What do i need to set my multimeter on to see the duty cycle of the bac???
And do i just probe both of the wires going into the bac???

 

"FREQ" or "Frequency"?
Usually designated in "Hz", if your DMM can measure that sorta thing...


-Ted

 

If your meter does not have the freq feature, then try this: Back probe pin 3Q on the ECU and watch it while you idle your engine (fully hot engine) with no lights on, no a/c, no brake pedal pressed, no heater fan on etc.

So while watching that voltage, now turn on the heater fan or a/c fan. The voltage should change noticeably. Now turn the lights on and it should change some more. If it changes when you do these things then I'd guess that the bac is working. Doing that shows that the bac is reacting to loads put on the engine.

And please, no, it's not the battery or alternator voltage dropping that is causing the bac voltage to rise/drop. Please, nobody write that. Save yourself some grief.

 

one wire to the bac has battery voltage and the other one has a little less and does drop a little when turning on loads......but the bac will only react if the rpms drop below 750 right.

The only other question is making sure my idle is at 750, is that first big block at the bottom of the tach 500rpm or 0rpm?? or is the second big block 500??

 

The wire other than black/white should show a voltage drop. Actually, if you have air conditioning, I *think* the rpms will rise above 750 and go to approx 850. Memory thing.

Never seen a S5. A S4 has the two lower thick hash marks followed by two thin hash marks and then once again a thick hash mark. That last one is 500 rpm. Then it's four thin hash marks follwed by a thick hash mark that is 1000 rpm.

I think radio shack sells a meter with rpm, freq, duty cycle. I saw it several years ago. It seemed reasonable at that time. I own brand X.