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New alternator only charges battery when car is revved up

So I just had the alternator in my car replaced. I took the car home, and on the way into town the next day, the car just died. I had it towed back to the shop and they replaced the alternator again.

When I picked up the car, the mechanic told me that he doesn’t know what is wrong, but after I start the car, I need to “rev the engine to 2k and hold it for ten seconds”. I mechanic I work with said that revving the engine like that right away can destroy my car. HELP!

Your alternator starts working after going to 2000 RPM because that’s where the alternator’s self activation occurs. (Yes, this is a design feature of most Ford alternators.) It means that the control lines are not correctly connected to the alternator or are broken or shorted somewhere.
I think that you need to find a better mechanic.

Below is from the Ford Factory service manual (for a different car, so the speed is different.)
NOTE: If the I circuit 10-BA25 (GY/RD) is open or shorted to ground, the generator does not function. If the engine speed exceeds 3,000 rpm, the generator self-excites, and begins to function normally (the battery light turns off and the generator operates normally) until the engine is turned off. This is a limp-home mode for customer safety.

This is the generator I (ignition) circuit. This circuit is used to turn the generator on when the engine is started. Once the generator is turned on, the generator only turns off when the engine is turned off. "