I've asked about this before and got little response, but it's still bothering me so i'm trying again...

My Idle w/ electrical load is 850 rpms (pfc). This only happens with the headlights on. After the AWS is over, there is no "problem" for about 5 - 15 minutes. What I mean is that my vacuum is 410 - 400 mmhg with the headlights on. If I hit my brakes my vacuum hold constant. After the 5 - 15 minutes my vacuum is still 410 - 400 mmhg, but when I hit the brakes, it drops to about 350 - 360 mmhg!?! Again, this doen't happen for the first 15 minutes of driving (after the AWS is off)...

Now with no headlights on, my vacuum is 430 - 420 mmhg, and my headlights only lower it about 10-20 mmhg, so how could my brake light lower it twice as much as this?

Last time when I was getting low vacuum (on and off) it was my airpump (I assume b/c it injects air into the intake durring idle)... I replaced it and everything was fine for 'till now. Is my airpump going again (it's not making any unusual sounds. Is there any other way to tell?), is my battery dying? Maybe my alternator? I have a hyperground ground kit, so I don't think it's b/c of poor grounding...

thanks for any opinions.

-matt

 

Same Thing happens to me as well. It does lower it certain amount and for that time that i hold the brake, seems like my idle lowers as well. It bothers me, but i can live with it.

I think it has something to do with load applied on the Electrical system as the brake lights get turned on.
Not sure though.

 

When I turn on my secondary lights (the lower ones, not fogs) my vacuum drops between half an inch and an inch of vacuum. When the fans come on at idle, it's even worse.

 

Are you sure it's the brake lights, and not the vacuum assisted brake booster?

 

Does your dome light dim slightly when you apply the brakes?

 

Mine does this too. It's the booster charging it's vacuum because you are pressing the brakes. Something along those lines anyway. I think it is normal.

 

thanks for the replies.

nope, not sure. how is the brake booster vacuum assisted?

no, nothing dims inside or outside the car.

The strange part of the whole thing is that doesn't happen right away... it takes a little while (again, after the AWS is over) before applying the brake has an effect on the vacuum?!

 

My lights will very rarely dim when I push down on the break pedal. Bad ground?

 

When I converted my brake and signal lights to LED there was NO drop in idle RPM and vacuum.

The load is almost non existent.

 

The things some people worry about!

 

It's the brake booster.

 

i'm not loosing any sleep... just curious. we all pay a lot of attention to these cars, it's nice to know that everything is or is not working properly .