Mercedes Benz repair: 1985 Mercedes 300D Turbo Diesel, turbo diesel, diagnostic procedures

Question
The tach and the odometer (including the trip) do not function all the time, the tach was coming on about every other trip, I checked the wiring and it worked while holding one wire but has not worked since. The odometer works every trip for a few miles and then stops. I have checked all the connections with a "hot light" and they all seem to have power. Are these two connected some how? WHAT ADVISE WOULD YOU GIVE ON REPAIRING THESE PROBLEMS?
THANKS IN ADVANCE  

Answer
There are multiple possible answers to your question.
Suggest you join Diesel Discussion:
http://www.mercedesshop.com/shopforum/forumdisplay.php3?f=15
There are hundreds of threads covering both topics you are asking about, many have detailed diagnostic procedures.