Recently I purchaced a tach which came from a 240? with the intention of installing it into my 122. I was told by a local parts shop that I will need another "unit" to do so. Is that true?
Could someone please let me know how to install the tach into a 122?
Thanks.

Not knowing too much about the 120 series, I would say as long as it's a 4 cylinder car, the only thing you have to do is find a place to put the thing in your dash.

With just 3 terminals, I'd say +12v, ground, and signal input from the ignition coil negative, That's how it is in the 240. (the +12v should be from a "switched" source -- ie, not hot till Key ON.
--
Bruce Young,
940-NA (current)
'80 GLE V8 (Now gone)
'83 Turbo 245
'73 142 (98K)
'71 144 (track modified--and still here)
New 144 from '67 to '78
Used '62 122 from '63 to '67