Rocketman

I have a couple of questions regarding the operation of the BAC on a N/A 88 'vert:

1) How does the BAC actually convert the duty cycle signal from the ECU to the mechanical action of variably opening and closing the valve?

2) When doing the test of the BAC mentioned in the service manual where you jumper the innitial set coupler and then disconnect the BAC looking for a decrease in RPM, how much of a
decrease should I be looking for? It does drop ever so slightly (appro 100 rpm) but barely enough to show up on a tach. I can only tell by engine sound. It is certainly no where near the percentage increase in RPM that happens when I jumper the power steering pressure switch for example!

Rocketman

HAILERS

I'm not where or what manual tells one to jumper the initial set coupler and then to disconnect the bacs plug. Never heard of such.

When you do jumper the initial set coupler, to set the timing or the idle, what you do is take the bac out of the picture so the ECU won't influence the bac, as you adjust the mixture/air screw. Initial set coupler more or less takes the bac out of the picture. If you didn't take it out of the picture, everytime you made an adjustment, the bac would be trying to correct it and you'd go bannas.

If you took the bac off the engine and put a plate over the hole, then started the car, you'd see and feel the bac vibrating (opening anc closing). Vibrator.

The idea is to install the initial set coupler which takes the bac out of the picture, and then set the idle to as close to 750 as possible. Then remove the initial set coupler, which returns the bac as a functioning member. THEN when you take the bacs plug off, you should actually not notice much of a difference in rpm. Thats because you set the idle to 750, which is the rpm that the bac tries to maintain. If its already at 750....not much change. But if you had set the idle to say 500 rpm with the initial set coupler in, and then removed the initial set coupler and gone on to pull the bac plug off, you will have a more noticeable drop in rpm.

The idea is to set the idle with the initial set coupler, to as close to 750 as possible. That way, during normal driving, the bac does not have to work as hard. Makes sense to me. Ramble, ramble, ramble.

Oh. Once I measured the amperage of the bac as loads were put on it. Yes it increase/decrease depending on load. Also watched the duty cycle on a Fluke 88 as loads were put on and off the engine (lights on/off, air cond on/off etc.

From your post it sounds like your idle has been set fairly close to 750 seeing as how the rpms seemingly only drop a hundred or so if you pull the bac plug off. Put your initial set coupler in and adjust the rpm at idle to say 500 rpm. Pull the initial set coupler. The bac should huff and puff for a moment, then try to adjust the idle to 750. Its making up the two hundred fifty difference b/t 500 and 750. Should hold 750, but its the bac thats pulling it up. On some summer day with the a/c turned on, you'll pull up to a stop light, and the car will die. Because the bacs having to work too hard too fast.

Rocketman

Thanks for the response
What you say makes sense, but.......

The FSM definitely has a bold NOTE:

"Connect a jumper wire to both terminals of the initial set coupler, then disconnect BAC and check that the engine speed decreases"

Does the valve just open and close really fast (ie at the duty cycle rate), or is the duty cycle somehow converted to the valve opening a specific amount proportional to the frequency?

HAILERS

specific amount proportional to the frequency I assume that. Is that in the 88fsm??? I'll look at it tomorrow when at work. I've got the 88fsm on that computer.(only look at it during lunch, of course). Fuel section??? I've never seen that in the 87 fsm that I have.

I've set my idle such that if the bac is disconnected, hardly any difference is noted. Prefer it that way. Only checkout of the bac I remember in the 87 manual is a ohms check of the solenoid.

EDIT: I just looked at the 89 manual and on page FI 41 I did see what you wrote. Oh well. Shouldn't drop much if the idle was set real close to 750 in my opinion.

Well, duh, says the same thing in the 87 fsm.

P.S. Have you read this before? http://www.teamfc3s.org/info/articles/idle.html I know you have a n/a, not a turbo, but the bac part applies on the whole. Except no adjustment screw.

Rocketman

Ya,
I've read it!
it say's everything except how the damn valve works, and how much the rpm should drop.
I even e-mailed the author, but no reply yet

NZConvertible

I can answer this bit, it's pretty simple. The Bypass Air Control valve is just a solenoid valve that opens and passes air when you apply 12V and closes when you remove it. It has a constant 12V supply to one terminal and the ECU provides a switched ground to the other terminal. The solenoid is pulsed open and closed by the ECU at a particular frequency and varing duty cycle. By increasing the duty cycle (i.e. the percentage of each cycle the valve is open), the amount of air that can flow through it increases. The BAC valve allows air to bypass the throttle blades (hence it's name), so allowing more air to pass through it will increase the idle speed. Hope that helps.

slvr7

iTrader: (1)

here's something else that might be helpful........


https://www.rx7club.com/forum/showth...hreadid=116449

Rocketman

Bravo!

Finally, someone who knows what he's talking about. I can actually hear mine clicking with the key on the on setting but engine not running.
Must be prone to failure with all that action on the solenoid.
Do you know how much the idle should drop doing the FSM test of disconnecting the BAC?

Thanks for your help
Rocketman

HAILERS

Depends how close you set your idle to 750. What part of that do you not understand?

hugues

well, if you disconnect the BAC, the amount of air that goes down the intake now depends upon:

1) throttle plate clearance (at rest)

2) how far the 8mm nut (which happens to open secondary plates I believe) on top of intake is screwed in

so, you will see the rpm drop quite a bit assuming your idle speed is set correctly to begin with.

You can turn on ventilation or headlights and disconnect the BAC, you should get an even bigger drop.

if it does not drop much, I would think it is because:

- bad BAC

- bad ECU to BAC connection

- idle too high because 1) and 2) not in spec (too much air coming down the intake, which make the BAC pretty useless at least for idling with no accessories on)

BTW, if you see an increase in rpm when jumpering the power steering switch, the BAC system should be OK.

What leads you to ask this question ? Do you have a specific problem ? or are you just curious ?

Hugues -

Rocketman

When I jumper the PS switch (which is shot by the way) I get a big jump in RPM.

The reason I checked the BAC in the first place is cause I noticed the RPM dropping a bit with the lights, brake pedal etc.and I failed emission test so I was checking everything.(turned out it had nothing to do with BAC)

I don't have much of a problem and wouldn't change BAC. I'm just curious now.

Thanks