are the w123 diesel tachs the same
 

for the 300d(5 cylinder) and the 280e gasser (6cylinder)

Well on the diesel tachs you have two types a two terminal which is the early one and a three terminal which is the later on, (maybe only for 1985 models.) They are not interchangeable as they both have their individual electronic sensor or sender. On the face of the tach I would suspect there is a difference in the number of rpm's shown and where the red line is. Usually a higher rpm red line is allowed for gas engines. My suspicion is that the tachs are no way interchangable

Have a look in the vintage section for a thread I posted in. There was some discussion about 2 different tach signal types.

do you happen to know when it wa sposted?
my tach I have is a 2 post diesel tach

Look at post 6 for info and there is another link there too.

http://www.peachparts.com/shopforum/diesel-discussion/377740-how-w123-tachs-may-differ.html