I purchased a used 2005 Ford Taurus several years ago.
The dealer who sold me the car said that it was delivered new to a car rental company in California, then later on sold to private owner.
After a while I noticed that the lights - interior & exterior, were flickering and that it kind of followed the change in idle.
So I took the car to an electrical automotive repair shop and the first thing he checked was the headlight switch which turned out to be good.
Then he removed the wire harness off the alternator and the flickering, unsteady idle went away!
He checked the alternator and it was charging properly: 14.25 volts with no load, 12.5 volts with everything on.
He said that the PCM on my car controlled the voltage regulator and that it's causing the flicker, unsteady idle and I should have it checked at Ford.
I called Ford technical support, told them the problem and that the alternator checked out good.
The response from the Ford technician was that the PCM was not at fault, but in 2005, Ford had designed five different alternators for that car and that I might have one with the wrong voltage regulator in it.
Before I spend more time and money I thought I'd look toward the internet for help, have you ever come across this problem ?
The Ford Tech may be right about the different alternators but the difference may just be the amperage output. You can check the serial or part number stamped on the alternator to see which one you might have.
The problem just appeared after eight years; I would suspect possibly it could be the PCM which has the regulator built inside.
There are a few things to check before you replace the PCM. Have the mechanic check all the body grounds especially the ground cable from the battery to the engine block. The mechanic can check the voltage drop from the battery ground to the alternator case to see if the ground is satisfactory.
The mechanic can also use an oscilloscope to check the wave patterns of the alternator to see if there are any components inside that may be faulty causing this problem.
OK, I will check all my ground connections. If it turns out to be the PCM would you know who I can contact ?
When it comes to electronics in the motor vehicle I like to stick to the manufacturer. I have had problems with after market electronics and have learned to pay the extra money for the quality.
If you can afford the bill at the dealer I would go there for the repair.
If money is the issue then your local repair shop should be able to install a new PCM for a more affordable price.