1979-1987 GMC K1500 Alternator Replacement and Upgrade Guide
Keep your classic Square Body on the road by understanding its charging system and choosing the right alternator.
- Verify if your truck needs an internally or externally regulated alternator; this is the most important step before buying.
- Choose an amperage rating based on your electrical needs. A stock truck is fine with 63-100 amps, but added accessories require a high-output unit.
- Replacing the alternator is a straightforward DIY job that can be done in about an hour with basic tools.
- Reputable aftermarket brands like ACDelco, Bosch, and Powermaster are reliable choices for this vehicle.
Is Your Alternator Failing? Common Symptoms
A failing alternator on a 1979-1987 GMC K1500 gives clear warning signs. Because these trucks have simpler electrical systems, symptoms are often more obvious. Look out for these common indicators:
- Dim or Flickering Headlights: The most common sign is headlights that dim at idle and brighten when you rev the engine. This shows the alternator isn't producing enough power at low RPMs.
- Battery Warning Light: If your dashboard has a battery or "ALT" light, it will illuminate if the system voltage drops below a safe level, often around 12 volts. This is a direct signal that the alternator is not charging properly.
- Whining or Grinding Noises: A high-pitched whine that changes with engine speed often points to failing bearings inside the alternator. A grinding or growling sound can also indicate serious internal problems.
- Weak or Dead Battery: The alternator is supposed to recharge your battery while you drive. If you find yourself with a dead battery frequently, or the truck struggles to start, the alternator may not be doing its job.
- Smell of Burning Rubber or Wires: A hot, burning smell can indicate an overheating alternator or a damaged wire. This is a serious symptom that requires immediate attention.
Choosing the Right Aftermarket Alternator
For a classic truck like the K1500, aftermarket alternators are the most common and practical choice. Original OEM Delco-Remy parts are largely unavailable new. The good news is that aftermarket quality is often excellent and provides opportunities for upgrades.
OEM vs. Aftermarket Quality
For this generation of truck, a quality aftermarket alternator can be just as reliable as the original part, and often more affordable. Brands like ACDelco (GM's own parts division), Bosch, and Remy have a long history of producing dependable alternators. ACDelco offers different tiers; their "Gold" line is considered a high-quality professional-grade part, while other lines may be more budget-focused. Many owners on forums report good experiences with parts store brands that offer lifetime warranties.
Pro Tip: Internally vs. Externally Regulated
A critical detail for 1979-1987 models is whether your alternator is internally or externally regulated. Early models in this range may have an external voltage regulator (a small box mounted on the firewall or fender), while later models have it built into the alternator. An internally regulated alternator has horizontal, side-by-side electrical connectors, while an external one often has vertical connectors. Ensure you buy the correct type for your truck's wiring harness or plan to perform a conversion, which is a popular upgrade for older models.
Upgrading Your Amperage
Stock alternators for the K1500 were often rated between 63 and 78 amps. This was enough for the factory equipment. However, if you've added accessories like a powerful stereo, off-road lights, a winch, or an electric fuel pump, you should upgrade to a higher-amperage alternator.
- Stock or Near-Stock: A 63- to 100-amp alternator is sufficient.
- Moderate Upgrades (Lights, Stereo): Consider a 100- to 150-amp unit.
- Heavy Upgrades (Winch, Big Amps): High-output alternators of 200 amps or more are available from specialty brands like Powermaster and JS Alternators. These often require upgrading your main power and ground wires (the "Big 3 Upgrade") to handle the extra current.
Common Ordering Mistake
Pay close attention to the pulley type. Your K1500 will have a V-belt pulley. Many aftermarket alternators, especially high-output models, may come with a serpentine belt pulley. You may need to swap the pulley from your old alternator to the new one. Always confirm the pulley type before purchasing.
Cost of a New Aftermarket Alternator
The price for a new aftermarket alternator for a 1979-1987 GMC K1500 can vary based on brand and amperage output. Here is a general breakdown of expected costs:
| Part Type | Estimated Cost |
|---|---|
| New Aftermarket (Standard Output) | $50 - $150 |
| New Aftermarket (High Output) | $150 - $440+ |
| Remanufactured Aftermarket | $70 - $180 |
| Shop Labor for Installation | $100 - $200 |
Frequently Asked Questions
Is replacing the alternator a DIY job?
Yes, for most owners with basic mechanical skills. The alternator on a Square Body K1500 is typically easy to access at the top of the engine. The process involves disconnecting the battery, loosening the belt tensioner, removing a few bolts and electrical connections, and then swapping the part. The entire job can usually be done in about an hour.
Do I need a special "one-wire" alternator?
A one-wire alternator is a type of internally regulated alternator that simplifies wiring, making it popular for custom builds. It only requires a single wire connection to the battery. While you can use one on your K1500, it's not necessary unless you are doing a custom wiring job. A standard three-wire, internally regulated alternator is a common and effective upgrade from an older externally regulated system.
What are the best alternator brands for my K1500?
Forum members and mechanics often recommend sticking with known brands. ACDelco is a safe bet for GM vehicles. Bosch and Remy are also reputable aftermarket manufacturers. For high-performance needs, brands like Powermaster, Proform, and WAI Global are popular choices that offer higher amperage outputs.
Were there any recalls for the alternator on these trucks?
Searches on the National Highway Traffic Safety Administration (NHTSA) database show no specific alternator-related recalls for the 1979-1987 GMC K1500 model years. Recalls for loose alternator cables were issued for much newer GM trucks (2019-2020 models), but this does not apply to the Square Body generation.
Technical Specifications
<ul><li><strong>Common OEM Style:</strong> Delco 10SI, 12SI</li><li><strong>Stock Amperage Range:</strong> 63A, 78A, 94A</li><li><strong>Voltage Regulator:</strong> External (early years) or Internal (later years)</li><li><strong>Pulley Type:</strong> Single V-Belt</li></ul>
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The information in this article is provided for general reference and educational purposes only. Vehicle specifications, procedures, and part compatibility can vary by production date, trim level, and region. Always consult your vehicle's factory service manual and verify part numbers before purchasing or performing repairs. Safety-critical components such as airbags, seat belts, and braking systems should be installed by a qualified professional.
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- Is Your Alternator Failing? Common Symptoms
- Choosing the Right Aftermarket Alternator
- OEM vs. Aftermarket Quality
- Pro Tip: Internally vs. Externally Regulated
- Upgrading Your Amperage
- Common Ordering Mistake
- Cost of a New Aftermarket Alternator
- Frequently Asked Questions
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