Choosing the Right Alternator for Your 1975-1978 GMC C35
Keep your classic GMC workhorse charged and on the road with the right replacement alternator.
- For this vintage, a quality new aftermarket alternator is a better choice than trying to find a rare OEM part.
- Assess your electrical needs; if you have any accessories, upgrade to a higher amperage alternator (100A+).
- Replacement is a simple, one-hour job for a DIYer with basic tools.
- While replacing, inspect the drive belt and battery cables for wear and tear.
Signs of a Failing Alternator
An alternator rarely fails without warning. For a 1975-1978 GMC C35, the signs are usually straightforward. You might notice your headlights and dash lights dimming, especially at idle. A whining or grinding noise from the front of the engine often points to bad bearings inside the alternator. The most obvious sign is a "GEN" or "ALT" warning light on your dashboard, or a battery that repeatedly dies and leaves you stranded.
OEM vs. Aftermarket Alternators
For a vehicle from this era, finding a brand-new Original Equipment (OEM) alternator is not a practical option. Your choices are typically remanufactured or new aftermarket parts. A quality new aftermarket alternator is often a significant upgrade over the original 1970s technology, offering better reliability and higher output. Many owners of classic GM trucks find that a modern aftermarket unit provides more consistent power, especially if they've added electrical accessories.
Choosing the Right Aftermarket Alternator
With dozens of options available, picking the right one depends on your truck's needs and your budget.
Performance / High-Output Brands (Powermaster, Proform): If you've added a powerful stereo, extra lighting, a winch, or other high-draw accessories, a high-output alternator is a smart investment. Brands like Powermaster specialize in alternators that produce significantly more amperage than the stock unit, ensuring your system has enough power at all times, even at idle. These are built for demanding applications and are a popular choice for modified trucks.
Premium Replacement Brands (Bosch, Remy): For owners who want long-term reliability for a daily driver or a stock restoration, brands like Bosch and Remy are excellent choices. They have a long-standing reputation for quality and produce alternators that meet or exceed original factory specifications. A new alternator from one of these brands is a dependable, fit-and-forget solution.
Standard Replacement Brands (BBB Industries, MPA, WAI Global): These brands offer a cost-effective solution for getting your truck back on the road. They provide new, functional alternators that are a direct fit for your C35. While some users report success, others have noted inconsistent quality control or lower-than-advertised output on some budget-friendly units. These are best suited for stock trucks with minimal electrical load where budget is the primary concern.
Aftermarket Alternator Cost
| Part Tier | Price Range |
|---|---|
| New Aftermarket (Standard) | $56 - $120 |
| New Aftermarket (Premium) | $120 - $190 |
| New Aftermarket (High-Output) | $190 - $263+ |
Frequently Asked Questions
What amperage alternator do I need?
The original alternators on these trucks were often rated around 63 amps. If your truck is completely stock, a 63-78 amp replacement is sufficient. However, if you have added any modern electronics, it's highly recommended to upgrade to a 100-amp or higher model to prevent dimming lights and ensure proper charging.
Is it hard to replace the alternator on a '75-'78 C35?
No, this is one of the easier repairs you can do. The alternator is typically located at the top front of the engine and is easy to access. The process involves disconnecting the battery, loosening the belt, removing a couple of bolts and electrical connectors, and then swapping the part.
Do I need a "one-wire" alternator?
Many high-output and aftermarket alternators are "one-wire" models, which simplifies installation. However, your C35 originally used a multi-wire setup with an external or internal regulator. Most replacements will come with an adapter or clear instructions to connect to your factory wiring. If you are using a battery isolator for a dual-battery setup, a one-wire alternator is generally not recommended.
How do I know if it's the alternator or the battery?
A simple test is to jump-start the truck. If it starts and runs but dies as soon as you disconnect the jumper cables, the alternator is the likely culprit. If the truck won't crank at all or cranks very slowly even with a jump, your battery may be unable to hold a charge. Most auto parts stores can test both components for free to confirm the problem.
Technical Specifications
OEM Part Numbers: 1102392 1102474 1102478 1102483
The original alternator for these trucks was typically a Delco-Remy 10SI series, which is internally regulated. Common stock amperage was 63 amps, with optional higher outputs available. The key to replacement is matching the V-belt pulley and ensuring the amperage meets or exceeds your truck's electrical demands.
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.