Jim Smart
June 12, 2014
Photos By: The Manufacturers

Mustangs have had alternator charging systems since August 1964, when all Fords and Mercurys became so equipped. Alternators are clearly better than generators in that they maintain a steady charge at all engine speeds, including idle.

In the beginning, Autolite 1G alternators, which were rated at 38-55 amps, didn’t have much to keep up with. There was the radio, heater, lighting, occasionally horns, and the ignition system.

Today, alternators have a much larger job to do thanks to all the accessories we like to install in automobiles that offer the comforts of home. When you start adding subwoofer sound systems, power windows and locks, high-intensity headlights, electric cooling fans, and the rest of it, it can make an older Autolite 1G alternator sweat with anxiety trying to keep up with the load.

Whether you’re restoring a classic Mustang or building a hot restomod, it’s important to know a bit about Ford-based alternators and how they have evolved over the last 50 years, then make the right selection for your application. Cool thing is, you can uprate an older 1G 38-amp to 100-amp with the right parts, or fit your classic with an uprated 1G for a stealthy improvement.Alternator selection boils down to electrical demand. Output must be greater than demand or you wind up with dim headlights and a dead battery.

Showroom stockers can get by with original equipment—the externally-regulated Autolite 1G common from 1965 to 1986 are all interchangeable. In 1982, Ford stepped up the charging system demand with the internally regulated 2G alternator, which looks basically like the 1G, only with a 2G-specific wiring harness. The 2G was common to all Mustangs from 1986 to 1993. It was replaced by the 3G in 1994, then the 4G and 6G later.

Not only were there differences in amperage rating, but also pulley sizing, width, and type, in either one-groove or two. Most small- and big-block Fords had a 2.62-inch-diameter, single-groove pulley. The 289 High-Performance V-8 alternators had a larger 3.87-inch- diameter, single-groove pulley to reduce rotor speed at high rpm. Dual-groove pulleys are 3.15 inches in diameter—slightly larger than the single groove. Pulley size, number of grooves, and amp rating depended upon application.

Alternator fan type is also very important to both identification and function. We see so many rebuilt Autolite 1G alternators out there with incorrect fans for the production time frame, and we’d like to set that straight.

According to Jack Brooks at www.deadnutson.com, fan type depends on when the alternator was manufactured, and this is easy to see at a glance. Those first 1G alternators had the flat 13-blade fan used before March 1965. Beginning in March 1965, a more sculptured (stronger) 13-blade fan was used on the 1G. Beginning November 17, 1969, the 1G got a 10-blade fan (fewer, wider blades), which was used through the end of 1G production in 1986. This 10-blade fan was also used on the internally regulated Motorcraft 2G alternator from 1982-1992.

1G Alternator

Alternator use and application gets complicated with the 1G. Because these 1G cores have been so scattered through rebuilds and salvage yards over the years, expect to see a wide variety of mismatched parts. The rounded case 1G Ford/Autolite alternator was used from mid-1964 through the ’71 model year. Beginning with the changeover to Motorcraft in 1971-1972, Ford redesigned the 1G case with a square corner housing, which was used through the end of 1G production in 1986.

This is the Mustang’s original externally-regulated 1G alternator in 38-, 42-, and 55-amp ratings. Most rebuilt 1G alternators are 55-amp today with new rectifiers, stators, and rotors. These alternators originally were identifiable by a color-code (prior to February 1969) and alphanumeric markings in the casting; purple for 38-amp, orange for 42-amp, and red for 55-amp. From February 1969 forward, alternator identification was stamped into the case.
Early 1G alternators have a simple harness with four terminals and protective boots. Beginning in 1969, Ford conceived this multiplex Y plug that fits over terminals and is protected with caps over each terminal. This is a square-case Motorcraft 1G alternator with the convenient multiplex Y plug.

You will see mix-matched round and square housings, 10-blade fans with round cases, 13-blade fans with square cases, and so on. If you’re not concerned with originality, none of it matters—it is all interchangeable and will function just fine. However, if you’re looking for correct period parts, that’s a challenge. It’s a crap shoot when you buy a 1G from a discount auto parts store. You may get exactly what you want or wind up with a mix-matched case combination.

If you have an Autolite or Motorcraft 1G alternator original to your Mustang, there are excellent resources where it can be rebuilt and massaged to perfection. You may also buy a rebuilt 1G at a discount auto parts store and swap its components into your original case, especially if you’re concerned about losing your case to a rebuilder.

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Most reputable rebuilders perform a complete replacement of all 1G parts. You get what you pay for. Some clean up existing parts, dress slipper rings, test all components for continuity, and put the alternator back together with new bearings and brushes. Not all rebuilt 1G alternators are new inside.

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The 1G is pretty consistent throughout its production history, along with the Mustangs it bolts into. Where the 1G gets tricky is the ’84-’86 Mustang SVO, which is 1G-equipped but wired differently between the alternator and voltage regulator. You are better off having your 1G rebuilt by a trusted source.

Although purists and restorers have spent a lot of time trying to sort out 1G alternator case differences, there are no pat answers. Some have Autolite on them, while some don’t. Some have the teardrop at the rear of the case, while others don’t. There are aftermarket 1G cases as well adding to the confusion. Ideally, you will find a complete 1G core or castings appropriate for your restoration.

Autolite versus no-name boils down to whether it was factory installed or a service replacement, though that isn’t even completely true. Assembly plants installed whatever was in the parts pipeline and no one cared.

And again there’s the confusion of aftermarket versus genuine Ford. It takes savvy eyes to spot what’s correct and what isn’t.

2G Alternator

The Motorcraft 2G internally- regulated alternator (known as “firestarter” and “flame-thrower” among enthusiasts) was introduced in 1982 on full-sized Fords and Mercurys, later working into the Mustang beginning in 1986, with 5.0 liters and fuel injection.

This Motorcraft 2G internally-regulated alternator common to ’86-’93 Mustangs. Unless you are doing a concours restoration, it is suggested you step up to the Ford 3G alternator and the appropriate wiring, which is a simple conversion with great benefits. The 2G struggles with a marginal multiplex plug to begin with. When you add corrosion issues and poor continuity, you get resistance and heat.

Although we’re showing you the 2G, it is strongly suggested you step up to the 3G unless you’re performing a concours ’86-’93 Mustang restoration. It is also suggested you periodically inspect the 2G’s multiplex plug and wiring for heat and decay.

The 2G’s biggest problem is under-capacity and wire size that just isn’t up to the job. When you add corrosion issues to this connector, resistance increases and heat becomes a major (and dangerous) issue. Do not use your 2G’s wiring and plug for the 3G conversion—it is not of sufficient capacity to handle the electrical load and will start a fire. Rick Harmon of PA Performance suggests upgrading to the 3G if you’re in a position to do so and don’t mind replacing the factory 2G unit.

3G Alternator

Ask anyone about 3G and they might get it confused with the older cell phone network. However, Ford’s 3G high-amp alternator is the sweetest solution for anyone looking to boost charging system output in an afternoon.

This is the Powermaster 3G (PN 47760; PN 47759 for 200-amp model) natural-finish high-output alternator. It puts out nearly 100 amps at idle and 130 at speed. Dual internal cooling fans keep the Powermaster 3G at a safe operating temperature. The 3G is available either in serpentine or V-belt drive pulley configurations. The voltage regulator is integral and part of the brush holder, which makes service easy.

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The Motorcraft 3G alternator is available from a number of sources. PA Performance, Powermaster Performance, Performance Distributors, and Summit Racing Equipment are all excellent sources for 3G alternators and conversion kits.

There are plenty of reasons why you should replace your 1G or 2G with the 3G. The 3G will handle just about any size electrical load you can imagine, whether you have a classic or late-model Mustang. The beauty of the 3G is idle speed output and fitment. It delivers plenty of amperage at idle, and can deliver as much as 230 amps depending on where you source your 3G.

4G Alternator

The Motorcraft 4G pancake-style high-output alternator, which looks similar to the 3G, is available from PA Performance and Performance Distributors for Modular engines, small-blocks, and big-blocks. It is more compact than the 3G and produces more amperage.

This is the super-high-output 4G alternator designed strictly for Modular V-8s from Powermaster Motorsports. The beauty of the 4G is its compact size and power output. The 4G is optimum for Modular applications with imposing induction systems.

6G Alternator

The Motorcraft 6G alternator arrived on the Mustang for the ’99 model year (4.6-liter two-valve) and has been a mainstay ever since. The 6G is a super-high-output alternator designed more specifically for late-model Mustang applications with the 4.6L, 5.4L, and the Coyote V-8. There is virtually no reason to apply it to your classic Mustang application.

This is the Motorcraft 6G alternator, which arrived first on the ’99 Mustang GT and went on to be used on the all-new ’05 Mustang. The 4G really is engineered for late-model Mustangs and shouldn’t be considered for vintage installs. The 3G remains the best vintage Mustang alternator upgrade. If you’re considering a 6G upgrade for your SN-95 Mustang with a Modular engine, give it a lot of thought because the factory unit is fairly stout.