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Old 07-23-2013, 06:54 PM   #1
jpnovak
Scooby Newbie
 
Member#: 221948
Join Date: Aug 2009
Vehicle:
1975 Porsche 914 TT

Default Vehicle Speed Sensor (VSS) waveform?

I swapped a ej20TT and 5MT into my 1975 Porsche 914. Its running Megasquirt and boosting quite happily to 18psi. The car was built on the cheap and competed in the GrassRoots Motorsports Magazine $2011 Challenge. Unfortunately the original motor spun a few bearings due to oiling issues. That is all fixed. It will go back again this year and will be my track car for a while. Now its time to finish sorting the details on the installation.

I need a speedometer. I am currently running a temporary bicycle computer running a magnet off the CV joint. Worked to pass inspection but not optimal.

I want to run a later Porsche speedometer in the dash. These should have a >5V input with 8 pulses per revolution. Its just a reed switch with a magnetic disc in the transmission.

Can anyone tell me what the waveform is from the 5MT? Its circa 1998-99. I only have electronic outputs from the transmission. No cable drive on this one.

I need to know if I can just do voltage conversion or if I have to convert the signal via PWM correction or duty cycle for accuracy. Thanks
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Old 07-23-2013, 11:48 PM   #2
Teryn It Up
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Member#: 225919
Join Date: Oct 2009
Location: Land of Kirk
Vehicle:
2000 2.5rs
Psh... F Kartboy brackets

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I found a schematic that shows its a hall sensor that generates 4 pules per revolution. square wave form. It looks like the power is 12v. Schematic just says it comes from ignition. So I am assuming its 12v. Someone will need to verify this for me.
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Old 07-24-2013, 12:04 AM   #3
jpnovak
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Member#: 221948
Join Date: Aug 2009
Vehicle:
1975 Porsche 914 TT

Default

Thanks, I will probably just put the scope on it while the car is in the air and read the signal from the sensor.
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