Idling and lights

PUZZLED ONE

MEMBER
2005 CHEVROLET TRAILBLAZER
4.2L
6 CYL
4WD
AUTOMATIC
111,000 MILES

When you first start it it idles way up then back down for around four or five minutes and the lights get really bright then dim while it's idling up and down. It does this every time you start the vehicle until it runs a few minutes and then it's fine.

7 Replies

JACOBANDNICKOLAS

MECHANIC
109,700 POSTS

Hi and thanks for using 2CarPros.

First, when the vehicle settles and is running properly, are the lights as bright as they are when you first start it and they get bright? I need to know that to determine if there is a power regulator issue. However, you can check it for me using a simple volt meter.

Here is a link that shows how to test an alternator. It is easy to do. You only need a volt meter or multi meter. I will provide directions on using one later in this post.

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

Now, what I need you to do is simply check voltage at the battery (cold) with the engine off. Have a helper start the engine. Watch the voltage readings you get as the engine idles high and low, at the same time the lights are getting bright and dim.

The bottom line is this. The change in how bright the lights get can be simply the result of an exceptionally low idle (where the alternator doesn't produce enough voltage) to a higher voltage (when it does its job).

Now, if the voltage goes too high, it can have an affect on the idle air control valve, causing the idle speed to increase. We will determine if that is a possibility once I know the results of the alternator test.

You will need to use a volt meter to check this. Here is a link that shows how that is done:

https://www.2carpros.com/articles/how-to-use-a-voltmeter

Let me know what you find or if you need help. Additionally, feel free to ask more questions. I would also like to know if the check engine light is on while the engine is running. That can be helpful.

Take care,
Joe