ECU/BAC control transistor fix?
03-12-03, 02:38 AM
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ECU/BAC control transistor fix?
Okay, so there have been a few threads about the ECU/BAC control transistor "fix" lying around, and a page or so on the subject. One of those pages (from teamfc3s.org) mentions putting a 10k resistor in the BAC plug, and with the ignition ON but not started check the voltage across that resistor to see what the ECU is outputting. They say less than 5V is a Bad Thing. I personally concur, but solicit your opinion on the subject. When I first measured it, I got 4.8 V. I took the BAC off the manifold, and using my test bench power supply put 4.8 V across it. It clicked, but hardly wiggled the piston inside. 12V across it makes a nice pop and moves the piston to its full open position (about 1/3rd the way across the gap).
Since the recommendation was that <5 V is bad, I went ahead and bought a Radio Shack TIP120 NPN Darlington transistor as recommended, and opened my ECU. Since I have an S5 N/A ECU the transistor is labelled differently and is in a different place (I forget the label, but its near the input pins, on the left-hand side looking from above the ECU. Its the left-most of two fairly large transistors on the lower board). I traced the pin 3Q (BAC control output) to the collector (middle pin) of this transistor, and promptly desoldered and removed it, replacing it with the TIP120.
The end result? I now get 4.08 volts across the 10k resistor instead of 4.88. Great. It not only didn't fix my problem it made it worse. So my question is, are there any other N/A people who have tried this 10k resistor thing (don't worry, if you don't short the two leads you won't hurt a thing, I promise. Just be careful not to short them), and what voltage did/do you find across it? How is your idle? When you unplug the BAC does it affect your idle? Mine doesn't, and my idle sucks, regardless of how I set the TPS and idle speed screw. I have no vacuum leaks (just pulled the entire intake manifold to check), and my timing is good. So, tracing things to where they should be, my BAC control circuit is most likely AWOL. I'd just like some additional feedback and input here. If we can get enough info going I might be able to play around with the circuits some more and come up with something better.
Since the recommendation was that <5 V is bad, I went ahead and bought a Radio Shack TIP120 NPN Darlington transistor as recommended, and opened my ECU. Since I have an S5 N/A ECU the transistor is labelled differently and is in a different place (I forget the label, but its near the input pins, on the left-hand side looking from above the ECU. Its the left-most of two fairly large transistors on the lower board). I traced the pin 3Q (BAC control output) to the collector (middle pin) of this transistor, and promptly desoldered and removed it, replacing it with the TIP120.
The end result? I now get 4.08 volts across the 10k resistor instead of 4.88. Great. It not only didn't fix my problem it made it worse. So my question is, are there any other N/A people who have tried this 10k resistor thing (don't worry, if you don't short the two leads you won't hurt a thing, I promise. Just be careful not to short them), and what voltage did/do you find across it? How is your idle? When you unplug the BAC does it affect your idle? Mine doesn't, and my idle sucks, regardless of how I set the TPS and idle speed screw. I have no vacuum leaks (just pulled the entire intake manifold to check), and my timing is good. So, tracing things to where they should be, my BAC control circuit is most likely AWOL. I'd just like some additional feedback and input here. If we can get enough info going I might be able to play around with the circuits some more and come up with something better.
03-12-03, 02:48 AM
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Oh, yeah, it IS an AC square waveform with a ~25% duty cycle I believe (depends on load)... so I don't know if my cheap meter was giving me RMS (it was set to AC) or peak, or pseudo-stupidity. Again, more info would be nice from somebody with better equipment or more experience with this Thing.
03-12-03, 08:41 AM
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I don't really know about the test they describe (putting 10K R and measuring voltage) [maybe HAILERS would know]
but I kinda know how the ECU is supposed to react when load is applied to the engine at idle.
If you look at DC voltage at the ECU pin corresponding to the BAC valve, it should read somewhere around 11.5 V when idling with no load, then as you put more load (like turn on the lights, put on ventilation to max, turn on defroster), that voltage will decrease.
So, for example, if you turn on the headlights, the voltage will go to 11V, then if you turn on defroster, it wil go to 10.5V.
Note that the voltage values I give are imaginary but they are pretty close to what I measured when I was into that stuff (I think HAILERs did similar things and got similar numbers).
It's best to have a meter that can read square wave (it will give you the pulse width) but you can do with a DC voltmeter, which merely reads the average square voltage. That tells you if the pulse is lengthening or shortening, which is in most cases all you need to know.
So, to sum it up, if the DC voltage goes down as you apply electrical load, it means the ECU is reacting correctly and is at least giving proper signal to the BAC valve.
All the above refers to the S4 NA but should be applicable to the S5 as well (at least I hope so).
BTW, what were the symptoms that made you think your BAC system was not up to par. sometimes it is the charging system that is at fault, not the BAC. Sometimes also, the engine is tired and low end torque is not as it used to be.
Hugues -
but I kinda know how the ECU is supposed to react when load is applied to the engine at idle.
If you look at DC voltage at the ECU pin corresponding to the BAC valve, it should read somewhere around 11.5 V when idling with no load, then as you put more load (like turn on the lights, put on ventilation to max, turn on defroster), that voltage will decrease.
So, for example, if you turn on the headlights, the voltage will go to 11V, then if you turn on defroster, it wil go to 10.5V.
Note that the voltage values I give are imaginary but they are pretty close to what I measured when I was into that stuff (I think HAILERs did similar things and got similar numbers).
It's best to have a meter that can read square wave (it will give you the pulse width) but you can do with a DC voltmeter, which merely reads the average square voltage. That tells you if the pulse is lengthening or shortening, which is in most cases all you need to know.
So, to sum it up, if the DC voltage goes down as you apply electrical load, it means the ECU is reacting correctly and is at least giving proper signal to the BAC valve.
All the above refers to the S4 NA but should be applicable to the S5 as well (at least I hope so).
BTW, what were the symptoms that made you think your BAC system was not up to par. sometimes it is the charging system that is at fault, not the BAC. Sometimes also, the engine is tired and low end torque is not as it used to be.
Hugues -
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