'97 F150, newish battery (about a yr old) was completely dead the other morning, recharged, cranked a started just fine, Next day, dead again. recharged again. Strangely, I heard a beeping coming from behind the glove box, would beep a couple of times then make a "scratchy" noise (like a fax machine) after a few minutes it stopped, next morning, dead! I hope you aren't gonna tell me BCM! Just put a new one in a year ago (it was a real PITA). Any ideas???

 

This is a common problem on most Ford products. It's most likely the alternator. Inside the alternator is a final diode that is subject to fail. The alternator will charge the battery when the engine is running but as soon as you turn it off, the juice flows back out of the battery into the alternator and to ground. The final diode acts like a one way valve allowing the juice to flow out but not back in the alternator. Here's how to check it. With the key in the off position, take the main lead wire off of the top/back of the alternator. Watch for any small arching of electricity. If you see any- the alternator is bad. Do not buy a rebuilt unit as most rebuilders do not replace the final diode. Buy a NEW alternator only. I would suggest staying with an OEM unit. This should solve the issue.

 

Why did you change the GEM? if it was becasue of corrosion due to water intrusion did you fix the leak? If not it might be corroded again.

.