How To Test A Mustang Alternator | 5 Simple Steps

Created by Jay Walling / 6 min read
Date Created: 9/20/2018
Last Updated: 9/1/2022

Having charging problems or see a battery light pop up on your dash? LMR explains how to check & test your alternator with only 5 simple steps.

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When your car's charging system is not performing up to par, you run the risk of being left stranded on the side of the road. When you see a battery light pop up onto your dash, you know that there is some type of fault in your charging system. If your battery is fairly new, the most likely culprit is your alternator. Follow along as LMR walks you through the quick and easy steps on How To Test Your Alternator.

Tools Needed

  • Voltmeter

How To Test An Alternator

  1. Remove battery terminals and make sure these are 100% clean from any buildup of corrosion. Clean these to ensure proper continuity for the charging system. Reconnect for testing.

  2. How To Test A Car Alternator: 5 Steps  - How To Test A Car Alternator: 5 Steps

  3. Test the battery voltage, at rest, this should read between 12.5 and 12.8V on average. If the batter is low, then charge the batter to meet these specs.

  4. How To Test A Car Alternator: 5 Steps  - How To Test A Car Alternator: 5 Steps

  5. Check overall vehicle grounds to ensure these are up to par.

  6. How To Test A Car Alternator: 5 Steps  - How To Test A Car Alternator: 5 Steps

  7. Inspect for proper belt alignment and any possible belt slippage, these can affect the overall output of the charging system.

  8. How To Test A Car Alternator: 5 Steps  - How To Test A Car Alternator: 5 Steps

  9. Test the battery and look for 13.5V-14V with the engine on and all accessories off including lights, a/c, and radio. This can be done at the battery terminals with a quality digital volt and ohm meter. This should be a normal reading for these setups. If you do not have access to a DVOM, then you can remove the alternator from the car and most local auto parts stores will test these for free.

  10. How To Test A Car Alternator: 5 Steps  - How To Test A Car Alternator: 5 Steps

That's it! These 5 simple steps are sure to help with checking and testing your alternator. Take matters into your own hands by avoiding the inconvenience that it takes for a professional to diagnose something so simple.

For More Information On SVE Alternator Warranties, Please Visit : Musang SVE Alternator Warranty Page

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Thumbnail image of the author of this article, Jay Walling.

About the Author

Jay has written content for Late Model Restoration for over 10 years, producing over 120 articles. Jay has an extensive 25-plus-year background in automotive and is a certified Ford Technician. Read more...

79-93 Mustang 130/200 AMP Alternator Troubleshooting


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. **