I fixed mine in a 1977 shorty Chevy , had to replace both speedo cable and the transducer , servo was good . You can check the servo by unplugging the vacuum line and holding your finger over the vacuum tube while pushing the carb side of the servo into the servo . If it holds vacuum it’s good . There are no wires to my servo just a vacuum line . The only wires are to the transducer and it is a 2 wire plug in , 3 wire if you have a light in the dash . I found a guy that rebuilds transducers on eBay , they are all the same in Chevy,s so finding one is possible . That’s really where my problem was .