VR timing issues

[bartholins]

So, I've had issues in the past, : http://64.50.169.36/~micro/viewtopic.ph ... 85&start=0 Basically I haven't touched the car for two years and thought, well, I'll flash to the latest code, see where I'm at. To make it so you don't have to read the above link Here's what I've got : a 350 Chevy with a lumpy cam, big heads, running MS-II on V3 board with MT2.25 and latest code as of 3 days ago (2.89 I believe?). It has a Mallory 50 series distributor (VR pickup) that MS reads the tach signal from (took the Mallory electronic piece out of it as I fried two of them, so I'm tied directly to the coil that the reluctor flies by). I have the mechanical advance on the distributor locked out and there's no vacuum advance as I want to control timing through MS. The MS then drives a MSD-6A box to get spark.

Now the actual issue : I had the VR polarity backwards before, so timing was not real solid, but it would signal all the time(not drop signal)... I reversed it lastnight, and *when the car runs* it's rock solid... I'm just getting random pulses occasionally now. Above 2500 RPM it gets reliable again (almost like it's a voltage from the pickup issue). All pots (R56 and R52) are fully counter clockwise. I can't get the blips in tach signal out with the hysteresis pot at all. Here's the fun part, one of the about 10 times I was playing with the car, it fired up, had the usual bad trigger points from the pickup, but then everything smoothed out and it ran beautifully! I have NO idea what caused it to be happy that once... as best I can remember, nothing changed from that time to the next one (and 6 or 7 restarts after that).

Any ideas on what to look at? Could the pickup / coil just be having issues? Is there a cheap / easy way to go to a crank trigger and still use the distributor / MSD setup?

Thanks a ton in advance.

--Sam

[Bernard Fife]

Sam,

This seems like electrical noise to me. Since it happens mostly below 2500 you may be able to tune it out using the time mask/percentage mask under 'tach signal masking' in the tuning software. There is more on that here: http://www.megamanual.com/mt28.htm#i

In that doesn't help, the next thing I would do is make sure all the grounds are good, and none of the wiring pases near the spark plug wires, etc. If those are good, then you might have to shield the pick-up wiring in the engine compartment (I find the easiest way to do this 'after the fact' is to wrap the wiring in aluminum tape, with a short bare wire touching the aluminum and grounded to the block).

Then I suppose you would have to look at the pick-up, etc. You can go to a crank trigger - you just need one with 1/2 as many pulses as cylinders (for a 4-stroke) and it would be just like a distributor to MegaSquirt. If it was mine, I would work with what you have, though.

Lance.