Logging Vehicle Engine RPM and Speed


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I have already posted this on the LabJack forum however they suggested that I also post it here.

Labjack forum post

I recently purchased a JabJack U3-HV and also a JabJack U3-HV OEM board. The OEM board will eventually be incorporated into a vehicle to monitor and log various data from the Engine Control Computer as well as other sensors that have been installed.

I am having trouble getting a sensible readout from a timer recieving the signal from the Tacho output from the engine management computer. This signal comes from the camshaft and therefore puts out 1 pulse per 2 engine revolutions. I have confirmed this with an oscilloscope. eg 8.3333 Hz for 1000 RPM.

The main problem I am having is that the displayed number decreases as the RPM increases !!!! The 'Speedo' display also decreases as speed increases. I do not understand how this is occurring.

Please note that I know absolutely nothing about programming. Everything so far has been a


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