My 2000 5.4 Expedition AWD with 147,000 miles has a problem. Just today I noticed that when I floor the accelerator from about 30MPH, it downshifts ok, accelerates ok, but the battery light comes on. I did it twice to be sure.

What gives? The battery is about 5 years old, and I have never had the alternator replaced. Is this a serious problem or simply what to expect with these miles and age??? No other problems, and the truck runs great otherwise.

Thanks in advance for any help.

 

Well, I just got back from my local auto parts store where they put a battery/charge tester on it. The results are good. In fact, the battery appears to be as good or better than when purchased. 12.87V, 845CA, Rated 720CA. Charging system: loaded 14.37V unloaded 14.33V Starter: 11.22V 238A Drain test: 0.39A passed.

When I got in the driveway at home I reved it up and at 3,600 RPM the battery light comes on - every time! I can hear no slipage of the belt.

 

this is usually a bad alternator even though it is working.

 

Doesn't this happen when the bridge rectifier in the alternator starts going bad?

 

I'd go with a belt slippage problem. This could be the result of either a worn belt or a weak tensioner.

 

I've seen similar things on 3 different Expeditions. On all three, the alternator was getting close to dying. Could be the rectifier but I don't know anyone that's gone inside to replace that.

 

At that mileage and the problem occurring at high RPM, I would suspect that the brushes are worn and losing contact. It's actually a common problem and doesn't show up on the test bench.

You can remove the alternator and pull off the regulator/brush holder assembly. The brushes will probably be about 1/4" long. If the rotor spins freely and quietly you can lube the rear bearing and replace the brush holder and regulator. This is a case where you want to use Motorcraft parts, the regulators sold at the "chain" parts stores are often...well...junk.

Before removing the alternator, check that the belt tensioner moves freely and springs back fully with the belt removed. If there is binding or play, address that first.