Trigger Wheel VR Sensor Question

[Spooky_Intentions]

Hey everyone, 

 

I am using a VR sensor and trigger wheel with my megasquirt ECU. I was hoping someone could answer my question regarding this set up. I currently have the VR sensor mounted near the bottom of the trigger wheel. I was reading up that the sensor needs to be about 5-7 teeth BTDC. I was unable to find a good mount online to put it at the timing mark mount, can I just bring the engine to TDC and rotate the trigger wheel accordingly? Any answers or information would be greatly appreciated. I tried the search function but wasnt finding an exact answer to this particular issue. Thanks in advance!

[Chickenman]

The sensor has to be mounted so that the missing tooth passes the sensor 60 to 70 Degrees MINIMUM before the piston reaches TDC. That is because a programmable ignition system does not actually ADVANCE the ignition timing. 

With an old style points system, the breaker plate physically moves the points ( or VR Reluctor ) vian Mechanical weights and Cams and the Vacuum pot. That physically does move the sensor location and ADVANCES the ignition. 

With a programmable system, the sensor location is fixed. So you have to start of with a figure that is higher than your maximum advance ( which can be up to 45 to 50 degrees BTDC in light load cruise mode ) and add a margin for the CPU to make calculations. So the ECU has to know when the piston is APPROACHING TDC ( of #1 cylinder ) 

Lets say you want a timing figure of 10 BTDC. Sensor is positioned so that missing Tooth pases sensor at 70 BTDC. 70 - 60 = 10 BTDC. Now lets say you want 40 degrees of advance at cruise. 70 - 30 = 40 BTDC. The ECU just delays or RETARDS the timing less.

Always remember that the CPU needs some time to make these calculations. If you place the  VR sensor too close to TDC, so that the missing Tooth only passes the sensor at say 45 degrees  BTDC, the ECU may not have time to do all the calculations and subtract the necessary timing amount. Then timing fluctuates.

Then you have to Sync the ECU Timing to the engine Timing with a Timing light. Usually to a Locked Timing of 10 BTDC. This is done in the Software.  by adjusting #1 Tooth angle or by using the Trigger Wizard., depending on type of Ignition system used. All this is is a fine Tuning. You are adjusting the basic idle with software, instead of twisting the dizzy as with an old style Mechanical system. That's the only difference. Once you wrap your head around that it becomes easy peasy. 

What version of MS do you have. MS2 and MS3 are pretty simple. MS1 can be a bit tricky as it's ancient. 

[Chickenman]

To answer your first question. Yes, bring #1 to TDC. Then rotate the Trigger wheel backwards so that the Missing tooth passes the Vr sensor at least 60 to 70 Degrees ( 70 preferred ) before TDC.  That will be your course setting. Then make the " Fine " setting by adjusting #1 Tooth angle. 

I take it your trigger wheel is held in place by a Center bolt and can be infinitely adjusted?