, and removing it.

Loosen the alternator pivot bolt and adjustment bolt on the support bracket for older vehicles that use a V-belt. Swing the alternator inwards to release the tension on the V-belt, and then remove the belt from the pulley.

3. Disconnect The Wiring At The Back Of The Alternator.

Disconnect The Wiring At The Back Of The Alternator.

While it may be more convenient to detach the wiring after removing the alternator bolts, turning or rotating the alternator for better access. It’s important to avoid twisting the wires excessively and letting the alternator hang by its wires, as this can cause damage to the wiring connectors or terminals. For this reason, it’s advisable to disconnect the wires before removing the alternator.

Note: If the alternator has multiple wiring terminals or connectors at the back, note which wires are connected to which connectors. Incorrectly reinstalling the wires on the wrong terminals may prevent the alternator from functioning or potentially cause damage to it or other electronics in the vehicle.

4. Remove The Alternator By Unbolting It. Typically, alternators have either two or four mounting bolts.

Remove The Alternator By Unbolting It

The standard configuration comprises a large pivot bolt (often with a hex nut on the inside end) and a smaller adjustment bolt on the alternator support bracket for models where the alternator position can be adjusted to tighten the belt. The alternator is generally secured to the engine or front cover with four bolts on models equipped with an automatic tensioner to regulate belt tension.

5. Before Purchasing A Replacement, Get Your Old Alternator Tested.

Starter And Alternator Tester Bench

Many auto parts stores offer free testing of alternators using a bench tester. It is advisable to test the old alternator to confirm if it has failed and requires replacement. Sometimes, an alternator suspected to be faulty may test “good,” indicating that the charging issue is caused by another problem such as loose, damaged, or corroded wiring connectors, a faulty external voltage regulator, PCM, or a wiring problem.

6. To replace the alternator in your vehicle, you can purchase a new or remanufactured one or opt for a good-quality used unit that has been tested.

However, ensuring that the alternator you choose is suitable for your specific vehicle model is important. Many alternators may appear similar but can differ in power ratings and wiring connections.

7. Ensure that the replacement alternator you select has the same type of pulley as the one in your old unit.

In newer vehicles, special one-way overrides or decoupler pulleys are often used to decrease noise and vibration. Cheaper replacement alternators may only come with a standard pulley, which may cause noticeable changes in engine noise and vibration.

If the replacement alternator does not have a pulley installed, you must transfer the pulley from your old alternator to the new one. This process may require a puller tool to remove the old pulley from the alternator shaft. Applying heat with a propane torch to the pulley hub may facilitate loosening and easier removal.

8. Install the new alternator onto the engine and attach all the mounting bolts, but refrain from fully tightening them if the alternator needs to be rotated to tighten a V-belt or a serpentine belt not feature an automatic tensioner.

Install The New Alternator

9. Reconnect the wiring at the back of the alternator, ensuring all wires are routed correctly, and all connections are securely fastened and clean. This step may be easier to perform before mounting the alternator onto the engine, particularly if access to the back of the alternator is restricted. Note that inadequate contact in this area could lead to the alternator malfunctioning.

Reconnect The Wiring

10. Next, reinstall the belt by placing it over the pulleys and ensuring it is properly aligned and routed.

Reinstall The Belt

If an automatic adjuster is not used, carefully pry outward on the alternator housing until the belt is taut, then tighten the pivot and adjustment bolts. Having a second person present for this step may be helpful, as an extra hand can make the process easier.

For applications with an automatic belt tensioner, rotate it inward and hold it in place so that the serpentine belt can be fitted over the last pulley. Choose the pulley that is the easiest to access for the final step.

To complete the job, reconnect the battery and start the engine. Use a voltmeter to check the charging output and ensure that it falls within the range of 13.5 to 14.5 volts, which indicates that the alternator and charging system is functioning correctly.

Voltmeter To Check The Charging Output

Here are some extra tips for replacing an alternator:

Do not attempt to test the alternator by disconnecting a battery cable while your car engine is still running. This action can create high-voltage spikes that may damage the alternator and other electronic components.

Before starting your vehicle, recharge it using a portable battery recharger if it has been drained. This will lessen the load on your new alternator and reduce the likelihood of overheating.

If the battery fails to hold a charge or if a charging issue is suspected, always examine its condition to ensure it still functions correctly. The problem could stem from an old battery requiring replacement rather than a faulty alternator.

If your belt has over 60,000 miles on it, replace it with a new serpentine or V-belt to prevent slippage and minimize the risk of wear. Replace any belt that displays signs of cracking, damage, or contamination with oil or grease.

Inspect the automatic belt tensioner. If the automatic belt tensioner is corrupted, weak, or immobile, it will not maintain proper tension on the serpentine belt, resulting in slippage.