Can someone please inform me as to what the BAC valve does and what the effects are if its not working?

 

The BAC will compensate for varying loads on the engine during idle.

 

It's the primary idle speed control.
It's a duty (vibrating) solenoid.
The ECU pulses it more or less as needed to maintain idle speed.

 

This brings up an interesting point. I maintain that its frequency never changes, just the amplitude of the duty cycle. Reason I say this is because if you ever took a BAC off and ran it through its paces, the frequency of the pulsing remains the same, but the amount the plunger moves always gets more and more.

I may be waaaay off base here, but thats what I think goes on.


HAILERS?

 

With the Fluke set on duty cycle, you can see the duty cycle percent change and the time open/shut in MS change. I've looked at the freq before and it is around 120hz, but to tell you the truth I don't remember staring at the freq when putting on/removing a load. I might look tomorrow....the day after.....sometime. My butt is stuck to the lounge chair right now.

Anyway, the bac does what was said above plus some. Go the S5 FSM and look at Output Devices (I think) and the chart shows what and how and when the bac works.

Not mentioned above is that when going to START and holding the key to START, the bac open and stays open fully as long as you are in START. Let go and it's not full open anymore. It's then in the duty cycle business.

The BAC functions during the AWS by going open for 17 seconds for a high idle speed of about 3000rpm (Air Bypass Solenoid working in conjunction with the bac). Only about 2300rpm if the Air Bypass Solenoid (also called the AWS solenoid on S5, I think) is unplugged.

EDIT: IF the BAC is not working, like the plug off or its transistor is kaput in the ECU, then the engine idle will fall down when a load is put on the engine and stay down. On a car with a functional BAC the idle (should be close to 750rpm) might for a moment fall down from 750rp when a load (headlights turned on, defroster turned on, heater fan turned on etc) is put on the engine, but if the bac is good it will be a momentary drop and the idle should almost instantly recover back to 750rpm. Also when the air conditioner is turned on the normal 750rpm idle should go to approx 850rpm or so.

I recommend the series five FSM, Fuel section and the section/page/chart/whatever called Devices (I think). The FSM is free, online and a lot more accurate than me.

 

Most BAC's are stepper motors. They go full in, usually, then take steps out, slowly, and once they're at the correct step (however far open or closed they need to be) they stop moving. A lot of the time they keep the throttle plates from fulling closing, so a small amount of air can be let into the motor to keep from stalling.

Say if your AC and Defroster are on, there's more load on the engine, so the idle would drop a little bit because things are turning slower. With the BAC operating, it would let a little more air into the engine, by keeping the throttle plates open a little more, which would show your TPS that the throttle is maybe 2% higher than at normal idle, which would then send a signal to your injectors to inject more fuel with that air...until ideal idle is met.

Kinda a crash course, but hopefully the gist of it is there.

James

 

Sorry, I disagree. Take YOUR BAC off the engine and leave the plug on. Turn the key to ON and look at the bac. EVen better, get a spare bac and put the bac plug on it and start the engine and take a look.

With the initial set coupler installed the bac's DUTY CYCLE is approx 40% (memory). Remove the initial set coupler and the duty cycle will rise and fall with the load seen by the ECU from the alt, lights, fan speed etc.

BAC...........BYPASS..AIR...CONTROL as in bypass air going around the throttle plates.

 

The BAC valve is not a stepper motor, it's a pulsed solenoid valve as already explained. As J-Rat said it's pulsed at a constant frequency and it's duty cycle (percentage time open) is varied by the ECU.

A stepper motor idle control valve is completely different. It's a rotary valve that is rotated to varying opening angles by an ECU-controlled stepper motor.

 

Thanks, my bad for spreading false information.

 

OOO man, im still trying to learn about these rotaries but yall totally lost me. i get what it does and how it works now(thank you all by the way) but what i was mainly asking is what is the down fall to not having a properly working BAC and simply what it is ment for. thank you all again, you've been more than helpful

 

It's meant to keep the idle speed up, even if you turn the airconditoner on or the headlights.

 

"The BAC functions during the AWS by going open for 17 seconds for a high idle speed of about 3000rpm (Air Bypass Solenoid working in conjunction with the bac). Only about 2300rpm if the Air Bypass Solenoid (also called the AWS solenoid on S5, I think) is unplugged."

COOL, I get to argue with Hailers again after checking in for the first time in months

I've told you before, Hailers, the BAC doesn't need the ABS or valve on the car to hit the 3K. BUT....just in case you forgot....my '88 only has the BAC, NOT the bypass valve system, installed, and she still hits 3K for the aforementioned 17 seconds... Every time...with outside temps over 59*F, anyway...

Good to see you're still helping the guys out with somewhat reliable information, though

And NZ, hello to you down under, also

 

This thread is above me, but Hey WAYNE! Long time no see i was just talking about you. Welcome back bud.

 

Actually this doesn't have much to do with rotary engines, pretty much all EFI vehicles made in the last 20+ years have an idle speed control valve of some type.

Without it you'll have no way of controlling idle speed and compensating for accessory loads. That's what it's meant for. Any time the alternator, power steering pump or A/C compressor put a load on the engine the revs will drop.



I see your oddball car is still doing funny things...

 

Well i'm having problems with my idle. I idle low at 650 when warm. I sometimes have bouncy idle from 1000 RPM - 1500 RPM. Also, when I turn on my headlights or AC my idle drops down a few hundred RPM for a few seconds, then goes back up to normal *650*.
I was told it is probably a BAC problem. Does that sound right? What should I go about doing to fix this problem?