DIY MSD tach adaptor

[impmad2000]

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

[impmad2000]

Here's the circuit, sorry, I can't get the diagram the right way up !

[shambly]

Timely and interesting... (honest!)

We made GaugePilot's RPM input require just such a feed, but have already seen a customer running a MSD box. In retrospect we should have built in a 12V square-wave input as well as the high voltage one, but we have to live with what we have for now (most will run kettering ignition) so i've been researching such adapter circuits on the web including the MSD 8920 adapter...

I'm just building a similar circuit that uses the primary of a mains input transformer rather than a relay coil and i'll report back.

Simon

[MikeS911]

Hi Tim.
Any idea if this would be needed on a 76 car 2.7, or is this only for the earlier cars
Thanks Mike

[impmad2000]

Most likely only for earlier cars Mike

[smallspeed]

nice work tim! and dead easy to hide in the standard relay format

i just got over a similar headache in my car trying to convert a modern digital tach output from the Engine ECM to a 90's dash board - all up and running now, although I suspect what I did was the opposite of what you've done here..

..never been so pleased to see a tacho work, however I'm now on to worrying about the fact idle "hunts" between 785 and 805 rpm when cold (I suspect this is only an issue in my head, brought on by the fact I can now see this happening )

[MikeS911]

Hi Tim Hope you are keeping OK with this virus thing. Would this adaptor work with Megasquirt 2 and a 76 Tacho? Thanks keep safe Mike

[impmad2000]

I think a ‘76 SC would use a 6-pin CDI. If so, then the CDI generates a switched 12V pulse. This type of adaptor would generate high voltage spikes (see scope trace). Replace the coil with a resistor (try between 1k and 10k ) to generate a 12V pulse from the 5V MS output. Look up low voltage tach adaptor on the MS information. High voltage adaptors like this should only be needed for a few years ‘69 to ‘71 I think.

[Jonny Hart]

High voltage adaptors like this should only be needed for a few years ‘69 to ‘71 I think.

...plus any car that originally had an 8 pin box - notably Porsche race cars and the early Turbo. The 8 pin box has an in built high voltage tacho drive.