2006 Mercury Mariner - Just blew the second alternator, please help!

Vehicle 2006 Mercury Mariner AWD

Engine Duratec 3.0 V6 w/156k miles

This problem started last December. Was an hour away from home and noticed that my headlights were dim. Have a Scan Gauge hooked to the vehicle as the stock gauges don't give that much info on the base model. The voltage is at 12.5V, not going up at all. Try to limp it home, freeway is closed for a major accident. Had to stay on side roads to get home, battery is ~11V when home. Throw the battery on a charger for the night. Drive to AutoZone the next day, buy a new battery as the old one was 6 years old. Voltage still won't get up to 13v+, alternator must have failed.

This is not an easy job, book calls for 6-8 hours as you have to go in from the side to get to the alternator. Live in an apartment, tools are in storage, no garage, and neighbors will report me for working on the car. Most shops quote me $1,300 with the genuine Ford part. Wife works for a Honda dealership, they say they can replace it for $800 something.

Car goes in, alternator and serpentine belt are replaced, I'm ~$900 poorer. They did mention that the serpentine belt is throwing a small amount of oil onto the underside of the alternator. A few days later while driving at night I notice the headlights and dash light alternating between dim and bright. The Scan Gauge doesn't show voltage fluctuations. I ignore it as the car is running. The voltage fluctuations seem to be worse if the headlights and blower motor are on. Being the middle of winter, the blower motor is almost constantly on.

A month later I am on the freeway and the battery light comes on. Scan Gauge has the voltage at 15.8V. This went away after a minute or two. It did happen again at the same stretch of the freeway two weeks later. Do some reading on this vehicle and find a lot of problems with the alternators. A few posts where they kept losing alternators point to the battery or bad ground wires. The battery is holding 12.5V at rest, so I believe that it's ok. I cleaned the ground connections on the firewall and insides of the fenders in the engine compartment. This didn't change the symptom. I then hook jumper cables between the engine, battery, and a chassis bolt on the firewall. This too had no effect on the symptoms. I'm forced to believe that I got a bum alternator

Take it back to the dealer as the part is under warranty. They swap it with another Ford part. I get the car back and the voltage now stays between 12.5V-13.9V. This is better, but the lights are still going dim-bright, especially with the blower motor on.

At this point I figure I'll just drive it till it dies. A few weeks later the battery light comes on while I'm on my way home from work. This time it's because the battery is at 12V and falling. Parked it, got a cheap commuter car for now.

I'll be replacing this alternator with another Ford part as I have the warranty on it. I don't believe I got two bad alternators in a row. Has anyone seen this type of problem before and successfully repaired it?

Thank you for your time!