Good observation about it going to 18 volts. Without that important information, the most common cause of a charging problem is worn brushes due to high mileage. That would cause an undercharge condition.
There are only four things that can cause an overcharge condition, and you replaced two of them already. The first is a grounded brush in the alternator, or more likely, a frayed wire on that terminal that is touching the case. The second, less likely cause is a shorted voltage regulator inside the engine computer. They do not fail very often because there are too many safeguards built in. The third cause is a break in the system voltage sensing wire going to the computer. Often other things won't work because they use the same circuit for their power supply. The fourth, and possibly the most likely cause of the problem is a grounded wire.
The first step in finding this intermittent problem is to measure the voltages on the two small wires bolted to the back of the alternator. One will have full battery voltage, but only two times. First, for about two seconds after you turn on the ignition switch, then it will go back to 0 volts. The voltage will appear again when the engine is rotating, (cranking or running). You will need the engine running to take these measurements.
The second wire will have less than battery voltage, but it will not be 0 volts. The voltage regulator will draw the voltage down, typically to 4 - 11 volts. This is the wire you must monitor. If the problem shows up very infrequently, tie a long wire to this terminal and run it inside the car to a voltmeter with the meter's black lead grounded to the body. Observe the voltage when the problem occurs.
Even if the voltage regulator is shorted, voltage on that wire will not go down to 0 volts. Anything related to a defective regulator or the voltage sensing wire will result in 2 - 4 volts on that wire. If you find 0 volts when the problem occurs, inspect the wiring harness for a place it is laying on a hot exhaust manifold or is cut on a sharp metal bracket. The wire's insulation could also be rubbed through and the wire is touching bare metal. You might be able to make the problem act up by moving the harnesses around or rocking the engine. For convenience, turn on the headlights so you can see the change when you make the problem act up. You will also hear the alternator whine when it becomes "full-fielded".
Monday, February 8th, 2010 AT 7:35 PM