GMC Alternator Diagnosis and Replacement

GMC Alternator Diagnosis and Replacement: A Practical Guide

Prashant Salla
Prashant Salla6 min read
GMC Alternator Diagnosis and Replacement: A Practical Guide

The alternator is a crucial main component of your GMC vehicle's electrical system. It's in charge of sparking up the battery and powering the electrical components while the engine runs; a malfunctioning alternator can lead to various electrical issues and even cause your vehicle to break down.

In this practical guide, we'll explore the process of diagnosing and replacing the alternator in your GMC, empowering you with the knowledge to identify potential problems and ensure the reliable operation of your vehicle's electrical system.

Replacing Your Alternator in Your GMC with Goodhood Mobile Auto Repair Services

If you happen to notice signs of a faulty alternator, reach out to our team at Goodhood Mobile Auto Repair Services for a reliable replacement.

The Goodhood technician team can quickly identify the issue and provide an efficient alternator replacement, getting you back on the road in no time!

Understanding the Role of the Alternator

When the engine is running, the car alternator converts mechanical energy into electrical energy. This energy is then used to charge the battery and power the vehicle's electrical components, like the lights, radio, and HVAC system.

Additionally, it provides the necessary power to keep the engine running smoothly.

Diagnosing Alternator Issues

Before proceeding with alternator replacement, it is essential to diagnose the problem accurately. Here below are some signs that may indicate a faulty alternator:

Dimming Lights

If you notice that your GMC's headlights, interior lights, or dashboard lights are flickering or becoming dimmer while the engine is running, it could be a sign of a failing alternator. Your charging system is not providing enough voltage to power the electrical components, resulting in an insufficient electric current.

Engine Speed Problems

If your engine speed is inconsistent or slower than usual, it could indicate a faulty alternator. An insufficient electric current will not make the engine run at its optimal speed, which can lead to inefficient performance.

Voltage Regulator Failure

The voltage regulator is responsible for ensuring the alternator output remains consistent. The electrical components will not receive enough current to function correctly if it fails. Check the battery terminal for an abnormal voltage level to identify a voltage regulator failure.

Electrical Failures

Malfunctioning alternators often result in sporadic electrical failures, such as faulty power windows or door locks or the radio and infotainment system turning off and on unexpectedly.

Warning Lights

Pay attention to warning lights on the car dashboard, such as the battery light or the "ALT" (alternator) light. If these lights illuminate while driving, it indicates a potential issue with the alternator.

Strange Noises

A failing alternator may produce unusual noises, such as grinding, whining, or squealing. These noises could be caused by a malfunctioning internal component, like a worn-out bearing or a faulty pulley.

Battery Issues

While the battery itself is not the alternator's responsibility, a faulty alternator can lead to a drained or weak battery. If you find yourself frequently needing to jump-start your GMC or experiencing difficulty starting the vehicle, it could be because of a failing alternator.

Doing a battery test can help you to determine whether the alternator is causing the issue or if a failing battery causes it.

Electrical Failures

Malfunctioning alternators often result in sporadic electrical failures, such as faulty power windows or door locks or the radio and infotainment system turning off and on unexpectedly.

Your electric power steering may become stiffer or less responsive. This is a sign of the alternator not providing enough power to keep your GMC's electrical system running smoothly.

Replacing the Alternator

Once you have diagnosed a faulty alternator in your GMC, it's time to replace it. 

Here is a step-by-step guide to assist you through the process:

Gather the Necessary Tools and Parts

Before starting, ensure you have the appropriate tools, including wrenches, sockets, pliers, and a multimeter for testing. Also, purchase a replacement alternator that matches your GMC's specifications.

Disconnect the Battery

Begin by disconnecting the negative terminal of the battery to prevent any electrical shorts or accidents during the replacement process.

Locate the Alternator

Refer to your GMC's owner's manual to locate the alternator. In most cases, it is mounted on the engine and driven by a belt connected to the crankshaft.

Remove the Serpentine Belt

Loosen the tension on the serpentine belt using a wrench or socket, then slip the belt off the alternator pulley. Remember to note the belt routing or take a picture for reference when reinstalling it later.

Disconnect Electrical Connections

Carefully disconnect the electrical connections from the alternator. This may include the main power cable, the exciter wire, and any other harness or connectors.

Remove Mounting Bolts

Use a socket or wrench to take off the mounting bolts that secure the alternator to the engine or bracket. Keep track of the bolts and any washers for reinstallation.

Extract the Old Alternator

Gently wiggle the old alternator out of its mounting position. Be cautious not to damage any surrounding components or wiring during the removal process.

Compare and Prep the Replacement Alternator

Compare the new alternator with the old one to make sure they are the exact copies in size and configuration. Transfer any necessary components, such as pulleys or brackets, from the old alternator to the new one. Also, double-check that the replacement alternator matches your GMC's electrical specifications.

Install the Replacement Alternator

Carefully position the new alternator into the mounting bracket and secure it with the previously removed bolts. Tighten the bolts to the manufacturer's recommended torque specifications.

Reconnect Electrical Connections: Reattach the electrical connections to the new alternator, ensuring a secure and proper fit. Follow the markings or notes made during the disconnection process to ensure correct wiring.

Reinstall the Serpentine Belt

Referencing the belt routing diagram or the reference picture taken earlier, route the serpentine belt back onto the alternator pulley. Use a wrench or socket to apply tension to the belt, allowing for proper alignment and tension.

Reconnect the Battery

Once the alternator is securely installed, reconnect the negative terminal of the battery. Ensure the connection is tight and secure.

Test the Alternator

Start the engine and monitor the performance of the newly installed alternator. Use a multimeter in order to check the voltage output at the battery terminals. It should read around 13.8 to 14.2 volts, indicating that the car alternator is charging the battery properly.

Final Inspection

With the engine running, check for any signs of abnormal noise, such as grinding or squealing, from the new alternator. Also, verify that all electrical components are functioning correctly and that warning lights have turned off.

By following these steps, you can successfully diagnose and replace a faulty alternator in your GMC vehicle.

But, if you are uncomfortable performing the task yourself, it's advisable to seek the assistance of a professional mechanic or technician who can ensure the proper installation and functionality of the new alternator.


The alternator plays an important role in keeping your GMC's electrical system running smoothly. By understanding the symptoms of a failing alternator, you can identify potential issues and take preventive measures to avoid costly repairs.

If you do end up requiring a replacement, contact Goodhood Mobile Auto Repair Services for reliable services at competitive rates. Our experienced technicians will meticulously diagnose the problem and ensure the safe installation of the new alternator, so you can get back on the road with peace of mind.

Frequently Asked Questions

What are the signs of a bad GMC alternator?

Some common signs of a faulty alternator include dimming lights, electrical failures, warning lights on the dashboard, strange noises from the engine bay, and battery issues.

How long does it take to replace a car alternator?

The process of replacing a vehicle alternator typically takes around 2-3 hours for experienced mechanics or technicians. If you decide to do it yourself, allow extra time for any potential complications.

Can I drive with a bad alternator?

Driving with a failing alternator is not recommended, as it could cause further damage to other components in your GMC's electrical system. It is best to have the alternator replaced as soon as possible.

How much does a GMC alternator replacement cost?

The cost for a GMC alternator replacement depends on the model and make of your car. Generally, you can expect to pay between $200-700 for parts and labor. Contact Goodhood for an accurate estimate.