Don't disconnect the battery when the car is running it can damage the electronics.
It sounds like you got another bad alternator (cheap) get an ACDelco or you have a wiring issue. Here is a guide to help test the wiring and thwe alternator wiring diagrams below so you can see how it works.
The thinking is that if you disconnect either cable and the engine stays running, the alternator must be working but a lot of them will stop working due to the voltage regulator responding to the dips in the "ripple" voltage being produced. That will make a perfectly good alternator appear to be bad so that test is not valid.
Three things are needed to generate the output current. They are a magnet, (electromagnet, in this case), a coil of wire, and most importantly, movement between them. That's why the belt needs to make it spin. One thing that can save you from doing damage by removing a battery cable is not raising engine speed. Generators are relatively inefficient at low engine speeds and their output voltage is less likely to rise to dangerous levels, . . . as long as you don't raise engine speed.
One other thing to keep in mind is batteries give off explosive hydrogen gas. Regardless if your generator is working or not there is going to be a big spark when you remove a battery cable with the engine running. Either the alternator's current will be recharging the battery, and that can be up to 20 amps, or the battery is going to be supplying the car's electrical systems, and that can easily be over 30 amps. That kind of current is going to create a big spark when a connection is broken or reconnected. Small arc welders run as low as 40 - 60 amps and look at the sparks they create. The reason we don't hear about more battery explosions is because people are careful to not disconnect the cables when there is current flowing through them. It's also why there are huge warning labels on all battery chargers to be sure they are turned off before connecting or disconnecting them from the battery.
The second most common failure is the main suspect when you have what appears to be multiple alternator failures. That is a break in the wire from that second smaller terminal to the voltage regulator inside the Engine Computer. For this you'll find exactly the same voltage on both smaller wires, typically around 12.2 to 12.6 volts. That means the difference in voltage between the two wires is 0.0 volts, and that means no electromagnet is being developed. This is almost always caused by corroded or spread terminals in the connector between the engine and body.
If you do find 4 - 11 volts on one terminal but output voltage is also low, as in 12.2 to 12.6 volts, suspect a defective diode inside the alternator, but those are rarely intermittent. It would be more likely there's a bad connection inside, but that would be really rare.
Check out the diagrams (Below). Please let us know what you find. We are interested to see what it is.
Monday, October 6th, 2014 AT 2:06 AM