Choosing the Right Alternator for Your 1979-1987 GMC C3500
Keep your classic heavy-duty truck powered up by understanding your alternator options.
- Upgrading from a stock 10SI to a 12SI style alternator is an easy and beneficial improvement for reliability and output.
- For trucks with added electronics like lights or winches, a high-output alternator from a brand like Powermaster is recommended.
- Replacement is a very DIY-friendly job on these trucks, typically taking less than two hours with basic tools.
- Always check battery cable connections and belt condition before condemning the alternator, as these can cause similar symptoms.
Signs of a Failing Alternator on a Square Body Truck
On a straightforward truck like a 1979-1987 C3500, the signs of a weak alternator are usually pretty obvious. These trucks don't have complex computer systems, so you'll notice the real-world effects first.
- Dimming or Flickering Headlights: This is the classic sign. You might notice your headlights get dim at idle and then brighten up as you press the gas.
- Battery Light: If your dashboard has a battery warning light, it will likely come on when the alternator's output drops below what the system needs.
- Whining or Grinding Noises: Worn-out bearings inside the alternator will often make a grinding or whining sound that changes with engine speed. A squealing noise could also point to a loose or worn belt.
- Slow or Dead Battery: The alternator's main job is to charge the battery. If it fails, the battery will drain while you drive and eventually won't have enough power to start the truck.
- Weird Electrical Issues: On these trucks, this might show up as a radio that cuts out or gauges that read incorrectly.
Choosing the Right Aftermarket Alternator
For a vehicle from this era, a new aftermarket alternator can often be a significant upgrade over the original 1980s technology. Original OEM parts are generally unavailable. The market consists of new and remanufactured aftermarket units. Here's how to think about the brands available:
Aftermarket Brand Tiers for Your C3500
| Tier | Brands | Best For |
|---|---|---|
| High-Performance / High-Output | Powermaster, Proform | Trucks with added electrical accessories like winches, powerful stereos, extra lights, or EFI conversions. These are built to provide more power, especially at idle. |
| Premium Replacement | Bosch, Remy | Owners looking for reliability that meets or exceeds original factory specs. These brands are often trusted OEM suppliers for many car manufacturers. |
| Standard Replacement | BBB Industries, MPA, WAI Global | A budget-friendly, direct replacement for a stock truck with no major electrical upgrades. Quality can be inconsistent, with some users reporting early failures or lower-than-advertised output. |
Common Ordering Mistake: 10SI vs. 12SI
During this generation, GM transitioned from the 10SI alternator to the improved 12SI. The 12SI offers better cooling and higher amperage output, even at idle. They share the same physical dimensions and mounting points, making the 12SI a direct, easy upgrade. When replacing your alternator, opting for a 12SI version is a smart choice for better performance and reliability.
How Much Does a New Alternator Cost for a 1979-1987 GMC C3500?
The cost is reasonable for these trucks, with labor being straightforward for any mechanic.
Estimated Costs
| Item | Cost Range |
|---|---|
| New Aftermarket Alternator | $47.74 - $262.86 |
| Shop Labor | $100 - $200 |
| Total Estimated Cost | $147.74 - $462.86 |
Pro Tip: Check Your Connections
Before you replace the alternator, make sure your battery cables and the main wire from the alternator are clean and tight. A corroded or loose connection can mimic the symptoms of a bad alternator and cause charging problems, even with a new part. Also, inspect the alternator belt for cracks and proper tension.
Frequently Asked Questions
Do I need a high-output alternator?
If your C3500 is mostly stock, a standard replacement with 60-94 amps is likely sufficient. However, if you've added high-draw accessories like a heavy-duty winch, a large sound system, off-road lights, or an electric fuel pump for an EFI conversion, you should upgrade to a high-output alternator (140 amps or more) to handle the extra load. Powermaster is a popular brand for these upgrades.
What's the difference between a one-wire and a three-wire alternator?
Your stock C3500 came with a three-wire alternator. This setup uses a main battery wire, an ignition/exciter wire, and a voltage sensing wire. A "one-wire" alternator, popular in the performance market, simplifies wiring by only requiring the main battery connection. While easier to install, some one-wire units may require you to rev the engine after starting to begin charging. For a stock or near-stock truck, sticking with the factory three-wire setup is often the most reliable choice.
Is replacing the alternator a DIY job?
Yes, for most people with basic tools. The alternator on these Square Body trucks is usually mounted on top of the engine and is easy to access. The process involves disconnecting the battery, loosening the belt, unbolting the alternator, and swapping the electrical connections. It's considered a beginner-friendly repair.
Are there any recalls for the alternator on these trucks?
No, searches of the National Highway Traffic Safety Administration (NHTSA) database show no recalls or specific technical service bulletins (TSBs) for the alternator on the 1979-1987 GMC C3500. Any issues are typically related to age and normal wear and tear.
Technical Specifications
The original alternators for this generation were typically the Delco Remy 10SI or 12SI models. Standard output ranged from approximately 42 to 78 amps, with some higher-output options available. The 12SI, introduced around 1983, offered improved cooling and higher output ratings up to 94 amps.
We Have This Part in Stock
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.
- 🛍️ Shop This Part
- Signs of a Failing Alternator on a Square Body Truck
- Choosing the Right Aftermarket Alternator
- Aftermarket Brand Tiers for Your C3500
- Common Ordering Mistake: 10SI vs. 12SI
- How Much Does a New Alternator Cost for a 1979-1987 GMC C3500?
- Estimated Costs
- Pro Tip: Check Your Connections
- Frequently Asked Questions
- 🎟️ Get 5% Off