Erratic RPM readings

Race Capture Pro hardware installation- power, wiring, physical installation, etc. See the dedicated forum for Sensor related topics

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marcus hall
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Joined: Sat May 21, 2016 8:57 pm

Erratic RPM readings

Post by marcus hall »

Hooking up the RPM sensor input for my MR2. The tachometer line from the ECU sends out a 5V pulse signal. My logic analyzer says that it is about 210 Hz at idle. But, through the racecapture input, I get totally erratic RPM readings.

I don't quite understand the 210 Hz, actually, though. At cold idle, it's running about 1500RPM, so I guess if I divide by 8 that comes out about right, but it's a 4 cylinder motor, so best guess may be that it's some standard for Toyota for tachometers or something.. The frequency does go up with engine speed, so that at least seems reasonable.

Thinking that there might be something weird in the voltage levels, I am now passing the signal through a 74HC14, but I get the same erratic RPM readings from the racecapture. It's bouncing around all manner of values, from over 100K RPM to a few 100 RPM and everything in between. But my logic analyzer shows variations between 204 to 212 Hz.

Any suggestions? Any experience with the tachometer circuit on mid 80s Toyotas?

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Post by brentp »


If the tach signal is sourced from the ignition coil primary winding, which is typical for 80's vehicles, then you may need to filter it through something like this:

Are you able to look at the signal on a 'scope to match up against what's in the wiki, or can you trace the source of that tach signal?
Brent Picasso
CEO and Founder, Autosport Labs
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