Testing a mfwd or flywheel speed sensor

wuntun · Dec 7, 2017

Can these sensors be tested stand alone/without being plugged in for what values to determine if good or bad?

CobyRupert · Dec 7, 2017

Not 100% sure, just a guess, but:
I imagine it's just a proximity switch (magnetic) that opens or closes when it senses the metal target passing by.
Does it just have 2 wires (or 2 connections)?
Not sure if you can test for continuity (from one connection pin to the other) while passing a metal (ferrous) target by it, or if you have to apply ~12VDC to one side and check for a pass-through voltage when the target is near.

aczlan · Dec 7, 2017

CobyRupert said:
Not 100% sure, just a guess, but:
I imagine it's just a proximity switch (magnetic) that opens or closes when it senses the metal target passing by.
Does it just have 2 wires (or 2 connections)?
Not sure if you can test for continuity (from one connection pin to the other) while passing a metal (ferrous) target by it, or if you have to apply ~12VDC to one side and check for a pass-through voltage when the target is near.

If its like the flywheel speed sensor in most cars, it probably senses the difference in the magnetic field when the teeth of the flywheel pass it.

Aaron Z

the old grind · Dec 7, 2017

What Aaron said. ^^ If it's a Hall-effect device there should be a coil inside with some resistance that can be measured. If you're troubleshooting a system or tach, you might more easily, if not more cheaply, swap it out to eliminate the sensor as a problem. (cheap, ergo logical start-point :thumbsup

If it's two-wire, this widget likely works perfectly or not at all. If three-wire there may be a 5v reference, too, and resistance testing would want a 'how to' from maker or a reliable expert.

If you just don't trust rpm readings and suspect this component you might just calibrate the o'all setup. I do such with HFT's <$40 digital tach:.
Digital Photo Sensor Tachometer ... & I take the batteries out if it might not be used every year.

btw, this 'photo' counter has been a Godsend when re-gearing a wood-bandsaw for metals with a jack-shaft intermediate reduction and 130v DC treadmill motor. Easy to calculate blade SFM precisely with a mathematical constant for each wheel/pulley I take an rpm reading from. My point is to suggest the versatility of application and that this tool will pay for itself in time saved within a scant few uses, and perhaps this is one. t o g

wuntun · Dec 7, 2017

Problem is, that it is not easy or cheap to just swap out (drain 6 gallons hydro fluid, and cost is $75 and 4-5 hours). I also hate when I, or even a 'professional' mechanic resorts to swapping things out just to see if that 'fixes' it or not. To me, a proper repair to anything, is to find the cause of the failure, not just the failure itself. That can give one a way to prevent the problem from occurring again prematurely, either by preventative or re-engineering (and a few other ways). ^^^^^ It is a three wire, uses a 5 volt reference and ground. Third wire i believe goes to the controller with the returning value. What is that value? The tech manual states a running value, but i cannot replicate that if something else is the cause, HENCE my desire to want to test values non-running. _ OLD GRIND that is pretty cool you can do that with the tach, and def. a good item for the tool collection, but that wouldn't help at this point. Thanks for every ones input, just hoping some one will have done the bench test and post the values, or test procedure.