300 SD Tach Problem
I just recently purchased an '82 300 SD that had this intermittent tach problem. After isolating the problem to the TDC sender, and pricing the replacement part at $61.00 to $130.00, I decided to take the TDC sender apart.
Once you remove the actual unit from the cylindrical screw down cover (the part with the tether), you can take the actual sender apart. I figured; What The Hell? The sucker doesn't work, I can't break it.
So, here's what I found. The "top" of this unit has a half dollar sized circuit board under a layer of latex. After carefully scraping most of the latex off of the circuit board, it can be carefully pried up out of the cylindrical housing. The components are "potted" in latex, and are on the underside of this circuit board. I then removed as much of the latex as I could to expose the lower portion of the device, which is a plastic housing for the contact pins that plug into the socket on the inside fender/wheel well and are attached to the circuit board by seven wires. Removing the majority of the latex above the pins on the inside of this cylinder will expose the three plastic clips that hold the contacts in place. Carefully squeezing the clips with a needle nose pliers, I was able to extricate the unit from the housing.
What I saw was about $10-$15.00 worth of components including an IC, a transistor, an electrolytic capacitor, a couple of tantalum capacitors, about 8 or 10 resistors, and a few diodes. In the process of removing the latex, I managed to damage the transistor, and break one of the wires. So, with the circuit board carefully scraped, I set to work with a brass wire brush to remove the remainder of the latex so I could inspect the circuit board.
What I found was several loose "cold" solder joints as a result of hasty dip soldering and poor QC. After resoldering the bad joints, replacing the transistor I broke with a new one, and reattaching the broken wire, guess what? It works just fine.
My final analysis? The cigarette butt trick works because it mashes down on the underside of the circuit board and as a result, the loose components make contact with the bad solder joints, not because it makes a better contact at the connector. Essentially, Mercedes should recall all of these "defective" TDC senders and replace them at their cost. Instead, they are creating a huge profit for resellers (and themselves) by selling their costly replacements.