one week ago I put a new alternator on my car. The alt. came from a reputable rebuild shop(120 amp). The battery is about three months old. I just ran my car across town and when I was pulling up to a store, I noticed the amp guage was down on 8. Anyway I went in the store and when I came out It wouldn't start. It would just barely turn over. I figured the alt. went bad and I was running off the battery and the battery had gotten drained. I took the alt. back to the shop and they tested it and said it was charging good, but gave me another one anyway. I had the battery charging while I was gone. I hook the new alt. up and crank it and go for a ride and the same thing is happening. I'm slowly losing my elec. power. I tested the battery with a multi-meter from post to post and was getting 13 volts. Can anyone point me in the right diredtion to correct this?

 

It should be more than 13 volts at the battery with the engine running. How old are the cables? As far as I can remember there are three, and they are all subject to corrosion and as a result, severely limit your alternator's ability to get it's charge to the battery. It's a bit like a bottleneck in your charging circuit. There's an earth strap at the rear of the passenger side cylinder head, a fat positive cable down to your starter motor, and an earth cable to your alternator mounting bracketry. Check their condition and replace if necessary, as they are a popular thing to overlook.

 

Check the plug in its self to see if any of the wires have pulled out. If you can hook up you tester to the battery and run the car while you wiggle it and see if the voltage jumps.

 

I checked wires and cables, they seem to be in order. I cranked the car while I had the multi-meter hooked to it and the voltage jumped up over 13 but after a while of running it dropped down to 12 and acted like it was gonna go down more so I shut it off. So I'm back to square one. BTW although my battery is not old I'm not ruling anything out. Could a bad battery cause something like this? The battery came from Walmart. It was the best one they sell but that's not saying much either. I did stick a test lead down in the ground wire on the plug that plugs to the alt. and ground it to the frame, it reved up and the meter was up around 14 then it just idled down and started losing power again.

 

There should be a small gauge wire (brown?) that is the switched 12V on for the alternator. Without that being plugged in the alternator will not charge, just spin spin spin.

 

The readings on the battery and the wire that bolts on to the stud on back of the alt have the same reading. The the plug has 2 wires coming from it. A heavy guage red wire and a smaller guage black wire. The red wire was hot at all times and the black was hot only when the key was in the ON position.

 

Ok so you have voltage to and from the correct places. The small black wire sometimes brown is the wire that goes to the regulator. Sounds like the alternator has a bad regulator. If you were to jump the two wires on the plugin (engine running and plug plugged in) it would turn the regulator on and the system would charge full blast. The parts stores do this on the machine and say yep you gotta good one here look somewhere else for the problem. You could try this on your car and see if it starts to charge or just take the POS alternator back and get one that works correctly since that will be the end result anyway.