That older external voltage regulator will work great to run the alternator, but the Check Engine light will turn on because field current is monitored. The fault code will "Field current not switching properly". You can add a 4 - 10 ohm resistor to trick the Engine Computer into thinking it's still running the alternator.
Before you do that, measure the voltages on the two small terminals on the back of the alternator while the engine is running. One will have full battery voltage. The other one is the key to the problem. If you find 4 - 6 volts, suspect the voltage regulator circuit inside the Engine Computer, but that is very rare. If you find 0 volts, the dark green wire is grounded. That is much more common; in fact, another person just found that wire rubbed through on a sharp metal bracket a few days ago. The voltage regulator is incapable of drawing the voltage all the way down to 0 volts.
Another clue that you observed is the problem was intermittent at first. That is not typical of the voltage regulator circuitry, but it IS typical of grounded and corroded wires. If you find the regulator has drawn the voltage on the dark green wire down to 4 - 6 volts, it is more likely the system voltage sensing wire to the computer is corroded. When that happens, other things don't work because that sensing wire is the voltage source for other circuits.
Friday, April 15th, 2011 AT 7:02 PM