Q: battery voltage falls when engine fan kicks in

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I changed the battery out of the vehicle today, I had a dead truck Friday, boosted it and worked fine for 2 days, died again on sunday, deader then dead. so I went in and bought a battery. Now my problem is when the truck is started and I turn the a/c on or the fan the truck is darn near ready to die, but doesnt happen when the radio is on, this is scary - nor do I feel comfortable driving it anymore. Would it be the alternator or? I have a diagnostic machine and it says there is nothing wrong with my vehicle. I am at a loss on what to do.

My car has 14300 miles.
My car has an automatic transmission.

The alternator may not be charging; when you put the fan on, it could be drawing too much power so the engine needs to stay running and will stall and kill the battery if battery gets too low. Check the alternator output with an amp meter, and you will likely see that it is not charging.

It may be a good idea to have a qualified mechanic inspect your loss of power condition.

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