Voltage problem?

[gumby5.0]

Im having a problem with the voltage on my car. Itll push 14.4v at idle with everything on except the a/c. As soon as I turn the a/c on the voltage goes down too 12v and everything dim's. What would cause such a issue? Both the alternator and battery are known good!

 

[302GTS]

Alternator could be getting old? I know this was an issue with the Foxes, because their, I believe, 95 amp alternator. Our cars had a 130 amp in them stock I'm pretty sure.

 

[TrueBlue95GT]

Do the RPM's drop? When the A/C turns on, the idle should go up a little to sustain the voltage.

 

[gumby5.0]

I think the rpms stay put. I cant really remember though?

 

[HISSIN50]

Is this at idle or at like 1500 or 2K RPM?


The AC compressor, HVAC fan and cooling fan all burden the system greatly.
It's normal to see a decent sized drop, but it should stay above 12.0V.

How long ago was the alt tested? The load test indicated that the alt had not dropped a diode? It kinda feels like you did.

And V8 fox III's have a 65 Amp alternator.

 

[gumby5.0]

It is at idle. Everything dims as soon as I cut it on. The car in question is a 95 gt. It was tested yesterday.

 

[HISSIN50]

There are too many variables at idle.
What is the system voltage at 2K RPM with a full load? Be sure you're using a DMM (the stock gauge is not accurate enough for diagnostics).

If the voltage is fine above idle, it probably just means the alt isn't spinning fast enough at idle for one reason or another. Variables include underdrive pulleys, idle speed, alternator pulley diameter, etc.

 

[gumby5.0]

There are too many variables at idle.
What is the system voltage at 2K RPM with a full load? Be sure you're using a DMM (the stock gauge is not accurate enough for diagnostics).

If the voltage is fine above idle, it probably just means the alt isn't spinning fast enough at idle for one reason or another. Variables include underdrive pulleys, idle speed, alternator pulley diameter, etc.

The car does have underdrive pulleys on the crank and water pump, but the alternator pulley is stock.

 

[Ryno8705]

I already posted a response on corral for this issue, but I have to ask, what is the diameter of the alternator pulley you are using? Some replacement alternators have a larger pulley than a stock Mustang pulley.

 

[gumby5.0]

I already posted a response on corral for this issue, but I have to ask, what is the diameter of the alternator pulley you are using? Some replacement alternators have a larger pulley than a stock Mustang pulley.

The alternator pulley is infact a little bit bigger than the factory pulley. I am not sure on the exact diameter at the moment tho.