Can anyone tell me how the tachometer on a 84 300D gets its signal? I know the 84+ cars had a different system than the 83 & previous cars. I unscrewed the black round tach sender on the fender well, but there is nothing under the cap. Likewise the same on my 84 300SD, but the tachometer works perfectly on it. Also, is there an in-line fuse for it. The fuse in the fuse box has been replaced.

Richard