Engine RPM on BMW E30

[fdv99]

I have a C125 Dash logger with the i/o upgrade, and am having trouble getting an RPM signal to the logger. The car is a 1989 BMW 325 so it has no OBD port or easy way to get RPM. There is a wire coming from the ecu that fed the stock tachometer in the dash. This wire shows 12 volts DC when the engine is off and decreases as the RPM increases. The best measurement I have shows roughly 8 volts at idle and continues decreasing as the RPM increases.

I was told that I could run it to a digital input and calibrate it, but when I go into the Dash Manager and select RPM for a digital input I can't figure out a way to calibrate it.

Any help is appreciated.

Forrest

[mstech]

I do not think that you can use a digital input as they do not read voltages, only frequency of change. Use one of the AV inputs, allocate it to the engine RPM channel and then make a table that is based off of your V vs. RPM readings.

I have enclosed a quick file showing what I believe you need, just rename the file extension to .c2u as these files are blocked by the forum software.

[fdv99]

With it going up to 12 volts, and around 8 volts at idle, will the AV input handle it okay and not damage anything?

Thanks for the help.

[SprinterTRD]

I doubt it would be a varable voltage!

I would put your meter on frequency measurement and see what it reports.

What you see is the multymeter averaging the frequency.

[fdv99]

Okay, I'll check again with it on frequency in the morning.

If this doesn't work, someone suggested picking up a pulse signal from the negative side of the coil, would that work to a digital input?

[SprinterTRD]

No, Do not put coil neg into the digital input!

You will blow the input up as it is not designed to handle the 400V coil neg pulse.

[Hoolio]

The wire from the ECU which feeds the standard tacho is most definitely variable frequency (conventional one pulse per cylinder firing) so that is the connection you need to make to your dash logger. You just need to figure out how to calibrate your dash logger; sorry but I can't help with that.

[fdv99]

Ok, great. Thanks for the help everyone, I'll hook it up and play with the calibration

Forrest