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2003 Ford focus battery light mystery

I have a 2003 ford focus wagon, automatic transmission, 140,000 miles. The battery light has been going on and off for 5 months, and the car has shut down several times. I’ve had the following replaced/repaired:

Spark plugs replaced. Two days later, car died.

Battery replaced. Two days later car died.

Alternator replaced. 4 days later car died on a vacation trip. Before it died, the battery light came on when I had to accelerate, and also when going down hills. Frequency increased and power shut off several times. If I waited 10-15 minutes, it would start up again. Finally would not start up - completely dead. Computer diagnostics first said the alternator was bad. Then, while still hooked up to computer, the bad alternator indicator disappeared and the alternator reported no problems. Technician ran the car for a few hours every day for a week and took it for test drives but light never came on.

When I got back from vacation, the new alternator was replaced with a ford alternator - the theory was that some 2003 ford focus computer chips have trouble recognizing alternators that aren’t ford’s.

All was good for about 4 months. Then battery light started going on and off again - sometimes 20-30 times in 40 minutes, sometimes only 5-6 times in that time span. Computer diagnostics found nothing - light never came on.

Car was vibrating. Had engine mounts replaced 4 days ago, hoping the electrical problem was caused by something vibrating against something else that connected to the alternator, and if the vibrations stopped, the battery light would stay off. First 35 miles - no battery light! Then it started coming on again, but not as much as before the engine mounts were replaced.

Battery light seems to only come on now 1) Every time I start up and begin driving and I think the car shifts to higher gears; 2) whenever I go up hills - it has also come on a couple of times when going down a hill (just like before, when the alternator was replaced); 3) when I accelerate beyond 55 mpg.

I can get the light to go off if 1) I floor the gas pedal; 2) I pump the brakes; 3) I put the car in park.

Mechanics are stumped. I love my car, but every time I drive it now, I worry the entire time about the battery light and about the power shutting off. I’m ready to give up.

Is the belt still original?

I’ve seen some junky genuine Ford rebuilt alternators . . . sometimes it’s a roll of the dice

I would check and clean all the grounds. They are just as important as the power. Make sure the battery connections are clean and good. I have seen corrosion in the cable at the battery end cause bad connection. Hopefulle the belt was replaced when the alternator was installed. I would replace the tensioner too.

Thank you, db4690 and knfenimore!

I’ll ask my mechanic if the belt was replaced, and will ask about replacing the tensioner. I’m pretty sure the battery connections, cable, and grounds were all checked and cleaned when the battery was replaced, and checked again when trying to find the source of the problem, but I will ask about these, as well.

When the engine mounts were replaced last week, the mechanic again looked again at connections to the battery and alternator - everything looked ok to him. I suspect the problem is one that is not easily visible.

When the car broke down on vacation and the mechanic started the computer diagnostic, he continued with it even after the alternator changed from bad to good as he was going through the diagnostics. He tested all the wires and connectors. Everything tested as ok.

I think the ford alternator replacement was new - not rebuilt - I’ll confirm. Even if it was new, though, I understand those can sometimes also be bad, so I guess that’s still a possibility.

Thanks for your help - I’ll discuss with my mechanic tomorrow. He is very good, and very open to input.