RPM reading from MSD ignition box

[20B]

Can anyone tell me if I can read an rpm signal which is output from an MSD ignition box which is normally used as a tacho output ? How do I go about doing it ? Digital input or ref input ?
It states in the manual that the MSD ignition box outputs a 12V 20% duty cycle square wave signal for the tacho output, can I get the ECU to read this as RPM as its installed on a Carburetor engine with points ignition which I'm just using for some Lambda logging.
Here is the link to the MSD box http://www.msdignition.com/Products/New_Products/6425-Digital-6AL-Ignition-Control.aspx

[Holmz]

MSD make a tach driver that hooks to the coil and provides a 5V (or somewhere around there) level.
I have one in the box that I have not installed - I think I even know where I put it...

[20B]

Thanks for the response.
So I would need to buy this tach driver and input it to what pin on the ecu ? Ref or digital input ?
How do I go about setting up the ecu to read it as rpm ?
Do you have the model of tach driver you are talking about ?
Is there no way to just use the output on the MSD box which is 12V square wave ?
This one is a bit beyond my experience so thanks for all the help

[Holmz]

Thanks for the response.
So I would need to buy this tach driver and input it to what pin on the ecu ? Ref or digital input ?
How do I go about setting up the ecu to read it as rpm ?
Do you have the model of tach driver you are talking about ?
Is there no way to just use the output on the MSD box which is 12V square wave ?
This one is a bit beyond my experience so thanks for all the help

A zener diode and some resistors should do the trick for you.
Probably will want to try it on a scope.

[20B]

A zener diode and some resistors should do the trick for you.
Probably will want to try it on a scope.

Hmmm that is another language to me unfortunately

Didn't realize this was going to be such a difficult problem to solve.

[SprinterTRD]

If you need to read RPM I would connect the MSD tacho output to the REF input. Set the Ref input up as a mag input
so you can set the trigger & noise levels.

The MSD output may be a noisy signal which may cause wrong rpm readings. If you have maxed out the filter value in the ECU then some external filtering may need to be done. This is easy as its only one resistor and capacitor.

[20B]

SprinterTRD,
Excellent idea, it is OK to input 12 volts to REF ?
Will the wave form just look like a normal magnetic square wave trigger output but be 12volts instead of the normal 5 volt you would see on a pickup ?
Is there a particular capacitor and resistor I should use or will it be specific to the conditions of the signal ?
Many thanks for help on this

[Holmz]

SprinterTRD,
Excellent idea, it is OK to input 12 volts to REF ?
Will the wave form just look like a normal magnetic square wave trigger output but be 12volts instead of the normal 5 volt you would see on a pickup ?
Is there a particular capacitor and resistor I should use or will it be specific to the conditions of the signal ?
Many thanks for help on this

If you are afraid ("ascared") that the level will be higher than 'X', then the zener is what you use to limit the peak.
http://en.wikipedia.org/wiki/Zener_diode
A resistor between mag output and zener - which limits the current (maybe 10k ohm which will give you ~ 0.9mA)
With the zener tied to the resistor and ground - which clips the voltage.
Then at the junction of the resistor and the zener go to the ECU or sensor.

For a low-pass-filter replace the zener with a capacitor.

Do you have a scope?

[SprinterTRD]

The MSD spec is a 12V square wave. The 20% duty means that at any frequency the output will be on for 20% and off for 80% of the frequency period.

A magnetic sensor with even sized teeth and gaps will be a sine wave with a 50% duty.

Magnetic sensors can generate a voltage up to 90V so 12V wont be a problem for the ECU.

The output of the MSD will still look like a square wave.

If the MSD signal is noisy then the filter will depend on the noise duration on the signal. You could probably start with 1000 ohm resistor and 100nF capacitor.

[20B]

I'm pleased to say after taking the advice of those suggestions in this post that everything is now working with the rpm. Thanks for your advice guys