Tach Specs

[Sands]

Does anybody know what the specifications are on the signal that goes to the tach on the dash display? Is it a pulse or a voltage?

Either that or the crank sensor?

I need to pick up RPMs for a little side project.

Thanks!

[johnf]

The engine speed signal out of the DME should be a clean, 12V, active low square wave with 1 pps per 20 rpm, if I have calculated correctly. Let me know if I haven't.

[Sands]

The engine speed signal out of the DME should be a clean, 12V, active low square wave with 1 pps per 20 rpm, if I have calculated correctly. Let me know if I haven't.
Cool, just what I needed, thanks!

[Bob Clevenger]

John, do you know what the speedometer signal is? I'd like to build a signal conditioner to correct my speedometer. 11% error is ridiculous!

[Sands]

John, do you know what the speedometer signal is? I'd like to build a signal conditioner to correct my speedometer. 11% error is ridiculous!

A vendor that does one:
http://www.dakotadigital.com

[Bob Clevenger]

Thanks for the link Ian. I guess I'll just have to figure out where to connect it so that it will drop the speedo read by 11% but not affect the odo read. More research,,,

[johnf]

I actually built and tested such a device that digitally reduced the speedometer reading in 2.5% steps and was all set to help you guys. It was simple, accurate, easy to hook up -- and because it had to make the odometer underread, also illegal. I was not happy to discover it appeared to violate a U.S. statue and that all I had to do was to use or sell just one such device to suffer a fine and a year in prison.

[Sands]

Gov'ment sucks. :cry:

I was looking at a Frequency to Voltage converter for the LED shift light project. I also found a Voltage to Frequency converter. In theory you could take a F>V then a V>F converter with a trim pot between or a linear voltage regulator to scale the spedo input pulse down 5% or so.

F>V is a National semiconductor LM2907
V>F is a NS LM231

Should cost all of $10 to do.

All for off-road, non-commercial use.

[Sands]

Thanks for the link Ian. I guess I'll just have to figure out where to connect it so that it will drop the speedo read by 11% but not affect the odo read. More research,,,
I'd just splice into the feed to the instrument cluster - might need to look at the cicuit diagram or board for the cluster - I don't know where the signal slits to avoid running afoul of DOT requirements for odometer.

[johnf]

I'd just splice into the feed to the instrument cluster - might need to look at the cicuit diagram or board for the cluster - I don't know where the signal slits to avoid running afoul of DOT requirements for odometer.Neither did or do I. I think you would have to add your circuit within the instrument cluster as I believe it internally splits and transforms the signal to feed to the odometer and speedometer.

[Ron Stygar]

I'd just splice into the feed to the instrument cluster - might need to look at the cicuit diagram or board for the cluster - I don't know where the signal slits to avoid running afoul of DOT requirements for odometer.
Moving the needle worked for me.

[johnf]

Sometimes a simple, mechanical solution makes the most sense.