DIY MSD tach adaptor
Posted: Sun Jun 07, 2015 6:56 pm
I was asked by theorangeperil about tach adapters for the high voltage, voltage triggered tacho. These will be pre about pre 1970. If you have a later tacho, the MSD will probably drive the tacho directly.
Since I had just done something similar with a Megasquirt for KS I thought I'd try the same here. I had the parts, a transistor, resistor and a Boxster relay, and it worked. So here's the write up for anyone wanting to try the same.
Here's the parts.
Remove the spring and actuator to disable the physical part of the relay. This uses the relay coil to generate a high voltage pulse. (Back EMF).
Now remove the diode, not required.
Then I fitted the transistor, resistor and wire as shown
And done
Here's the result on the oscilloscope, I'm using a x10 probe so the display is 20v per square, so 80v pulse.
The transistor is readily available from Maplin, the resistor too. The relay was a spare boxster one, but I'm sure any car relay would work.
This would work with any aftermarket ignition that produces a 12v square wave output that need to work with a high voltage triggered tacho.
Tim
Since I had just done something similar with a Megasquirt for KS I thought I'd try the same here. I had the parts, a transistor, resistor and a Boxster relay, and it worked. So here's the write up for anyone wanting to try the same.
Here's the parts.
Remove the spring and actuator to disable the physical part of the relay. This uses the relay coil to generate a high voltage pulse. (Back EMF).
Now remove the diode, not required.
Then I fitted the transistor, resistor and wire as shown
And done
Here's the result on the oscilloscope, I'm using a x10 probe so the display is 20v per square, so 80v pulse.
The transistor is readily available from Maplin, the resistor too. The relay was a spare boxster one, but I'm sure any car relay would work.
This would work with any aftermarket ignition that produces a 12v square wave output that need to work with a high voltage triggered tacho.
Tim