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Nuts & Bolts: Alternator Upgrades for Winching

Posted in How To: Tech Qa on June 21, 2018
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Dear 4-WHEEL & OFF-ROAD: I have a 1978 Chevy Blazer that I plan to do some four-wheeling with and will be adding a custom bumper with a winch. I was thinking I need to upgrade the alternator to handle the load of the winch. It’s my understanding that a winch can draw over 200 amps when in operation. What would you recommend in terms of an amp rating, and what upgrades are available?

Jeff H.
Via nuts@4wor.com

A winch can draw in excess of 400 amps when in use, so on paper a winch can suck a battery flat in no time. However, in the real-world use that’s not usually the case. The amperage your winch draws depends on the amount of load placed on it and how long it’s in use. With no load (just spooling in and out) a typical 9,000-pound winch will draw 60-70 amps. It will draw about 250 amps under a 4,000-pound pull, and nearly 480 amps at 9,000 pounds. Actual amp draws varies by the winch and the manufacturer, and the cheaper the winch, typically the greater the amp draw. These are big numbers, but remember that the pulling power a winch applies is often brief. In most situations, a quick tug is all that’s needed to get a vehicle unstuck. It’s only in deep mud or situations like pulling a dead vehicle up a long hill that a winch will draw large amounts of amperage for a long time.

When high amperage is required, your vehicle battery acts as a reservoir for power and it supplies the necessary amperage to the winch by dipping into its reserves. The alternator contributes what it can and then fills the battery back up once the winch stops. The amperage rating of the alternator determines how much reserve is used during the actual winching and how quickly it fills the battery back up. This is why it’s important to pause often during those really long pulls; you want to give the winch a chance to cool down and the alternator a chance to recharge the battery.

Your Blazer is most likely equipped with a Delco 10si alternator, as most GM cars and light trucks used the 10si through the 1970s. These came in a variety of amp ratings depending on the vehicle and how it was equipped, but we’re guessing that your Blazer’s alternator is capable of producing somewhere around 60 amps. While certainly on the lower side of ideal, 60 amps is adequate if you only plan on using the winch occasionally and don’t do a lot of mudding.

If you wanted to upgrade, you can swap in a 94-amp 12si alternator without much trouble. The 12si was used on GM vehicles starting in the mid 1980s and had a higher amperage rating to handle the growing number of computers and other electrical accessories on vehicles of the day. These later alternators share very similar physical dimensions to their 10si predecessors, so swapping one in is easy. If you want to know more about GM 10si and 12si alternators, our friends at MAD Enterprises (madelectrical.com) have put together this great resource: madelectrical.com/electricaltech/delcoremy.shtml. There are also many higher-amp aftermarket alternators based on both the 10si and 12si cases, so it would be fairly easy to swap in an alternator capable of 200 amps if you wanted to (though MAD makes some good points about why this might not be a great idea). Remember that a significant jump in amperage will also require wiring modifications to handle the higher current.

If you plan on using the winch rarely, we would just upgrade the battery to a higher reserve capacity than stock and make sure both it and the alternator are in good shape. Remember to stop often during the rare long winch pulls, and you should be fine. If mud is your terrain of choice and you find yourself winching nearly every weekend, then an alternator and even dual batteries would be a wise investment.

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