I have a B18C alternator and I'm using an ex wiring harness and have noticed that on the ex harness the alternator plug is different, in which it has three wires whereas the B18C its plug has four wires with the extra being a Alt Control... Wiring advice please thanks

 

Are you in Canada? You'll have to add the 4th wire to the ECU

 

In Australia, using the P73 ecu

colour coded the wires to match the three presently