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I've got an AEM EMS in the car for the engine computer. I found that the original vss was a reed switch in the speedometer head and that the EMS did not like the signal from the first gen Mitsubishi. They say the second gen cars don't exhibit the problem. Several years ago I added an aftermarket hall effect in-line speed sensor that it mounted directly on the transmission and then the original speedometer cable is screwed to the top of it. My understanding of the speed signal was 4 pulses per rotation of the speedo cable. I have never found what that was supposed to be per mile. When I got the speed sensor from the Speedo shop, they said it should match so I fed the output of it to the EMS and it was the same as the original except that it was very stable and didn't flucuate like the original reed signal did. Earlier this year the speedometer got to wiggling and the cruise wouldn't stay engaged for more than a few feet, replaced the cable and it worked fine. The EMS vss reading was still stable and didn't flucuate, course the sensor for it is before the cable. This is the sensor that I attached the to the MPGuinio.
When I tried the MPGuinio on the reed sensor wire that had originally gone to the factory ecm, I didn't get any reading at all. So I just hooked it to the aftermarket one feeding the EMS because the pulse count should be the same as stock. I'm wondering if the flucuation I'm seeing is simply that I've probably put 35-40 thousand miles on it since I put this sensor on it and it now has some wear in the bushings allowing it to wiggle?
The EMS still shows steady speed up and down but maybe it isn't reading + to + pulse but just + or - and that is how it determines speed.
Can you think of a way I can get you a good picture of the scope screen to make a determination from? I don't think this one will let me save it in a manner that I could download the trace and send it out.
Mark
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