# car a/c problem, need help?

what causes voltage to drop when a/c is on if battery and alternator is brand new. that was the reason battery and alternator were replaced in the first place. a/c still causes voltage to drop

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• Sean K
Lv 5
1 month ago

Depending on your type of voltmeter, just putting the leads on the battery will cause it to drop. If that isn’t the case the bearings or windings on the compressor might be bad. If it’s a slight drop that’s totally normal due to the extra load on the battery.

• 1 month ago

Need more input. Why are you replacing the battery and alternator when the ac is causing a possible problem? If it is doing this at an idle, that's normal.

• M.
Lv 7
1 month ago

Which vehicle do you have?

Since you have a voltmeter and you measured something, give us some numbers (voltage) and an explanation.

Dont know?

Then give us these voltages:

What is the battery voltage when the car is idling?

What is the battery voltage when the car is idling with the headlights on?

What is the battery voltage when the car is idling with the headlights on and the A/C on?

What is the battery voltage when you are driving a constant speed with the headlights on and the A/C on?

• Dan
Lv 5
1 month ago

The a/c on at idle will always cause the voltage to drop, you didn't say how much?

If it stays above 13v there's nothing wrong.

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• 1 month ago

You need a vehicle sparky to test the system off load and under load to establish what is going on.  Could be something as simple as a high resistance from poor connections, in fact that could have been the original problem which caused you to replace the kit.

We can all guess but no-on can really tell you without testing.  Good luck!

• Anonymous
1 month ago

Year make and model.  What is the voltage when the AC is on and the engine is running at 1500 rpm?  What is the voltage when the AC is off?

AC clutch coil could be shorted (low resistance) or the clutch pulley or compressor is seized or alternator is bad.  Get a clamp multimeter and check amp draw at the AC clutch power wire when AC is on and engine running.  Remove belt and check if the clutch pulley is seized or compressor is seized.

If everything checks ok, check for voltage drops from the B+ terminal of alternator to the battery positive terminal and from alternator case to battery negative with the engine running and AC on.  There could be wiring issues.  Load test the alternator with a carbon pile tester.  Many alternators now are controlled by the ECU.  Check ECU inputs and outputs to alternator before condemning alternator.

https://www.youtube.com/watch?v=fYjuP8FBUu8

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