Battery discharge light comes intermittently?

TBANDA

MEMBER
1999 LEXUS SC 400
V8
2WD
AUTOMATIC
23,000 MILES

With the AC on the battery discharge light comes intermittently and when it does the battery charge drains. Also, with the AC on the car idles at 500rpm and with the the AC off the car idles at 650rpm shouldn't it be the opposite.

Thanks

1 Reply

SQM

MECHANIC
6,383 POSTS

Hello,

First you need to get the alternator tested, as when the battery light comes on it mean that the battery is not getting sufficiently charged.
It could be due to the idle speed drooping below recommended level but then again you could have an issue with the voltage regulator in the alternator, which is unable to support the extra load when you turn on the A/C.

Here is a helpful video of alternator test:

https://youtu.be/Z-p4dMJUpUw

https://www.2carpros.com/articles/how-to-check-a-car-alternator

The idle on this vehicle is control by the idle speed control module, which works directly with the ECM for optimal idle. When there is an issue with the module the check engine light is typically illuminated. You will get an error code stating the issue with that module.
Is your check engine light on?

https://youtu.be/vMwTzURWXmk

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

I have attached diagrams for your reference.

Please let me know of any questions.

Thank you.

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