My 1990 240 DL alternator bearing is making a bit of noise, so I brought it to the local alternator and starter shop. They gave me a rebuilt alternator.

But when I tried to put it back on my Volvo I noticed that it is missing a connection where I plug a third wire. My original alternator has three wires connected to it: a fat red one that is hooked up to the battery, a smaller wire that is the ground, and a third wire that connects to a blade on the alternator.

My question is: what is this third wire for?

I tried to drive the car with the new alternator and a unconnected third wire, and I am getting erratic idle and jumpy speedometer.

I called the shop and he said the third wire is useless.