Yes, I noticed this morning when I started reading the responses, I forgot to state which one. It is the 79 SD W116.

The speedo works fine.

I am worried about the possible grounding problem if its not the amp. I tried a tach amp I thought was working and it didn't fix the problem. That's when I broke down and ordered a new one.

I tried the search and couldn't find the help I needed. I did find one thread that talked about cables to the transmission. That is why I was questioning the cable.

Thanks for the responses.

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1984 300SD 148K Gold
1979 300SD 142K Cream
2001 Audi TT Roadster 60K
2005 Chevrolet 3500 13K