furgo
Samba Member
Joined: September 06, 2016
Posts: 944
Location: Southern Germany
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 Posted: Wed Nov 22, 2017 4:31 pm Post subject: Re: TDC marker unit questions |
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| raygreenwood wrote: |
As far as I know...the two pins in the flywheel are NOT magnetic. Of course I havd not checked any that I have to see of they are magnetic....but neither do they need to be.
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I cannot be certain either without having examined a flywheel or the sensor itself, but I've also come to the same conclusion. Here's how I think it works:
• The two TDC studs screwed into the flywheel are simply made of steel and not magnetized.
• The sensor is passive: of variable reluctance type, based on a coil with a magnetic core (i.e. the magnet is on the sensor).
• The output of the sensor is analog, zero-crossing (AC), sine-wave-like. A cycle is generated every time a TDC stud passes across the sensor (see the diagram I posted earlier). Peak to peak amplitude increases with speed, but I won't know the actual value until I measure it. It will probably be low voltage and will need to be amplified.
• Connector: one pin is the shield, which is most probably already grounded on the bell end. The other two (the output) are interchangeable (as long as you don't care about the output's polarity). Ground one of them and take the reading from the other one.
| raygreenwood wrote: |
The two pins do not have to be at TDC to detect TDC. You simply program in the offset
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Indeed, I was just wondering why the two of them instead of just one.
One reason could have been to calculate the engine speed using just the sensor output. But given the fact that the device used to measure the sensor output already had a dedicated input for engine speed, it's most probably not the case.
| raygreenwood wrote: |
Speed/rpm is measured due the known distance between each pin and the time hack between impulses can be divided out in the circuitry.
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Also true. Fortunately, having terminal #1 with the points signal available makes it even easier to just measure the frequency, divide it by two and convert the result to RPM.
| raygreenwood wrote: |
| .....but I know you can buy things like thia pre-made. |
Ahhh... but where's the fun in that? 
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