Q: Overcharging 18volts +

The shop was correct about how the ECU controls the field current of the alternator which will cause the alternator to charge the batteries. If the wire on pin 57, the dark green with orange stripe wire at the ECU, is getting battery voltage then it is normal. If not, then it is broken and needs to be repaired.

You should then be able to disconnect ECU pin 20, the dark green wire, and the alternator should stop charging. If the alternator does not stop charging, this wire should be checked between the ECU and the alternator field for grounding in between. If the alternator charges when you ground the dark green wire, and doesn’t charge when you don’t connect it to anything, then the wire is not grounded. Check to make sure the alternator is grounded. The ground is near or below the a/c compressor.

If the wires listed all check out to be okay, and the alternator will turn the charge with the dark green wire on and off, you will need to have the ECU replaced. If the shop put in an external voltage regulator then it should of worked if it was installed correctly. If the alternator overcharges the batteries even after the field circuit is disconnected, then the alternator is shorted and needs replaced.