MSII 3.57 2.890 36-1 MSD6AL cranks 3 sec. before ign or fuel

[HomeBlown57]

I am using a ford vr sensor on a 6-1/2" 36-1 wheel. I have the car tuned well up to about 90kpa. I have been driving it for tuning purposes with great results. HOWEVER!! When I am cranking it it takes about 2-1/2 seconds to get a tach signal. It will only give me ignition and fuel for 3-5 ignition events before dropping out again. Then the 2 second dead crank followed by several tach/fuel events begins again. My battery is about a year old and cranks at about 10.2 Volts when it starts to fire in can drop as low as 9.4.  Eventually it fires on those tach events it starts and runs great. I am not getting a processor restart through any of this.

On rare occasions while driving I will have the tachCount drop to zero and lose the ignition. The last time was during low rpm high load. I was trying to set my high kPa and low rpm bins I stomped the throttle and brake. I was at 90kpa and 750rpm with a 14.5 afr then nothing

Can someone check their cranking voltage and see if it is close to Mine.

350 chevy
MSD6AL
All grounds land on a terminal block and return to the battery on a 10ga wire
Power for the system (wideband and ecu) is  fed from the battery via a 10ga through a relay then to a small fuse panel
The same 10ga also feeds a second relay for power to the injectors via separate feeds in the same fuse block

All wires are either soldered or GM weatherpack terminations. My logging shows no noise in the system. Ignition wires are no less than 6 inches from any control wiring.

[HomeBlown57]

The only changes between the log and the current tune are VE table changes. My current logs all began with it running.

Thanks for looking

[Simple Simon]

Just a suggestion but it may be worth checking/resetting the air gap between the VR sensor and Trigger wheel just a thought and easy to check.

[grippo]

If you look at the Trigger+/- column in your datalog, it counts up pretty consistently through the entire run, both at cranking and at normal rpms. Each time it counts up it means an extra pulse arrived between 2 real wheel teeth. The ecu throws these out, but it will only throw out 1 at a time, not 2 or more consecutive. The software is only meant to throw out very occasional noise pulses. If you haven't already I would try a large audio capacitor across battery and ground. It might also be due to wrong polarity since it seems very consistent. What happens is the extra pulse comes in on the missing tooth gap, and this can often be fixed by changing polarity. You can try just changing the VR input polarity in TS and see if this helps. You will have to also change the trigger offset by 1/2 tooth worth to keep the same timing.

[HomeBlown57]

Thanks for replying!

I reversed the polarity on the VR sensor. It didn't make much difference. After logging with the sensor reversed I was comparing the new polarity to the old in regard to trigger +/-. With the old polarity the +/- would only add between 1500rpm and 2500rpm. This happens like clock work. As soon as it reached 2501rpm on an incline it stops adding. On the decline it stops adding at 1499. With the new polarity the rpm high range remains unchanged but the low range lowered to about 800. It adds from 800-2499 with the new polarity.
I changed the trigger input back and forth from rising to falling. It cranks with no ignition or fuel on falling edge. All of my tests tonight were with polarity changed. Tomorrow I will change the polarity back and change the input capture to falling edge and try that.

What is the trigger +/-? As I understand from your post it is an extra VR pulse. In 36-1 teeth it would send 35 pulses? If the VR sensor crosses the zero mark 37 times in one revolution would the +/- rise by 2? Please correct my terminology so that I can communicate my thoughts properly.

If there is a problem with my VR gap my gap would be too close. What gap should be looking for? Mine is about .030. Could this be a problem?
Do you think that this would go away by eliminating the 36-1. I can very easily change to the gm module in the distributor.

[grippo]

To see what is going on take a look at this picture.
http://www.megamanual.com//ms2/VR-polarity.gif

It explains why the polarity vs edge matters, but the has only to do with accuracy: a line that comes down near vertical cross the horizontal time axis at a consistent point each time, so you get consistent time differences. If you look at the side where the curve crosses the horiz axis at a very shallow angle, a slight difference in voltage can really move the crossing point all over and the ECU will see a wild variation in time. Something else happens that isn't clear from the picture, but is easy to visualize. On the section where the curve crosses at a very shallow angle what can happen is that a part of the curve that is very horizontal can poke up above the horizontal axis just a bit then come back down then make the real jump where the voltage gets very high. The ECU VR circuit doesn't look at the voltage, it just looks at the point where the voltage goes to 0 and crosses above or below the horizontal time axis. So if this small shoulder pokes above for just a bit, the ECU will see a tooth where the missing tooth should be and it will look like noise, so it resynchs. Except it shouldn't because there is logic in the code that throws it out ONCE. If it happens on the very next tooth it does resynch, but you can see that that is not likely on a regular tooth, because there both crossing are very near vertical. Trigger +/- is just a software counter that counts up 1 if it detects an extra tooth and counts down 1 on a missing tooth (except where it is supposed to be missing). I would really try to keep the trigger wheel, unless you have a distributor.

That said I now don't think this is your problem because when this problem occurs it will be consistent once you have exceeded a certain rpm. I have never seen it happen in just an rpm band. So I am thinking there is some sort of electromechanical noise getting in there. For example alternator brushes that spark over a certain rpm band - just speculating here. If you haven't already I would put a large audio capacitor across the battery pos/neg.

[HomeBlown57]

I will use the cap and see if it clears up. I will also verify polarity on the VR sensor.

Thanks again for the help!