I decided to put my BAC Valve back on after not having it on for about 4 years. I cut the old connecter for the S4 BACV off a while ago which is not a problem. I put a S5 BACV on the engine since I have the S5 mani and it just fits better. I got the S5 plug for it as well and wired it into the harness.

I tested the BACV in 2 different ways. Resistance and running a 12v wire to one pin and ground to the other. I heard a nice click. I also did this while the engine running and when I applied 12v to it, I got lots of vacuum on the BACV intake tube.

For Resistance, I got at first around 14.5ohm After clicking it a few times it dropped to 13.5ohm. Out of the range of the FSM which is 10.7-12ohm. Unless im missing something, I can't see this being an issue if im getting 12v at the plug? Let me know if im not thinking right here.

The wires I used were Black/White and Blue/Green on the emmisions harness. According to the wiring diagram, that seems to be right.

After soldering the connector to the harness I check to see if I had volts at the plug. I should have done this before soldering but o well. When I hook my DMM to the b/w and L/G wire I get 12v+, but if when I connect the plug to the BACV, it does nothing.

Now according to the wiring diagram, I should have 12v on the B/W wire which I do and ground on the B/G wire, which I do. So im wondering why the BACV doesn't do anything when I plug it in.

Is the ECU supposed to vary the ground to the BACV or what is it suppose to do? I figured I should have got vacuum from the BACV when I plugged it in since I had 12v+ at the connector.

 

BAN for putting member names in subject lines.


-Ted

 

11. Do not post a thread solely aimed at getting the attention of another forum member (i.e. posting ATTENTION FRED SMITH!) The forum provides a private messaging system for contacting individual forum members.

 

LOL im tellin the principle

 

Should have 12vdc on the black/white and the L/G with the key to ON. Actually it might drop a few volts down from there. Depends.

When working, the thing has a duty cycle of approx 120 hz, so the gnd from the ECU should be coming/going and the diapharam in the bac vibrating. If you had a meter on the L/G wire the voltage will drop from approx 12vdc down to ???? memory says in the eight volt range when it's real busy.

If you hold the BAC in your hand with the plug on, and turn the key to On, engine off, usually the bac will open say 3/16ths inch or so. I find if I reach over to the throttle linkage and open it a bit, the bac will now start to vibrate rapidly. Opening won't be very big, but it will vibrate.

Another way to see if the ECU will operate it, is to hold it in your hand and then have someone crank the engine over. When the key is HELD to start the bac will go full open and stay there til the engine starts.

So, with the engine off, key to ON, does the bac vibrate if the throttle is moved open a bit ???? Does it go open if the key is held to start? I usually just pull the small wire off the starter when doing this by myself so the engine does not turn over when going to start. It's holding the key to start that causes the bac to go full open. By full open, I mean maybe a 1/4 inch open and stays that way as long as the key is held to start.

 

Sorry, I didn't realize that was a bad thing. I don't understand why it is either. I could understand if it was to bash on him.

I was wondering why I got a PM from Icemark.

Thanks Hailers, I will give that a shot and see what happends.

 

Since its going on and off so fast, I probubly won't be able to see this with a DMM Since the reaction time is so slow right?

 

Whan it's working all you'll see is, is a voltage drop. IF your meter has a FREQUENCY function, then you could see the HZ it's working at. Most meters don't have FREQ though.

I'll step out to my car in a while and just put a meter on it and give you the results in vdc with the engine off and then with the engine on. On the whole I remember tapping into the output wire at the ECU in the past with the meter on dcv, and what I'd see is the voltage drop down the more busy the thing got. Like at idle I'd see a certain voltage. Then I'd turn the headlights on and the voltage would drop down to another figure. I'll write back in a while.

But........the bac is held on with but two 10mm nuts. Usually you can just undo those nuts and even with the water hose connected you can pull the bac off a bit and twist it up so you can see/feel the diaphram with your finger. It'll be vibrating. Even with the engine off. key ON, if not vibrationg you can reach over and move the throttle so the tps will move off it's dead ***, which will make the bac vibrate. That happens on my cars when I lift the throttle with the engine off key ON.

 

No one said it was bad.
It's just against the rules.
Who said the rules gotta make sense.


-Ted

 

Oh, I just realized something. I usually always look at the bac voltage from the ECU plug. The plug on the bac has a boot around it so you can't backprobe the wire. Uggggh.

I kinda remember you saying you respliced the two wires back together. Maybe you can get at a bare spot on the L/G wire at the splice? LIke I say, I usually acces the L/G wire at the ECU. Pin 2Q on the top row, far left.

 

You have a very good point

Well. I did shrink tube the connection to cover it, I could remove some of the wire covering with a knife and probe it that way.

Would the ECU know if the BACV is connected or not? This is on a Series 4. Reason why I ask is because I probed the BACV plug from the front while it was disconnected from the BACV and I would see the voltage go up and down sometimes and very little. 10ths of a volt would be seen to drop.

I wont be able to do anything on the car today untill late tonight, after 9pm EST. So I will let you know what I find out after then.

 

The BAC plug has to be connected up and it's the L/G wire that needs to be read.

With a hot engine but engine off and key to On, I see less than 2vdc. That means it's being commanded full open.

Engine at idle and fully hot, the voltage is around 9.5vdc. Turn the headlights on to put a load on, then the voltage goes to the high 8vdc.

Looking at duty cycle it is 45% at idle and if the headlights are turned on it goes to 51%. I can also read the MS on this meter but kinda forget the figures. Something like 4.5ms and it'll increase to 4.8ms or so if the headlights go on.

No matter. Even without a meter you should be able to determine if it's working or not. No meter needed. Just pull it off and leave the plug on. It should be vibrating. If not try moving the throttle linkage so the tps will move off its dead *** and now the thing should vibrate. If it does not, then the transistor in the ECU is burnt out and needs replacement (if all wiring is intact to the ECU). The transsitor cost about a buck fifty and is in the ECU internals.

 

You could use a sewing needle to ***** the LG wire or use a wire tap temporarily on the LG wire to see the action on that wire.

 

Excellent, I will test it out and see what happends.

You also spoke about the throttle sensor and I also put a question mark on my check list next to the TPS for the BACV operation. I did have to readjust the thermowax so I know the TPS is all screwed up now and is reading much lower then 1v.

 

Be very careful NOT to short the two BAC wires!!!!!!!!! You will need to resolder a transistor in your ECU if you do.

ask me how i know

 

I mentioned moving the trottle to move the TPS for a reason. On some rare occasions I've had the bac in my hand with elec plug connected and no vibration. I noticed if the TPS was moved via the throttle linkage, then bac would then vibrate as normal.

I wasn't suggesting the TPS is a problem if not set dead nuts on.

Basically if the key is to ON, both wires are connected at the bac and ECU, then the thing should vibrate. IF it does not then the transistor in the ECU is kaput. No meter really needed but is helpful.

 

Well my clumsyness had the B/W wire short on the block poping the engine fuse. Thankfully thats all it was.

For some reason if I can get 12v by probing both wires together, don't you think the transistor would be good?

I have yet to do any further testing. Havn't had time. I should be able to do it today. I have tried to feel it vibrate with the engine off, IGN ON, while moving the throttle but I get nothing. I will do some further testing.

 

You might try a rather generic way of checking the signal at the BAC harness connector. Use a 12v test light connected across the plug terminals and watch the flash at different rpm/ throttle settings. Of course if you accidently fried the ECUs transistor, you would have no flash. I haven't actually tried a test light on the BAC signal, but it makes sense to see a flash there. I use a hand-held lab scope to check signals like these. I can watch the actual waveform as well as get readings for frequency, duty cycle, etc.

 

******For some reason if I can get 12v by probing both wires together, don't you think the transistor would be good?*********

That's normal if the ECU isn't ever grounding the blue/green wire. You'll see 12v on both wires in that case. Your just reading the voltage thru the coil from the black/white to the blue/green. I suggest the transistor is kaput.

Like I say if both wires are coming and going from where they're supposed to, then the transistor in the ECU is kaput. Thing should have vibrated for you if it was good.

 

Alright. I think I remember years ago the S4 BACV not working either....

Is it easy to locate the blown transister? I have seen one in my life time and it was VERY clear since there was a burn spot in the middle of it.

 

At the very bottom of this site: http://www.teamfc3s.org/info/articles/idle.html

I used that site to replace a transistor in a 87 na about five years ago. Got the transistor at Radio Shack. If I can do it..............anybody can for sure.

 

Thanks, i'll take a look at that. I know I can do it. I just didn't want to replace a bunch of transisters if I didn't need to just to find the right one

 

I did it earlier this year and it is VERY easy to do. I did a few tests on the transistor to see if it blew. After some googling, you can find how to use a multimeter to test it.

 

Ya I printed, a few months ago, how to test transistor for some reason. I guess I though I would need to test one, one day.

HAILERS, another question, before when you answered back about me asking why I got 12v+ probing both wires together, since that happends wouldn't the BACV go full open or would I hear a click at least when plugging the connector in? Reason I ask is because when you apply 12v to the BACV, I hear a click.

I see you said the bacv opens more when less voltage is applied to it..... I kinda got confused there, I would think the more voltage, the more it opens.

I appriciate all the information you have been giving me, thanks a lot.

I have yet to remove the BACV. Wednesday im going to work on this along with the OMP. I just havn't had time and Wednesday I can't see my self doing anything that would keep me from what I want to do

 

No. The bac has 12c on it all the time the key is to ON. What makes it work is the ECU putting a ground on it. In the case of a BAC the ground is pulsed on/off 120hz. The voltage drops on the LG wire only when the ECU puts a gnd on it. The bac won't do anything unless a gnd is put on it one way or the other.

The ground from the ECU is pulling the voltage down. On most of the other solenoids the gnd from the ECU will pull the voltage down below 2vdc. In the BACs case it's pulsing a gnd and the voltage drop is only down to nine or so volts because the gnd is being pulsed to the bac, not a steady gnd like on the Relief solenoid. Something like that.