MSefi.com read only forum (Jan.15,2018)

I'm looking for a VR sensor with a smaller diameter tip to try on my custom smaller than stock trigger wheel. This is running a 1987 GM DIS waste fire ignition module with it's tach ouput going to the a MS1v2.2. The oem sensor won't work much above 4000 rpm although everything is OK at lower speeds. The oem sensor tip is 0.20 inches diameter (5 mm) and at higher speeds the signal does not fall far enough to get a zero crossing on the 7th notch which is the one with the least spacing. If anyone knows of a VR sensor with a smaller diameter metal sensing tip, I'd sure appreciate hearing about it.
thanks
Ken

Might be easier to use a hall effect sensor. There are ones available which are about the size of a small transistor. That is just the sensor itself - you'd need to devise a method of mounting it. They can be glued with epoxy. The type you need is known as a gear tooth sensor - other hall effect may need a magnet.

This sort of thing:-

http://www.allegromicro.com/en/Products ... TS616.aspx

The GM DIS ignition module is expecting an AC input with zero crossings and has worked well for several years as I haven't been pushing it beyond about 4000 rpm until now. The easiest and cheapest upgrade solution would to find a smaller automotive VR sensor. I would think that some of the sensors for 60 tooth trigger wheels might be smaller?? Failing that, I'll continue some attempts to apply some negative bias to the existing sensor or maybe look for an industrial sensor. I don't mind fabricating a mount for a Hall Effect sensor but I think I'd also have to upgrade MS to do the ignition instead of just the fuel and I don't have the time to do that right now.
Ken

What a zero crossing input is looking at is the AC sine wave changing from one 'polarity' to the other. A hall effect produces a square wave signal which gives a far more definite change from one state to the other. By use of an added pull up or down circuit, it should be easy for a VR input to read it.

The snag with a VR sensor is it basically just a coil of wire round a core. The smaller you make it, the lower the output. Within reason. I'd guess the smallest ones will be used on motorbikes. But your VR input on the DIS likely needs to see a signal pretty similar in every respect to the original.

Try a GM wheel speed sensor from a Blazer. Napa PN SC497.

For what ever it's worth, the MS-E standard practice for hooking up a Hall sensor is to the VR Input rather than the opto-isolated input, and it apparently works just fine.

Having played with the Allegro chip mentioned above, a word of warning: make sure you get a sensor that is intended for what purpose you are going to use it in.
I accidentally ordered one that was meant for a constant tooth wheel (ie: wheel speed tone ring) instead of one intended to be used with a missing tooth. It might have worked, but it also may have kept trying to re-calibrate each time it passed the missing teeth.

Thank you for the suggestions.

After a lot of bench testing with the 6+1 trigger wheel mounted on a variable speed router (0 to 8000+ rpm) and a digital scope, my conclusions follow:
Two new aftermarket GM waste fire DIS modules for a 1987 Chev Cavalier proved to be even more picky about the VR signal than the original unit. So I started playing with triggers that would work with MegaSpark instead of the GM ignition module.

I could not get the v3 VR circuit to function satisfactorily with the small 2.5" diameter trigger wheel. It seems that my VR sensor (from the Cavalier) has enough inductance to cause the current to lag the voltage and it is current that the transistor responds to. It was fine at low speeds but I was seeing significant timing retardation at high speeds in addition to some other less that rock solid performance issues.

However the v2.2 VR trigger circuit with the LM1815 chip is voltage triggered and it seems to work perfectly with my VR sensor. While not the simple solution I had hoped for, I now plan to ditch the GM ignition module and use the megaspark software with a new small trigger wheel.

A quick bench test of a low cost Hamlin 55505 Hall sensor also seemed to work just fine with my small wheel so that will be my fall back plan.

Ken