When I first picked up my 83 300SD the tach would occasionaly work in a eratic sense at best, I looked up all the "tricks" on the internet to no avail, futher investigation revealed that the tach sensor mount was bent about 45' and no way was the magnetic pickup going to work with the sensor being that far off, I think someone must have used a pry bar to adjust a belt and thats how it got bent, I straightened it out, started the car, still no tach, I removed the sensor and its pickup point was caked with crud, cleaned the sensor up, reinstalled it, started the car and the tach worked 90% of the time, did a different mod under the tach amp/diagnostic cap, I cut out a circular section of 1/8" fiberglass the same diameter as the inside of the tach amp/diagnostic cap to put even pressure on the pins and the tach has operated 100% ever since.