On my recently purchased 83 300td the tach was not working which became increasingly frustrating while trying to finess my shift points. After reading tons of old posts I thought I narrowed it down to a bad amp but switching the working amp out of my 79 300sd it still didn't work. I also put the 300td amp in the otherwise working 300sd which proved my wagon amp didn't work and led me to believe there was something else going on. I contacted Techguy512 to purchase one of his slick electronic amps because I new the amp was dead and the LEDs offer some diagnostic aids.
With techguy512s help I realized my TDC sensor was not passing the ohm test. It should read about 80ohms and I got nothing. Visual inspection showed that the sensor had been nicked by the spud on the harmonic balancer which must have killed it. Getting a new sensor was no problem, installing without removing anything else was a different story.
I used a small 8mm box wrench and some serious finger contortions to back the retaining nut out in 1/8 in. turns. Getting the nut started on the new sensor was equally challenging but it seems a lot of cursing helped my motivation.
With the new sensor and amp installed I got the rush of success. I must have stated at my rpms all the way to work grinning about knocking another to do off the list.
I just wanted to share my tale of success. This forum has been so helpful at solving these little problems as I try to get all my systems up to speed.

Thanks to all and big props to techguy512s awesome little amp and great costumer support!

Joey