Club Cobra - Electronic tach and speedo

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Electronic tach and speedo

Does anyone know what the electrical characteristics of the signals that drive an electronic tach and speedo are? Voltage levels, pulses width, pulses/sec, etc. We're building an electronic throttle control and plan to use the tach and speedo signals to control features like cruise control and rev matching.

Thanks,
Jay

Electric speedo's use Hall effect signaling.

Bounce that one off Petek. He's the electrical engineer in the group.

Get out the occilliscope to see what the signal waveform and amplitude really look like and how they change with frequency. Anything else would be a rough approximation of what should be happening.

I'll take a gander at this one.
I'm pretty sure the tach signal is straight foward 8 pluses per revolution (for an 8 cyl engine) with time being the constant. Voltage between 12 and 24 Volts. A VDO programmable tach shows a calibration range of .5 to 200 pulses/revolution. I suspect this is when the signal is pulled from an Electronic Ignition Control Box.
As far as the speedo, I know my VDO which uses a hall effect sensor at the tranny, indicates a calibration range of 500 to 3999,999 pulses per mile. I think the default is 16,000 pulses per mile.

Very helpful, thanks.

Campy - very helpful reply. Thanks. I just bought an oscilloscope off ebay (pretty good deals available, btw), but I won't be able to check out the actual signals on my car until after we're committed to the throttle control electronics. However, I can pretty easily change resistor values in the input buffers to deal with different voltage ranges, and the programming of the microcontroller for counting pulses. It would be nice to have a feel for the voltage range from the hall effect sensor, but I think this is enough info to get the circuit designed. Thanks very much.

- Jay

Jay - just go to the source...

The easiest and most accurate answer for you is to contact the tech support group of the Tach/Speedo manufacturer and ask to get/buy a copy of their maintenance specs used for calibration and repair of the instruments. Most electronic instrument service benches use fixtures on the bench for calibration, and the parameters should be available.

As an aside, I chuckled reading about your project; it reminded me of several projects I worked on as a young EE... I was designing digital instruments and built a digital speedometer for my '70 Barracuda. I used a strobe wheel from a tape transport, attached to the speedometer cable, as an interrupter for a photocoupler. Lacking any data, I had to experiment to find the conversion factors (# of pulses per unit of distance). (...aah, fond memories of SN74xx discrete chips).
I painted distance strips on the street in front of my house...marked the tires, made a wheel-shaped readout for the transducer and kept rolling the car up and down the street, taking data points.
Had all the neighbors peeking thru their curtains, trying to figure out what was going on.
I think the "great" revelation was that the cable spun 1000 times per mile, or something like that.:D

Campy,

You would only have four ignition pulses per crankshaft revolution, unless you are running a 16 cyl or a two stroke motor.

Thanks Mr.Fixit,
I had this gut feeling that I was wrong. Going back to the days I use to work with points ignition, I was trying to recall the how many lobes were on the cam ..... Anyway, thanks for the correction.