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Testing Alternators:
Don't pull the cable off the positive terminal on the battery, this is a bad habit. Alternators need power to create power, so if you remove the power it is possible for an alternator stop charging, and hence the vehicle will die, yet the alternator can still be good. Also pulling the positive cable and replacing it creates weird spikes in the electrical system, which can fry things like your ECM.
With the Jeep running, your voltage should be between 14-14.9 volts. 13.0-15.0 volts is acceptable, but it should be 14 something. If it is 14 volts it's a pretty good indication the alternator is fine. Every now and then I've come across an alternator that puts out 14 volts, but little to no amps. 14 volts @ 1 amp isn't enough power to charge anything, but this isn't real common.
If you don't have 14 volts, you need to check a few other things. Most all alternators have a wire harness going to them. One of these wires is for the field voltage, if you don't get power to the field, your alternator won't charge. On newer YJs and all TJs, the voltage regulator is controlled by the ECM. On these alternators there are two wires on the back, one constantly cycles between 10-12 volts to control the alternators output, while the other is a reference wire to tell the ECM how much power the alternator is putting out. On CJs and early YJs the voltage regulator is built into the alternator, so the field wire, will have a constant 12 volts. If you have power to the field, but aren't getting 14 volts, it's a pretty safe bet your alternator is bad.
If your alternator is putting out 16+ volts, stop driving the Jeep. Either your voltage regulator is bad, or (on newer Jeeps with the ECM controlled alternators) your battery could be bad. I've come across several newer Jeeps where the ECM will try and over compensate for a bad battery by boosting alternator output. Last WJ I worked on that did this was spiking to 22 volts!
If all else fails, check with your local autoparts store, almost all of them can test your alternator both on and off the Jeep.
Related Writeups: | Multi-Meter Basics |

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