Help with ford 2G- 3G alt upgrade

Jun 15, 2003
1990 E350 motorhome :rolleyes: with the 460. Has a dual battery setup but we're only talking about the engine battery to keep it simple.

Van has no alt idiot light, but an ammeter.

Came with a non functioning but correct 2G alt that did in fact test bad at autozone.

After seeing how all its output power goes through two 1/4 inch spade lugs I opted for the 3G upgrade, using parts from a 95 Mustang V6.

I picked up a used alt, not tested by me. Came with wiring pigtails. Spliced like colors to like with the vehicle harness. Black-white is the stator connection which loops around to itself. Yellow is the voltage drop sense wire that comes from the fusebox somewhere. It does see +12V all the time. Green/Red is the "trigger" wire. I see +12V from it when the ignition is on, only. As expected. The big output lug goes to a 100 amp breaker to the battery positive. I got it from a car audio setup until I get something better. This breaker passes current-- tested with a headlight.

Having not seen charging from the junkyard alt, I spliced into both sides of the green/red wire with a couple feet of hookup wire so I could do more tests. I took a 194 signal bulb and put it between the battery positive and the alternator's green/red input terminal. With the engine not running, the light is on. With it running, the light is off. It's as though the alternator is being correctly triggered but just doesn't feel like putting out.

Being at a loss, I shelled out for a new in box alt that behaves exactly the same. I'm stumped. Maybe my white/black wire is defective? Easy enough to check.

Here's a schematic of my simplified test rig:


  • 20210831_171646_resized.jpg
    70.5 KB · Views: 18
Hook the yellow to the output post of the alternator and try that.

Also make sure the white wire is making good connection on both sides.