To begin with, I have an AEM EMS with HKS Twin Power and HID head lights, I also have underdrive pulley on the alternator)
I am trying to diagnose a fluctuating low idle that appears when there is additional electrical load. The car idles fine with no additional e load (lights off, a/c off, stereo off), vacuum is at a steady 15-16. When any of the mentioned items are turned on (together or indiuvidually), the idle would fluctuate low and vacuum would reduce to 11-12.
My first inclination is the current draw from the items exceeding the alternator (using underdrive pulley) capacity. But have been told that an engine with low compression would do the same due to increased load (hoping this is not the case).
The a/c compressor does create additional mechanical load when on, but the symptom is the same when I just turn the lights on. I don't see how an increase in electrical load would cause an increase in the alternators physical load.

Any suggestions or additional tests that can be performed to determine one way or the other (besides a compression test)?

 

A couple of "easy" tests you can do to test if your alternator is not meeting the load at idle is to press the brake light while the doors are open. If the light dims a significant amount, it's probably your alternator. This will also happen if you press the brake pedal with the driving lights on (look at your gauges). Another way to test is to put your car in a dim place and turn on the parking lights and check if they flicker.

A remanufactured alternator from Malloy Mazda is ~$165 after you trade in your core. If you have a lot of miles (100K+) and an underdrive pulley on there, I would really consider getting a new alternator.

 

Electrical load (such as a/c) should drop your rpms down, therefore causing the lower vacuum. I notice that things such as pressing the brake or turning on the light does decrease the rpms, but only very slightly.

You may want to try looking at your connection to your battery and replace/clean your grounds. My car's rpms used to chop upon start up, but it went away after I cleaned the terminals and the grounds. these things can cause differing strange little problems. For instance, my battery terminal was badly corroded & my windshield wipers would stutter whenever I turned them on. After replacing the terminals with gold plated ones, the symptom went away.

It could be that you have a bad alternator or battery also

 

I'll try that, thanks.

 

Both contacts are clean and I also have extra grounding cable put in (both the battery and the positive terminal is brand new).