In his YouTube video Kent covers the most common problem areas
Tach not working or jumping around. There could be a number of things causing this problem. In this video Kent will show you some of the key things to check and possibly replace to get the tachometer woking again on your 1978 to 1980 W116 300SD, 1981 to 1984 W123 turbo 300D 300CD 3000TD and W126 300SD.
Problem & Solution
Understand the system and where the key components are located really helps to troubleshoot problems.