cbrinega

I have searched and used the FSM and these threads and sites to investigate my problem without success:

https://www.rx7club.com/showthread.p...highlight=idle
https://www.rx7club.com/forum/showth...hreadid=314040
http://www.geocities.com/huguesdc1/idlecontrol.html
http://www.teamfc3s.org/info/articles/idle.html

The signal going to the BAC was measured at 121 Hz with an AC voltage of 6.5 to 7 Volts. The BAC clicks (opens and closes) with the application of 12 V across its terminals.

The car idles well at 750 RPMs without the A/C on. Here are the symptoms:

1. Idles fine with the A/C on while in neutral

2. Idle drops to 200 RPMs with the A/C on and the transmission in first gear (clutch disengaged)

3. If idling in neutral with the A/C on and the gas is tapped rasing the RPMs to 1500 and then released quickly, then the RPMs drop down to 200 RPMs then later stabilize at 750

4. Idle drops in neutral with no A/C when the brakes are applied

5. Idle drops in neutral with no A/C when the steering wheel is turned (power steering activated)

6. Idle drops in neutral with no A/C when the headlinghts are turned on

7. If driving with the A/C on, a quick drop in RPMs to stop will often kill the engine

This behavior is really annoying especially considering how well the car runs without the A/C on. I fixed some vacuum leaks when I had my intake manifolds off and was replacing the rubber vacuum lines.

Any assistance would be greatly appreciated.

WAYNE88N/A

Tried cleaning the BAC? Also might want to check out the air bypass solenoid & relay- I think that's actuated by the steering switch (not sure)...All these "idle drops when..." you're talking about- does the idle return to 750-ish after a couple of seconds, or does it stay down?

cbrinega

When the A/C is on and I put the car in gear (clutch disengaged), the idle stays low (around 200) and does not return to 750 RPMs.

Some of the others like the headlights only temporarily lower the idle. The idle tends to stabilize after a second or so.

HAILERS

Check out what your *REAL* idle speed is. Install the initial set coupler per the fsm and see what the idle speed is. Or pull the bac plug off at idle and see what the real idle speed is. If it drops down considerably from 750.....then adjust the idle as close to 750 as you can without the bac being in the circuit. Then put the bac plug back on and go for a drive.

cbrinega

I'll check my actual idle speed tonight. I was using the stock tachometer before, but I can use my Fluke Meter to get a more accurate reading.

I checked over everything again later last night and did notice that the hose from the air intake tube to the intake of the BAC does not fit very tightly onto the BAC. I'll tighten up that hose with a clamp or something before doing anything else.

I don't want to go adjusting the idle just yet (without having a better idea of why the idle & idle recovery is not working correclty).

Also, this problem has existed as long as I've owned the car (about 2 years). I am the second owner.

hugues

If I were you,
I would check to make sure the ECU is sending the proper signal to that BAC when you turn the accessories on, AC, headlights, etc.

Just monitor the DC voltage at the BAC plug or at the ECU pin corresponding to the BAC ... Since the BAC acts on a square wave, DC voltage represents the average value of the square wave, meaning you can tell if the pulse width is getting longer and the BAC opening for longer times (raising up idle to compensate for load). In short, the DC voltage should go down as you put the accessories on. Then, you know the ECU is doing the right thing and that really clears things up.

So look at DC voltage with no acc on at idle, maybe it's gonna be 11.5 V ...
now, turn on the headlights, look at the voltage, it should be maybe 10.5 V ...
That would mean the ECU is sending the proper signal.

Do same thing when you turn on the AC and the compressor engages .. that will give you some clues.

I believe the S5 has switches for ventilation, headlights, AC, power steering which tell the ECU those systems are being turned on ... So you also have the issue of bad switches if the ECU signal does not show up. As a note, the S4 does not have all of those switches (no headlight switch, no vent switch).

IF the ECU is doing its thing correctly, then the problem could be, in any order, bad bac, improper idle speed, vac leak, poor charging, bad grounds, low compression, etc etc.

Hugues -

HAILERS

***I don't want to go adjusting the idle just yet (without having a better idea of why the idle & idle recovery is not working correclty).****

The main reason I suggested looking at the idle with the bac out of the picture is...............if the idle is really, really low, say around three hundred or four hundred, then, when the bac is in the circuit, the bac will have to work its *** off to maintain the 750 rpm.

But if the idle is close to 750 with the bac out of the circuit....then when the bac is in the circuit later and a load is put on the engine, then the bac won't have to work as hard to recover/keep a 750 idle.

The above makes sense to me. It may not to others. The bottom line is that the idle should be set with the bac out of the circuit and real close to 750 (initial set coupler installed OR the bac plug disconnected to set the idle speed).

cbrinega

I understood what you meant, and I agree that it is a good idea. From my experimenting yesterday it looked like the idle was fine even with the BAC disconnected, but I've not measured it to any great accuracy.

That's why I do not want to modify the idle yet. The idle seems okay (need to verify), and if it is then I'm going to look at other causes for the problem before trying to change the idle.

Thanks for the response though. I do appreciate your insight.

cbrinega

I agree that I need to do more verification of the input and output signals from the ECU. Maybe it never knows that the A/C is on? There are several possibilities here.

I would expect the DC voltage to be roughly equal to the duty cycle times the max voltage (12 V) or:

3.6 V without the A/C on (12 V * 30%)
7.8 V with the A/C on (12 V * 65%)

I'll post later after I try these things out.
Thanks again.

hugues

I am not too sure about the S5 but on s4 ....

The ECU grounds things to energize them .. in other words, the BAC sees 12 V on 1 side all the time, the ECU lowers the voltage to the other side (ECU pin in FSM) to energize it. The lower the voltage on that pin, the higher the pulse width. Well, at least, thats what happens on the S4.

I would think the S5 behaves the same way, but I could be wrong.

Hugues -

cbrinega

I believe I have fixed the problem with my car's idle. Thank you hugues and HAILERS.

Part of the problem was related to four separate vacuum leaks from old hoses. After these were fixed, the car ran better, but it still was not right. The main symptom is that at idle with the A/C on and car out of gear the idle would go up to around 1000 RPMs. If the car was put into gear, the engine would bog down and the RPMs would be around 200 or so.

After looking at the BAC signal and verifying that it is correct, I started verifying all sorts of items like the clutch switch, A/C cut-out system, and etc. (As a side note, the S5 BAC does operate by having the ECU ground one of the pins as hugues stated for the S4s.)

I looked at the table on page F1-77 of the 1990 FSB. The table shows when the BAC is operated and what it is doing. During the warm-up idle and normal idle, the BAC's duty cycle is determined with a feedback loop (presumably the RPM from the tack.). During cranking it outputs a "Large quantity of air." At all other times the BAC has a fixed duty cycle.

It appears that the BAC switches to its fixed duty cycle when the car is put in gear regardless of whether the A/C is on or not. The A/C compressor was putting too much of a load on the engine. The same condition could easily be reproduced by letting the engine idle with the A/C on and grounding the green 1-pin test connector near the battery. Grounding this connector signals the ECU to put the BAC into its fixed duty cycle mode.

I went through the procedure of setting the idle without the A/C on and the car ran better, but was still not right. After not being satisfied, I set the idle again with the A/C on and the test connector grounded. Now the idle is just great!

I do have some modifications from stock including:
Exhaust (high flow cat)
Port Matched LIM

Maybe the porting of the LIM caused the air flow to be far enough off at idle that the idle needed to be reset. Probably the duty percent of the BAC in its fixed duty mode (ECU resistor or capacitor timing circuit hack? or is this in firmware) should be increased so that the idle can be correctly set with the A/C off. Now the idle is a little high with the A/C off, but I don't mind. The idle is still under 900 RPMs.

I just wanted to provide my resolution to this problem. My thinking that the ported LIM has changed the idle may be off though. Feel free to add more information.