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Taurus electrical drain, unknown fan (?) running continuously

I had not driven my 1997 Taurus SHO V-8 in a week. I was surprised to find the nearly new battery was dead. Immediately upon hooking up the jumper cables I could hear what sounded like a fan running under the hood. I also hear a loud hissing sound that is caused by whatever the “fan” noise is. This is without the engine running, so it’s not manifold vacuum. The noise keeps going after it starts. It’s just muffled by the engine noise. When I disconnect the jumper, it spools down to nothing over a second or two like a fan coming to a stop. The noise is barely hearable inside the car, and no air is coming out any vents inside. The heat/AC is turned off. The noise remains when I switch the A/C on. I’m pretty sure it’s not part of that system. I’ve never heard this noise before. It seems to be coming from the area under the power brake booster, but the booster has no long lasting vacuum when the engine is off, and this will apparently go on as long as the battery is up. I’ve felt around under the booster, but can’t put my hand on anything that makes a difference or feel any vibration. It’s a bit tight for space with that big V-8 in there.

What’s going on?

Possibly the emission air pump which I think is located on the driver side below the ABS controller, tucked under the radiator and frame.

What would make it stay on as long as the battery was charged?

Could be the engine cooling fan … on because one of the fan relays (Hi or Lo) has failed.

A stuck relay, or short in the wiring.

Nope it’s definitely not the cooling fans.

If it is in fact the emission air pump, you can always unplug it at the pump motor. Thus preventing drain on the battery until further diagnostic can be performed.

The air pump being disconnected will only cause the check engine light to stay on.

Slightly higher exhaust emissions for the first few minutes until the engine warms up will result.

The alternator may be backfeeding voltage to the device. Disconnect the connection on the back side of the alternator and see if that kills the sound. If so replace the alternator.