hi everybody!!
i have an mc28 tachometer and i want to connect it to the mc21 because the mc21 has broken down. (i have mc21 electrics and harness). my problem is that the tachometers are not the sames in the connection. the mc21 works with three wires (ignition, ground, and another yellow-green) and the mc28 with two (ground and pulse). i made some connections with those three wires but i couldn t make it work. is it possible to connect it and how? do i need to take pulse from another place? thanks in advance!!

Connect a wire to the RH ignition coil and send it to the pulse wire of the mc28 tacho.
The wire will trigger the mc28 tachometer. In fact, this is how I'm using my mc28 along with a PgmIII thanks to previous owners hacks...

The LCD will not be of any use to you though... and I'm looking for one if it's for sale.
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2t-racelab.com

2Tracelab wrote:Connect a wire to the RH ignition coil and send it to the pulse wire of the mc28 tacho.
The wire will trigger the mc28 tachometer.

This sounds great! I've got a PGM IV with 010 & 030 HRC cards and 2 std cards, but the PGM won't make the tacho work. The PGM's a spare and I'd always assumed it was kinda junk! It's sat in a box for 5years!

It sounds like I might be able to get it to 'fire' the tacho using a separate wire to the RH coil. I assume you mean the coil to the lower cylinder?