Q: New battery,belt and alt but voltage drops and squeals under load

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When my truck's air conditioning or heater is on, the voltage drops below or at 12v and makes a load squeal sound. The battery,alternator and belts are new. What would cause this?

My car has 120890 miles.
My car has an automatic transmission.

Hello, It sounds like you may have a problem with the voltage regulator. As you know, when turning on an accessory like the A/C, this puts an additional load on the motor which results in the motor needing to create additional power to maintain the same consistent RPM. This sometimes takes a second or two resulting in a delay in the motor maintaining the proper voltage. The voltage regulator is what maintains this consistent voltage across the RPM range whether the motor is idling at ~800 RPM or at full power at ~6,000 RPM. The voltage regulator is a unit that regulates the charging of the battery by the alternator and maintaining a certain amperage while the engine runs. I would recommend having an expert from YourMechanic come to your location to diagnose and inspect your charging system.

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