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.
- Check the state of the battery.
- 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.
- Compare voltage at the battery to the voltage at the output post of the alternator.
- 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.
- 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.
- Check the voltage on the 3 wire Regulator Plug.
- 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.
- 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.
- 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. **
