I broke the shaft for the needle while trying to fix the clock which is on the same faceplate. So I got a tach from a w126 and put it in. I saw some of the part numbers and electrical components had different values from the original W123. Additionally I wasn't sure if the needle was put back on in the correct position. The question also arises because when cold it doesn't idle to well of course this can be attributed to many other things, but I have to calibtrate or verify the tach first so I can then set the idle rpm confidently. I thought the might be a plug in type of meter to test from the tach sending unit in the engine compartment.