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Fox Body Mustang Alternator Buyer's Guide

Created by Neal Jimenez
Date Created: 7/26/2022
Last Updated: 7/26/2022

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  • Fox Body Mustang Alternator Buyer's Guide

Selecting a proper alternator for your Fox Mustang build will be a crucial part that you will need to consider. This will also include all associated parts that go along with either a stock or aftermarket upgraded applications. Within this Fox Body Alternator Buyer's Guide, we will cover everything you will need to know for your build!

WHAT ARE THE MOST POPULAR FOX BODY MUSTANG ALTERNATOR UPGRADES?

  • 75 Amp - If you are looking for a 100% OEM replacement alternator, the 75 amp option is the route to go. This option will be a direct bolt-in part, and these do not require modifications to the bracket or any wiring connectors on the engine harness. There can be drawbacks, though, depending on your application. 1979-1993 Fox Mustangs are sometimes plagued with low idle voltage issues, especially if underdrive pulleys are added. But if you are shooting for true restoration and want an OEM look, feel, and operation, this is the route to go!
  • 95 Amp - The next step up from the 75 amp option will be the 95 amp alternator. This step up over stock will help eliminate some of these low idle voltage issues we previously discussed on the 75 amp configuration. Dimming lights, idle surging, and battery drain problems all are somewhat common on these older Fox Bodies. This 95 amp upgrade is recommended for engine combinations with mild upgrades like the underdrive pullies. This can help eliminate these concerns dramatically compared to the stock 75 amp alternator. Depending on your option, these can require modification to the stock wiring and bracket for fitment on your Mustang 5.0.

  • 130 Amp - Adding underdrive pulleys or running your accessories such as headlights, A/C, or heat will likely cause the lights to dim and the battery gauge to drop. Throw an electric cooling fan into the mix, and the stock 75/95 amp alternator simply can't keep up. This is where the 130 amp options provide the increased charging output it needs to keep all electrical systems running as they should. These larger alternators will require modifications to the bracket and upgrades to the associated wiring. It is also highly recommended to use the upgraded 4 gauge power wire for this one as well.
  • 200 Amp - The 200 amp upgrade will be the last we cover. This one is used in extreme duty electrical output needs for the Fox Body Mustang. When considering this setup, you will have everything we have previously covered on the 130 amp, along with an upgraded stereo setup. Aftermarket subwoofers and amplifiers can add additional strain that the smaller options cannot keep up with. If you are throwing the book at your Mustang 5.0, then the 200 amp upgrade is right up your alley. Just like the 130, you will need to modify the bracket and upgrade the associated wiring. But with the 200 amp setup, the 4 gauge power wire is 100% required.

Fox Body Mustang Alternator Upgrade Options

SUGGESTED USE FACTORY HARNESS MODIFICATION? POWER WIRE UPGRADE? BRACKET MODIFICATION?
75 AMP Factory Replacement No No No
95 AMP Step Up From Factory Yes No Yes
130 AMP Needed For Electric Fans or Aftermarket Stereo Yes Recommended Yes
200 AMP Great For Electric Fans + High Demand Stereos Yes Required Yes

Commonly Asked Questions

WHAT IS THE DIFFERENCE BETWEEN 1G, 2G, AND 3G MUSTANG ALTERNATORS?

  • 1G - The 1G alternator was an externally regulated alternator design, and these were used on all 79-93 Mustang applications. These have an average output of 70-75 amps stock and will utilize all OEM wiring harness connections. These tend to feature a smaller case design over the upgraded counterparts.
  • 2G - The 2G alternator was the updated second generation design and featured an internally regulated design. These were not equipped on the factory's Fox chassis Mustangs. This design was normally rated between 65-75 amps by Ford.
  • 3G - The 3G design was not found on the factory's 79-93 Mustang application. These featured 130 or higher amp ratings, updated sealed bearings, and an internal fan for upgraded cooling.

CAN YOU INSTALL A 3G ALTERNATOR ON THE 1979-85 MUSTANG?

A 1G to 3G upgrade is perfectly fine for the 1979-1985 applications. When completing this upgrade, a new replacement external regulator box will need to be added to capture the proper circuit and retain the stock wiring harness along with a 3-wire (3G Series) regulator plug wired into place. This route will allow you to retain the OEM warning lights and shunt-type ammeter on the vehicle dash.

DO YOU NEED TO UPGRADE THE POWER WIRE ON A FOX BODY ALTERNATOR UPGRADE?

Upgrading the power wire is not always needed in all cases. The 75 and 95 amp options do not require this upgraded power wire. However, installing this on any of the 130 amp options is recommended, and it will be 100% required for all 200 amp configurations.

 


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Alternator Diagnosis

 

SVE 130/200 Amp Alternator Calculator

Why is this important to know? Often, the combination of performance parts can result in your upgraded alternator not charging at its idle RPM. The 130/200 amp alternators need 1800-2000 idle RPM to make power. IMPORTANT: this is shaft RPM and not engine RPM. These alternators will produce maximum output at 6000 shaft rpm but will fail at 16,000+ RPM.
Steps To Calculate Shaft RPM

  • Measure Alternator pulley size (for example 2.5")
  • Measure Crankshaft Pulley Size (for example 5")
  • The Ratio is 2:1 (5 divided by 2.5 = 2)

To determine the max shaft RPM, take your max engine speed (for example, 5800 RPM) and multiply it by the calculated ratio you figured above (5800 x 2) = 11,600 shaft RPM. To determine the idle shaft RPM, multiply idle RPM x Ratio (for example, 1000 x 2) = 2000 shaft RPM @ Idle. Note: lower output alternators require less shaft speed at idle to perform.

The manufacturer recommends the following:

  • 95 amp alternator minimum idle shaft RPM of 1600
  • 130 amp alternator minimum idle shaft RPM 1800
  • 200 amp alternator minimum idle shaft RPM 2000 (2200 is ideal)


3G/4G Alternator Charging Diagnosis

If you have upgraded to a 3G alternator in your Fox Body Mustang and are still experiencing charging issues, here are some of the best places to start. Don't forget to check to ensure your alternator is also idling at optimum RPM to charge.

  1. Check the state of the battery.
    1. Battery in good working condition will show between 12V to 12.5V with nothing drawing on it. If your battery is below that level, chances are you have a bad battery.
  2. Compare voltage at the battery to the voltage at the output post of the alternator.
    1. There should be very little drop in voltage from the battery to the output post of the alternator. Any significant resistance on this wire could be detrimental to the operation of the alternator.
    2. If you show 0 voltage at the output post of the alternator, check the fuse or fusible link in the power wire going to the battery as it may be blown.
  3. Check the voltage on the 3 wire Regulator Plug.
    1. Green wire with red stripe - This wire is the ignition key on/key off. With the key in the on position (Car Not Running!), this wire should read between .5 volt to 1 volt less than the battery voltage.
    2. White Wire (3G only) - This wire is called the stator wire. This wire needs continuity from the voltage regulator plug to the grey clip that plugs into the alternator.
    3. Yellow Wire - This wire is called the sense wire and is a reference wire for the voltage regulator, so it knows how much voltage to produce. This wire should equal battery voltage at all times.

** Never under any circumstances remove the battery cable while the car is running! Doing this can damage the internal voltage regulator making the alternator no longer charge. This test only worked on early cars with a very low draw and an external mechanical voltage regulator. **

Fox Body Mustang Alternator Buyer's Guide Tech Info