The absolute best driveability mod I've done: Reinstallation of the BAC
Joined: Feb 2001
Posts: 29,798
Likes: 128
From: London, Ontario, Canada
Originally Posted by andrew lohaus
the BAC is not the ONLY means for the ecu to adjust idle.
the pressure sensor serves some role in this too. i know because i dont have my bac and the idle will slightly dip from 800 then rebound when i turn on my headlights. i belive this shows the sensor detacts the increased load and the ecu respdonds (only slightly perhaps) and manages to return the idle.
however ther is very little the ecu can do if it doesnt control thethrotle and thus the intake air, which is why the bac is needed for higer loads like the p/s and ac.
the pressure sensor serves some role in this too. i know because i dont have my bac and the idle will slightly dip from 800 then rebound when i turn on my headlights. i belive this shows the sensor detacts the increased load and the ecu respdonds (only slightly perhaps) and manages to return the idle.
however ther is very little the ecu can do if it doesnt control thethrotle and thus the intake air, which is why the bac is needed for higer loads like the p/s and ac.
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Joined: Feb 2004
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From: Coldspring TX
Vaughnc- just because the BAC coil is good, does not make the entire BAC system good. Only way to verify for sure is to put a meter on the BAC control output pin at the ECU, and watch her while driving...If the voltage is dropping at idle, decel, and applying electrical loads, then the BAC system is working...If no voltage drops, something is wrong...
Troubleshooting this from the ECU covers the entire spectrum- the wiring, the BAC valve, the ground, AND the ECU...All from one spot...Cool, huh?
Troubleshooting this from the ECU covers the entire spectrum- the wiring, the BAC valve, the ground, AND the ECU...All from one spot...Cool, huh?
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From: n
On a (Zenki) FC turbo, there's 3 idle bypass valves, for which the BAC valve is one of them.
I think the other two are air bypass valve and air bypass solenoid?
One is for power steering.
One is the black plastic thing that plumbs into the intercooler with the "?" shaped hose and mounted on the upper intake manifold.
The other one is lower and mounted low on the upper intake manifold close to the firewall.
-Ted
I think the other two are air bypass valve and air bypass solenoid?
One is for power steering.
One is the black plastic thing that plumbs into the intercooler with the "?" shaped hose and mounted on the upper intake manifold.
The other one is lower and mounted low on the upper intake manifold close to the firewall.
-Ted
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From: Coldspring TX
I thought the air bypass valve and the air bypass solenoid were parts of the same contraption? (the air bypass solenoid valve)...In other words, the relay activates the solenoid (because the current draw is too much for the ECU to do it itself), and the solenoid opens the valve- all the same system, basically...At least on S4's...This is the bad boy that energizes with the power steering switch and A/C switch to on, I would think...
knowledge junkie
Joined: Mar 2001
Posts: 5,595
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From: Atlanta, GA
Originally Posted by WAYNE88N/A
Vaughnc- just because the BAC coil is good, does not make the Troubleshooting this from the ECU covers the entire spectrum- the wiring, the BAC valve, the ground, AND the ECU...All from one spot...Cool, huh?
Also forgot the RPMS do drop to 500rpms with the brakes / lights on, so the first thing I'll probably do is clean all the chassis, engine, & ecu grounds. Also forget to re-connect the ground to my partial headers/downpipe.
But before any of this I have to solve my front suspension clunking (happens at lowspeed sharp turns like up a driveway), which started after a complete suspension restore including RB Vert springs, OEM bushings, OEM end links, & tokico shocks. This one's throwing me for a loop - everything's torqued down but I still get clunking from both sides... which I think is happening in the front strut assembly, but it could be in the steering system as well.
Rotary Enthusiast
Joined: Jan 2003
Posts: 1,255
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From: fl
Originally Posted by Aaron Cake
Care to explain how the ECU controls the idle without the BAC valve?
i assumed it did so by adjusting the advance and/or fuel which concievably could prevent the engine from bogging to an extent. maybe its just the engines own inertia...
all i know is that my iddle will dip and then stabilize when load is added and i dont have my bac or the aws or any of that crap.
perhaps the pressure sensor has something to do with this since the extra load would change the vacum, but thats just speculation.
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Jeff20B
1st Generation Specific (1979-1985)
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Sep 16, 2018 07:16 PM
