Thursday, October 20, 2011

MS3X cam input adjustment

If you read the MS3 hardware manual it says this about setting up the MS3X VR conditioner circuit for Hall inputs:

"Turn both pots (R11 and R32) full anti-clockwise - approx five turns. Then turn the top one (R11) two turns clockwise."

I did exactly that and chased my tail until I finally discovered that my cam input wasn't working.  The problem was that my car ran great.. the oscilloscope showed that it was running full-sequential and the spark plugs and injectors were firing exactly when they were supposed to, except that the engine was only starting about 50% of the time. 

I attributed the starting problems to bad cranking and prime parameters.  If it was running full-sequential, how could it not have a good cam signal?

The answer is that it was polling the cam at start and always seeing a "0".  If the engine happened to be in that position it started and ran fine.  If it was NOT in that position, it didn't start until it fell into that phase.  Once it started it was full-sequential by happenstance.

Here's what I found when I took a closer look at the MS3X VR conditioner circuit:

The VR conditioner circuit.

Input is a 0 to 5v square wave.  After it goes through transistor Q2 it becomes a 1 to 4V square wave.  Arrows show circuit test points.

This is the resulting signal and the threshold at the comparator (OpAmp U7A).  With R11 turned twice clockwise, the threshold isn't sufficient to trigger the output.

R11 turned 3 1/2 turns clockwise.  Threshold is now high enough to trigger and the output becomes correct.  The minimum turns on R11 for the threshold to trip was about 2 3/4.
The bottom line is that you should turn R11 clockwise until you have 2 volts on pin 3 of U7A

U7A pin 3.  R11 sets the threshold voltage on this pin, should be around 2 volts.

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