I noticed while sitting a stoplight the other night that when I depress the brake pedal, the engine is noticeably affected. I can hear the engine change and the tach drops about 150-200 RPM. Why in the world would pressing the brakes affect the engine?

 

2 reasons i can think of......brakes make the brakelights come on putting extra load on the alt....your's may be on the way out.....or, vacume leak.......

 

BAC should alow more air flow when putting more load on the car.. ie amps being taken from alt when putting brake light.. Mine goes down when I hit the brake at a stop.. but quickly go back up normal.. If your rpms just stay down when hitting the brakes.. maybe your BAC is out.

 

Def check the alt. Mine did the same before it crapped out. *shameless Silverrotor plug here*

 

Sorry, I am semi-retarded when it comes to cars (but learning). What's a BAC?

 

The brake lights normally draw about 11-12A, which will put quite a strain on a tired alternator and/or battery.

The BAC (bypass air control) valve is the main idle control valve. The ECU uses it to compensate for extra loads on the engine from the alternator and A/C.

 

Having no BAC on my car I feel I should comment on this.
The current drain on the alt. that the brake lights draw will NOT affect your idle(considerably).
I feel that the only reason you might experience idle difficulties is if there is a Vacume leak.

 

I feel it will not affect a "strong" engine as much. Tried my "new" engine without the BAC and very minor flucuation even with a/c on. The engine rpm's will drop more on a weak engine.

 

Your brake booster runs off vaccum.

 

Well im not tooo sure about that..
My engine was VErrrryyyyyyyyy tired

It stopped responding to throttle/ignition inputs shortly after that, hehe

 

my brake light is on all the time until i shift pass 4k rpms, but when i stop in a light it will come up... i resolder teh point sin the idiot lights and it still does it... it has new battery, new alternator, new belts... it just started doign this about 3 - 4 weeks ago... could it be teh BAC? the amps on the bac read okey... maybe i should take off the home made cartton box gasket i ghetto made... and use rtv

 

Mine does the same but I didn't notice if it stay low (rpm) if my feet keep on the brake. I not put the gear in N and use the hand brake at the light, that helps. I will see tomorrow have my AC on and keep my feet on the brake see if the RPM stay low. If it stay low it could be bad BAC right?!