Tachometer Intermittent 300 TDT

For a long time now the tachometer on my 300 TDT works intermittently. When it does work, it works perfectly. Sometimes it will work for hours, sometimes it will not work for hours, sometimes it will work for
just a few minutes and then stay dead for a long time and vice versa.
I know about the ribbed small barrel shaped unit on the drivers side fender under the hood and also the sender unit that is down by the front crankshaft pulley.
Does anyone have a guess as to which one of these is the likely culprit?
There is no apparent pattern to the problem. Bumps don't affect it or anything else I have been able to notice. Once when I used a pressure washer under the car it came on when I started the car afterwards and it had been dead for a while before that, but I haven't been able to repeat that, even when I really targeted the sender for cleaning.
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heav wrote:

winning answer said to pack some material, [foam]? under the cap thing on the fender. Trying to remember..., take the cap off, and somehow the thing it screws down onto needs lifted up, ie. the foam under it. now you screw the cap back down, and the connection is made better somehow. Worked for a bunch of us!! Good luck.. -Gar
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heav wrote:

Very high probability it's the ribbed barrel thing, which is called the tach amplifier. Not sure exactly what's in it, but it's likely no more than a transistor or two and a couple of caps. Like most of the rest of the electronics of this vintage, it ain't worth crap. And when it goes, it leads to erratic tach behavior. In fact, I need one myself, as mine is starting to act up. The original lasted about 22 years, the replacement from MB maybe 4.
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