stuck tach on 300D

I shut my '84 300D down this afternoon and the tachometer stuck at just under 1000RPM. Now when the car's running the tach goes up in proportion to the extra 1K, and now the car "idles" at about 1500RPM - of course
the engine is doing fine and actually is probably still right at 750... it doesn't jump around (like a bad speedo cable) and the gap between reality and what the tach says seems to narrow as the real engine speed increases.
Is my cable screwed? Is it hard to replace? or could the needle itself just have gotten jarred off the pin? Any thoughts on what to do (other than ignore the problem and compensate for it mentally) would be greatly appreciated.
Good New Year to All,
Terry Solomonson
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Hi,
I have a 300D Turbo CA model. The Tac is fed by electronics, so the bad cable is not a choice. I think that you are correct in the jumped the zero point on the pin. This might be fixed with a tap on the meter. The good news is that if it moves when the eng is running, that will indicate that the meter movement circuitry is not burnt open. I do know that for our year model, the tac is fed off the eng, sent thru an amplifier on the drivers side fender in the eng compartment. Then it goes to/thru a 10 amp fuse on the overvoltage protector located behind the glove box. It uses one of the "Newer Style" GM type bladed fuses. At least that is what I have found.
Good luck, Bill

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And only the 84 and 85 tachs will work. 83 and back 123.133 tachs are a different frequency.

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Could be a couple of mechanical reasons
Maybe the pointer has spun round a little on the shaft pin, especially as you say that the gauge appears to be turning appropriately for the revs you reckon you are doing. I think the 'stop' for the pointer is the end of the gauge rotation (controlled behind the gauge itself). Its the mechanical end of travel (or should be). Try something Engine off, give the top of the dash a pat (not too hard). See if the needle bobs below where it is now. Or (maybe better idea) run the car, rev it up and turn it off. If the gauge is electronic, it should shut off straight away. You are looking to see if it does bob below the final position it rests at now (due to gravity and inertia). If it does, there could be a problem with the watch type balance spring that provides the resistance and damping to the meter movement. There maybe two of these springs, one at each end of the meter shaft (behind the gauge face). One may have broken or become unnatched. If its the rear one, you can get at that (when you take the instrument binnacle out and dissassemble). This maybe able to be re-attatched. If its the one behind the face, good luck.
Anyway, if its just the pointer moved on the shaft, re-set it (you should be able to do this if its moved already) and re-glue it to the shaft.
I'm just thinking out loud here...
Cheers... Rob.

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Thanks to all who came up with ideas. As is often the case this problem seems to be healing itself. I gave the dash a gentle wack after revving the engine up, and the needle slipped down a bit - another wack, and a bit more... I can't get it to the endpin, but now it's only off by about 150rpm. Heck, I've got a good set of ears - if it falls the rest of the way, fine - otherwise, I'll just keep listening. Half the joy of owning these cars is the great sound!
ts
tillie solomonson wrote:

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