Wednesday, September 21, 2011

Adjusting VR bias and getting a nice idle

I got the motor idling and starting nicely.  Cranking advance changed to 8 degrees, cranking pulsewidth changed so that I got 8ms, same as what I measured on the stock ECU.  Adjusted idle VE slightly and it was starting and idling very nicely!   ...and it's not even closed loop yet.

Problem is that when I blipped the throttle it would bog and die.  Three or Four times in a row...  I changed the dashboard to show lost synchs and saw that the count was going up right as it was stalling.  I had a VR sensor problem.

This is where I discovered how powerful Megalog Viewer can be.  I logged one of the stalls and got this plot:

The very top line is RPM.  Right as I blip the throttle the RPM dips to zero but then instantly recovers.   Due to noise, I guess, but not sure why I was getting noise from blipping the throttle.  Anyway,  I was getting a lost synch reason 2.  Missing tooth out of sequence.  The reason it was stalling wasn't because of the lost synch, but because going to zero RPMs and back up was triggering After Start Enrichment (ASE) which you can see where the blue line goes up.  The ECU saw the rise from 0 to 750 rpms as a start and increased fuel!

I adjusted the R52 bias screw 3 1/2 turns to the right from full-left and tried again.  Revs nicely now and no synch drops!

O-scope of the VR conditioner with car idling.  The zero-crossing down looks great, but why is the ouput returning to ground at a negative voltage?  Isn't the hysteresis bias on the (+) OpAmp pin??

I took the Megasquirt back in my shop and ran the signal generator through the VR input.  The first thing I noticed is that R52 and R56 don't behave the way I expected.  That is, after turning both 6 turns to the left to ensure that they were on the stops it took around 3-4 turns clockwise on either one of them to get anything to happen.  I adjusted them so I have a good hysteresis margin, but not so much that a 4v peak-to-peak cranking signal would keep the car from starting.   The conditioner trips at about +1 volts and - 1/2 volts, well within the margin of a cranking signal but well above any noise (I hope)

This is only 40Hz and a few volts peak-to-peak.  The actual VR signal is near vertical at the threshold points.
Mystery of the negative voltages solved... this is a scope of an input signal and the output of the first stage of the VR conditioner.  The transistor biases the voltage of the input up by about 1.2 volts.  This is why a negative input voltage was tripping the zero crossing detector.. it was at zero volts by the time it got to the comparator.
I also found that the behavior of the hysteresis pot, R52, is very non-linear.  The first 3 turns did almost nothing to hysteresis.  Turn 4 starting increasing it.  Turn 5 started increasing it quickly.  After turn 6 it started to go off the chart very quickly.
The bottom line

I would take both R52 and R56 and turn them to the left about 10x to get them set to zero.  Leave R56 alone.  Turn R52 5x to the right.

The motor starts and revs with ZERO lost synch counts now.

1 comment:

  1. Thank you! Thank you! Thank you! Thank you! For this info! SYNCED :)