alternator problem please helppp

First, the post does not state which year car you have. Therefore, I can not tell you which fuses to check. So I suggest that you check ALL fuses as this could very easily be a blown fuse. There are two fuse panels. One in the engine bay and the other inside the car.

For the 1999-2004 MY, check fuses F1.20(20amp) and F2.5(15amp).

If all fuses are OK, check the cable between the alternator and the battery. There is a fusible link built into this cable. If blown, the alternator will not charge. Start by testing for +12 volts between the large alternator B+ stud and battery negative. Should be +12 volts (key off). Post the results.

Also, are the battery terminals clean and tight?

Check for +12 volts on the smaller alternator connector key in run position. The wire color is YE/WH. If no voltage the regulator will not function.

Check for a good ground to the motor. Measure the resistance between the alternator case and battery negative (key off). Should be low. Post the results.

Note, the days of jumping/push starting a car with a totally dead battery are OVER. The battery is needed to excite the alternator field coil. The alternator will work for a short while but with a large change in electrical load the field coil will collapse shutting it down. Without a battery to re-power the field coil, the alternator spins but doesn't generate anything.

How did the guy test the alternator - in the car? Did he have you rev it up and hold it at 2000 RPM or did he just read it at idle? With the car warmed up and a fully charged battery, the voltage can drop down to near battery voltage at idle and with underdrive pulleys, you may not see much more than that if you've got any electrical loads going.

A stock alternator should put out about 14 volts at idle (more or less depending upon battery charge and temperature). Underdrive pulleys are known to reduce the voltage output at low RPM's. However, the alternator should be putting out something. 12.4 sounds like battery voltage to me.

This is easy enough to confirm for yourself with a Volt Ohm Meter (VOM). Measure the voltage at the battery with the key off. Post.

Start the car and measure the voltage at the battery. Post.

Rev the car to 2000 RPM and measure the voltage at the battery. Post.

Checking Ripple Voltage
Ripple voltage or AC voltage can leak past the rectifier bridge diodes and actually cause the battery to discharge. It can be measured by switching your DMM to AC and connecting the black lead to a good ground and the red lead to the "BAT" terminal on the back of the alternator. Do not connect the leads to the battery, as the battery will absorb or "dampen" some of the AC. Run the engine at 2,000 rpm
and read the meter’s display.

Did someone say "ripple"...? Search on it.

ALTERNATOR - Ripple Voltage Test
Using a DMM

The Alternator produces AC voltage and current. The battery requires DC voltage and current to charge properly. Diodes located within the alternator change (rectify) the AC to DC. However, a small amount of AC can still be present and no harm is done. Problems can develop when alternator diode faults permit unacceptable amounts of AC to pass into the vehicle electrical system.

Set the meter to read “AC” Volts (lowest range) .

Connect the black COM lead probe tip to the battery negative post.

Connect the red Volts lead probe tip to the battery positive post.

Run the engine at 1500 RPM.

Turn ON the high blower and high beam lights.

Your meter reading should not exceed .09 volts (90mV) AC.

High ripple voltage readings imply faulty diode(s) which can cause:

Undercharged battery.

Stalling.

Rough idle.

NOTE: If your meter reading exceeds 90 mV AC, use an ADL7100 labscope or equivalent to verify that ripple voltage spikes do not exceed one volt peak to peak

I didn't think to ask before, but is the battery light on the dash coming on while the car is running and what's the voltage gauge doing?

If you're having trouble interpreting the results your getting, another parts store should be able to do a more thorough test of your alternator and battery then what this place did from the sounds of it. The Advance Auto Parts stores around here have a fancy machine they diagnose your charging system with that test output at different RPMs and loads and checks for A/C ripple and you get a nice printout with the results.

Basically though, if it'll put out 14 volts DC at just off idle with your U/D pulleys and a solid 14V at 2000 RPM with the high beams and fan on full blast then it is essentially working. Then check for AC voltage as mentioned above and make sure it's not too high as you can get strange behavior and it can drain the battery too - there's always going to be a little AC though due to the nature of how the AC is converted to DC.

There's a good chance that your old alternator killed your old battery or vice-versa so it's not too surprising that they both died close too each other. There is a slight chance that your old battery could've killed your new alternator too - constantly charging a dead battery can burn up the diodes - but hopefully, it's just fine. If not, you should be able to replace it under warranty since its new, right?

Glad to hear you got it figured out