I think I've made a rookie mistake in wiring up my alternator, but I don't know for sure.

Here's the background. Have an 88 780 with V6. The voltmeter never got above 12 volts, the car would not start intermittently, and when I looked at the battery wires, they looked pretty iffy.

So I went to the local pick and pull and found a wiring set from a 92 740 turbo. The battery, alternator and starter were on the driver's side of the 740 turbo, and the 780 has all of its electrical bits on the passenger's side so I figured the length of the cables would be pretty close to the same.

I decided to leave the original battery cables in place, in case I needed to revert back to the originals, and I just detached them at the battery and wrapped the connectors with electrical tape until I knew I had everything working well.

Ran the new cables.

Connected the positive cable to the main post on the starter motor. Still have the red cable going from the starter to the alternator from the original positive cable. Connected the back ring connector on the negative cable to a bolt on the rear of the engine block, (right beside the original negative cable), and connected the other negative ring connector to the frame rail.

Ran a new ground cable from the body of the alternator to a bolt on the body joint.

There's two female spade connectors that go to the alternator. One is a wire that goes back to the ignition switch. The other is a wire that comes from a wiring loom that goes in front of the engine block. That one just had a really poor wrapped connection (not soldered or anything) so I decided to make a fresh connection with a butt joint. But I didn't strip the wires that went into the butt joint, so I might not have any valid connection on this wire.

Now the car starts right up, doesn't have the intermittent no-start condition, but the car will cut out above 2500 rpm. Anyone out there tell me what I've done wrong?