Mindspin311

And when is it active?

trainwreck517

iTrader: (1)

when load is put on the engine while idle it raise the idle.. when you turn on the ac.. see it go down then go up? well when it goes back up thats the back kicking on.. also high load on electrical system.. etc..

WAYNE88N/A

Exactly what its name implies- allows bypass intake air around the throttle plates, primarily to keep the engine from dying due to a closed throttle. It does this electrically, using signals from the ECU...

It is "active" mostly near idle speeds. With a perfectly adjusted idle, with no extra loads on the engine, it theoretically should be de-energized, and only energize when loads are applied that would cause the idle speeds to drop below 750, as sensed by the ECU...

In real life, however, it's energized a bit during most driving conditions except cruise. It's not an "on-off" type of valve, but works under a duty cycle. A low duty cycle means that not much bypass air is being passed. You can "see" the valve in action by reading its output wire at the ECU with a voltmeter- when completely de-energized, system (alternator) voltage will be present. As the valve energizes, the voltage as seen on the meter will fall (due to the square wave signal, the meter can't display the on-off voltages fast enough for us to see visually, so the needle falling on the meter is an illusion).

With everything imaginable on (headlights, e-fan, etc), I've seen about 8v on mine, which means it's at about a 50% duty cycle (half of the time it's energized, the other half not).

That enough info?

staticguitar313

iTrader: (2)

thats a good set of info, thanks for clearing that up

NZConvertible

How about the simple answer? It's the idle control valve. Pretty much every EFI car has one.

HAILERS

It also goes and stays full open when the key is held to START.

WAYNE88N/A

NZ, are you getting old? Since when have you ever given simple answers?

Hailers, while we're at it, I can also state that it's responsible for the 3k warmup...

Which means when she's already warm and doesn't go to 3k, the BAC is not "full open" at START...Which kinda disproves your little theory, now, doesn't it? Wunderbar

HAILERS

The 89fsm say under OUTPUT DEVICES, that during START (not IDLE), the bac puts out a large quantity of air. Like I said START not IDLE. It also says with a cold engine. I call it a tie.

By the way, I've layed a spare bac on the engine, disconnected the starter wire, and gone to START and observed the bac going wide open. and staying there while the key was in START. Frankly I don't remember the condition of the engine, whether hot or cold. It probably makes a difference. We'll see.

HAILERS

Just refreshed my memory and went out and put the spare bac on the connector. With a fully HOT engine the bac goes wide open during START and stays there(key held to START) til the cows come home or the battery goes dead, whichever comes first. So it's not cold start related i.e. 17second jobber. This has been a fun posting.

Mindspin311

So would a bad BAC cause the car to die on cold start? My car will strat fine, rev to 3000ish but when it starts to drop, it will just die completely. Holds idle fine when warm.

HAILERS

The idle of a car is set with the bac disabled, so it follows that the engine should run with a bad bac. The bac is there to maintain a idle when a load is put on the engnine, like turning the lights on or the a/c , radio, heater whatever.

A bac will maintain a reasonable idle even if the idle was not set properly in the first place (within reason). Like right now if I remove the vac hose for my boost sensor off the manifold, if the bac is disconnected the engine will die. If the bac is connected up, it will maintain an idle even though that hose is off (I just did this as described and it is so).

I'd say try cleaning the bac with some carb cleaner as described in some posts in the SEARCH site on this second gen site. Do not pry the bac valves guts with anything. Just pour the fluid in the hole and wash around. Open and close the bac as described in the search posts or just remove the bac......invert it......turn the key to ON so it will vibrate....then pour/spray carb cleaner in and out of the bac.,

WAYNE88N/A

89 FSM? And just how does that pertain to our S4's? We can't tie, I'm keeping score, and I don't have a column for ties...

Hmm, spare BAC on the engine. Well, that's lab conditions if I ever heard of one. So the ECU wasn't getting any input from the AFM during this little experiment of yours? Which means you didn't exactly use "real world" conditions...Maybe that's why it went wide open, eh?

Tell ya what, do what I did- hook the meter up to the BAC output at the ECU, and drive around for a day with it there while keeping a watchful eye on the readings...This is also a good way to know how well your mechanical idle is set, BTW, because if the BAC stays beyond about a 10% duty cycle at idle with no loads, then it's having to make up for a misadjusted idle

WAYNE88N/A

The fun has yet to begin...As I've stated numerous times in the past, the BAC is ENTIRELY responsible for the 3K thing. I know this for a fact, because I have no ABP valve on my car, and she still revs to 2800-3200 on every cold start above 60*F. This is with my foot off the pedal too, in case you were wondering. Whether that's wide open or not, I can't know. But if it's even close, what you're saying is that with the key held to start, the BAC opens enough to produce a 3K+ idle. AND, as we all know, other than the warm-up thingy, our engines don't start and idle at 3k.

You sure it's not just a momentary thing (like it only does it with the key to start, then de-energizes as soon as the key is turned back)? I take it that's what you're saying, that the key has to be physically held to start the whole time to note this effect, eh?

HAILERS

Wayne, I did not stutter. I held the key to START and the valve opens wide open as long as the key is held to START and the battery holds up. Yes, it goes to a duty cycle when the key is let go to ON and the engine is hot/warm.

It's been a while since I messed with a non turbo, but he last time I looked the Air Bypass Relay pulls in on cold starting and in turn keeps the air bypass valve open for seventeen seconds to assist the bac on the cold start routine.

I believe I made the comment: THE BAC ALSO................BLAH, BLAH, BLAH meaning that in addition to the above remarks it does this too. Never did I say it causes the approx 3000rpm for seventeen seconds (by itself).

I made that comment to light up a light bulb in someones head that this is part of the start sequence. Trying to make note that the bac supplies more air during STARTING of the engine. As in the initial START of the car. Not what follows the starting of the engine.

You really need to disconect your bac and lay it on the fender and go to START while watching it. No guess work involved.

What do you think your air bypass valve does?????? Seriously. I'm always serious. humor

The 88 and 89 fsm's are more explanitory in some areas than the 87/86 although the 87fsm has superior wiring diagrams (rice paper and pretty colors).

HAILERS

Tell you what. Have someone start your car while you have the air bypass valves hose off (cold start). Now tell me that it does not supply bypass air for seventeen seconds.

WAYNE88N/A

Why do I need to set it on the fender? I'm smart enough to see what it's doing from the ECU without tearing anything apart. Just a little algebra involved to interpolate its duty cycle, no biggie.

Now let's assume for one second that you're right (a stretch, I know) about the ABP opening up for the 3K start. IF that's the case, then the ECU makes the BAC do it if the ABP's not available, or at least on my car it does...

What do I think the ABP does? I think it opens up when the AC button is pressed, and when the steering switch kicks in at low speeds. And that's it. Neither of these systems are on my car, so I didn't need the ABPV or relay anymore. So I removed 'em. Weight savings, you know. Yet I still have the 3K thing going on.

Real electrician's don't need pretty colors and explanations on our diagrams.

You're right, this is fun...OK, I'll call it a tie, just to be civil

HAILERS

You and your meter are not going to be quick enough to see the bac hold at wide open withe the key to START, imho. Unless you pull the starter solenoid wire off the starter.

Take a moment out of your busy day and read the 88fsm, page 4a-63 and you WILL see the light.......maybe. I'm starting to have doubts!

87 diagrams are better. You don't have to access another diagram to see the whole circuit. Unlike many places in the 88 and above. 87 rules the hwy and driveway.

I never said the air bypass valve is the cause of the approx 3 grand startup when the engine is cold. Never implied it. The bac and airbypass valve work together. Except on the light weight versions. That''s another view of your rust bucket. The more rust, the lighter the car. Hope you fix that hesitation problem of yours someday.

No. No ties. I win. Period.

WAYNE88N/A

You know what? I've come to the conclusion that we weren't on the same page here (I finally figured out what you were talking about- and I agree with you on your one point- shock...).

The original topic of my first post wasn't about what the BAC is doing during the second or two of the start cycle, but what it does at all other times. That's where our wiring is crossed.

And believe it or not, an analog meter will show quick transient voltages much better than even the best Fluke digital meter. I should know, I've been using both types for 20+ years now. You won't be able to tell exactly what the peak or trough voltage is with an analog, but the needle will fall towards 0v during start if the BAC is in fact wide open. Strangely enough, I don't think I've ever paid attention to it during the moment of start, even though it was hooked up for my drive.

And this is what I've been trying to tell you, so listen- THE BAC DOES NOT NEED THE ABP open to hit the 3K warmup. They DO NOT work together to hit 3K. Why do you keep insisting that? For the tenth time, I DO NOT have an ABP valve on my car, yet still hit 3K during the warmup. The BAC IS DOING IT ALL...What further proof do you need???

On a lighter note, I'll go get the schismos out of the car and review the page that you gave. If any revelations come from it I'll get back to ya...

A tie is the best you're gonna get- better take it while I'm dangling it

WAYNE88N/A

Oh, the test on 4a-63...

Hailers, you gotta remember that Mazda wants us to read voltages differently than the normal "American" way.

First off, the test doesn't say whether to disconnect the connector to check for voltage on the br/y wire to ground (the br/y is the ground side of the circuit, not the power side like we would normally check). So, it could be construed that the ABP valve would actually be de-energized for those 17 seconds, not energized. It all depends on whether that plug is connected for the test or not...

The voltage reading scenario is the same as the 5th gear switch you seem to have so much trouble understanding- when voltage is present at the point Mazda wants you to take it, the circuit is DE-ENERGIZED, and when voltage is not present, it's energized. Crazy Japs

I'm seriously wondering if the ECU makes up for the loss of an ABP load signal by making the BAC work harder now, you seem so adament about it...That would be kinda cool, actually, it would mean the Mazda engineers actually installed fail-safe software for more than just the sensor inputs...

HAILERS

The air bypass solenoid opens for 17seconds during cold starts. It does open, always has, always will during cold starts unless you disconnect the water temp switch at the bottom of the radiator.

Your not going to see the bac fully open when going to START with that bent needle old fashion meter you own, UNLESS you disconnect the starter solenoid wire and go to START. Then you'll see it. If you try to see it without disconnecting the stater solenoid wire, all your going to see is a voltage drop due to the starter dragging the battery voltage down.

I have no problem with the fifth gear switch now or ever. Stop it. The over the top switch puts a ground to the ECU which in turn puts a ground on the split air solenoid to pull it in to pass air to the split air pipe.

You have counted the time you BAC keeps the rpms up during cold start, right? Seventeen seconds.

WAYNE88N/A

Umm, no...But it does seem like 17 seconds to me. I'll have to actually time it just for you

The starter just drops the batt voltage down to about 10v or so, so if the BAC goes anywhere near full open, the meter should fall to about 0v, because at that point the duty cycle will be approaching 100%. There's a big difference between near 0 and 10v. And yes, I love my "bent needle" meter. Can't stand the Flukes with their digital updating every second, it drives me crazy...Flukes are OK for the airplanes, though...

That's funny, I thought I had to explain the fifth gear switch voltages to you a while back

You sure you don't want to declare a tie/ truce?

C-Murder

All I know is that some idiot broke mine before I got my car in the open position, and now the car idles high. That combined with bad thermo wax makes a rough idle at about 1600, hehe

WAYNE88N/A

OK, Hailers, just for you, I timed it. 17 seconds flat. BAC only...

I repeat, there is no air bypass valve or relay on my car, and the 3K warmup works as advertised using the BAC...For 17 seconds, nonetheless...

You like to tinker- go pull the plug off of your ABPV on your car and see if the 3K thingy still happens. Only then will you be convinced, because you sure as hell ain't listening to me

OR, if you're really adventurous, pull the plug off of the BAC and see if you get the 3K thingy with just the air bypass valve- I'll be waiting for your results with anticipation...

HAILERS

Wayne, Wayne, Wayne, I have not had my air bypass valve connected to my turbo engine car for over a year or two. I have the cold start when the engine is cold. I DID notice that when I disconnected it that the rpms did not go to three grand but now go around 2300. That is the difference I saw and noted on this forum some time back there.

Wayne, your just not listening to me (put simley face here). I'm saying the air bypass valve is part and parcel of the cold start system. If yours is still reving to a full three grand, I'm sorry your car is busted (place even larger smiley here). Done with this thread (snicker)

74trk

I'm working on a t2 into repu project. I have no power steering or a/c. So I don't see a need for the BAC. But with the thing removed and blocked off what have you guys done for an idle adjustment air bleed? refering to 4b-82 idle speed and idle mixture.