When I turn on the A/C the voltage guage drops 2 v. I checked the battery with car off and @ idle and it is fine. Anyone have any ideas?

 

Check the grounds, relays, and power wires for the electric fans. Also, make sure the plug on top of the compressor didn't melt the diode, they do that alot.

 

checked everything you suggested and checked voltage again; 12.6 w a/c on and 13.9 with it off. Alternator?

 

mine does that too.
i think since the drivetrain is on heavier load,
its normal to drop voltage slightly.

i never measured, not sure if mine drops by 2v, but
it does slips down a little bit.
it still stays pretty close to 13.. so i let it be.

 

Try raising the engine rpm with the a/c on, then measure the voltage. Compressors and fan motors draw considerable current, so like jekyll and hyde says, check all your connections. Are you measuring at the output terminal of the alternator or the battery terminal or where? Also, look for corrosion under the battery terminal connections, both hot & ground. Acid has a way of leeching into the cable, under the insulation, and will cause a higher resistance through the cables.

Good hunting,
Tim sends

 

If you have a *ahem* less than perfect *ahem* battery then I'm not surprised about the drop.

Use your meter in AMP mode in place of the chassis fan fuse to see how much current that draws. Then move to the blower motor, then to the A/C fuse. You'll be shocked.

Magnetic A/C compressor clutch draws about 10 amps. Since the radiator fan runs continuously when A/C is on, you can add another 12-15 amps. Now the blower motor is another 12 amps (these are my own readings, yours might vary considerably.) Non-A/C operation doesn't use any of these except for the radiator fan intermittently.

So having the A/C on pulls about 35+ amps. Your stock alternator or Autozone/Discount OEM replacement puts out around 40-50 at idle. Peak rated output is measured at significantly above idle. So your 110A alternator still doesn't quite put enough out Add your headlights, actually running the motor, a loud radio, and your exactly right - Your volts drop a bit as your actually pulling from the battery to keep your car running.

Psycho GM engineers figured nobody would be silly enough to idle the car with the air on for very long. So its made to kill the battery at idle. Most people eventually get moving again and the battery defecit is made up for when you hit the gas for a minute.

My temp solution - Use the second to highest fan setting, and cranked all the way sparingly. It's only slightly slower to cool the car down, (even deep in south florida) and uses a full HALF the power. The volt drop is less substantial on that setting.

Some simple logic lets you maximize whats available without needlessly destroying an otherwise healthy battery... MAX does nothing but pull the air from inside the car. The extra woosh you hear on MAX is just the air being pulled in from the passenger side. NORM and BILEVEL use outside air.

Use NORM only after you first start the car to get the hot air thats built up out of the car with the windows cracked, The outside air is way cooler than the 140+ commonly encountered in your car in summer. Since our HVAC can only get a temp differential of like 30-40 degrees or so... Well cooling 90 degree air to 50 degrees is better than cooling 140 degree air to 100 degrees. Once you feel its cool for a minute or two, close the windows up and use MAX to finish getting her cold. Put it back in norm, which should keep the temp comfortable as well as keep the accumulator and evaporator from frosting up.

Permanent solution is a larger battery, and an alternator that is rated to put out higher amps at idle.

 

I just had this problem with my car too. Have an 86 IROC-Z and I did everything I could, Upgraded wires, changed alt three different times to no avail. Finally went to junk yard and pulled a CS-144 Alt from a Buick Roadmaster and paid like 11.00 for it. It is a 140 Amp alt and now at idle with everything on my volts are at about 13.4 Volts when car is hot.