Strange tachomoter behavior

My tachometer does not work. Most of the time. But if the weather is hot and the air conditioner is running and the temperature guage goes above 100 C, the tachometer starts working.
Any thoughts?
The car is a 1981 300SD. Everything else works.
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    You've failed to mention the phase of the moon.... it might help
cheers :-)
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says...

Well, it is full moon.
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Your car needs a new "Tach Amp" - the black plug on the left fender. About $60 on line.
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says...

That's the thing you stick cogarette butts in? Sometimes that works too.
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There is a strange looking little barrel shaped object with ribs on it on the top of the driver's side fender, under the hood, not far from where the wire harness from the cruise control actuator plugs in. That is the tachometer amplifier. It has some connections inside that start to fail to make good contact, and then your tach starts to operate erratically.
There are some good instructions in the archive of this list if you do a search for "tachometer amp."
The top part unscrews and comes off. I took out the silicone gel thing inside the top and put a little bit of foam rubber in there to force it to come down a bit lower and make better contact once the thing was screwed back together and it fixed mine. I also took contact cleaner spray and sprayed all the contact prongs and connected and disconneted it a few times to get the oxidation off the contacts. My tach has worked flawlessly since I did the above steps.
Definitely try fixing your old one before spending the money on a replacement. Don't buy it from the dealer if you decide you need a new one. They charge more than double what you can find one for on the internet.
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snipped-for-privacy@inyopro.com says...

Thanks a lot. I haven't really paid that much attention to it lately. But the key does seem to be that silicon button. A cigarette butt that puts extra pressure on it also sometimes works.
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A new one for $64.95
http://cgi.ebay.com/ebaymotors/RPM-Tach-Sender-Sensor-Mercedes-Benz-300-D-SD-123-126_W0QQitemZ300113178865QQcmdZViewItem
A used one for $29.95
http://cgi.ebay.com/ebaymotors/Mercedes-tachometer-amplifier-W116-W123-W126-300SD-300D_W0QQitemZ250120692633QQihZ015QQcategoryZ46098QQcmdZViewItem

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