I have no way to know how they diagnosed a problem with the computer, but I can't help but wonder how they think it can be okay but needs TO BE replaced later. Most mechanics aren't psychic.
A failed diode is somewhat common. One bad one of the six will reduce the maximum output current to exactly one third of the generator's rated current. 30 amps from the common 90 amp generator is not enough to run the entire electrical system under all conditions. The battery will have to make up the difference until it slowly runs down over hours or days.
A rotor is one of the last things to fail. The commutator bars the brushes ride on could get chewed up, ... I suppose, but in 40 years of repairing electrical problems, I've never come across a bad one yet.
Ford had a really nice charging system throughout the '90s, but it's anyone's guess what problem they found a solution for when they added an unnecessary and unreliable computer circuit to it. The charging system on a 2007 model doesn't do anything they didn't do in the '70s.
As for the cost of that computer, that seems rather low, but I don't replace many computers. Wiring problems, like corroded splices and connector terminals cause a lot more trouble than computers.
To figure out where to go next, I need test results to analyze. You can start that process if you have a digital voltmeter. Measure the battery voltage with the engine off and with it running. With it running, you must find between 13.75 and 14.75 volts. If that is okay, the next step requires a professional load tester. Those will measure maximum output current and "ripple" voltage. Those values will point us in the right direction.
Sunday, January 12th, 2014 AT 12:00 AM