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Discussion Starter #1
I finally got around to installing the new tach cable I’ve had on the shelf for a while. It isn’t working properly, as the video below shows. What kind of problem would cause the tach to behave this way? I’m not really up to speed on how it works.

 

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did you remove and slightly grease the inner cable before fitting it, or fit it straight out the bag? (if it was say hanging about on a shop shelf for years, old grease might have hardened)
I'd check the cable on the water pump drive is correctly fitted
and also where it screws onto the back of the instrument itself

was it ever working in your possession?
 

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74 Spider 78 Spider 84 Spider
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do you use grease or a light machine oil on these cables?
 

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Discussion Starter #5
Thanks for these tips.

While the cable sat on the shelf it was still in its bag, so the ends of the cable were still greasy when I installed it, but I will try removing them and using more lubrication.

The tachometer was working in the past, but I can’t remember now why I removed the original tach cable in the first place. Maybe there was nothing wrong with it and it is the dial itself that is malfunctioning. I doubt that though.

Can you help me understand why a lack of lubrication causes this behavior?
 

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But Mad North-Northwest
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The cable binds in the housing and instead of spinning smoothly will jerk. Grease isn't a great choice because over time it gums up and collects dirt. Graphite lube doesn't have this problem.

Remove the cable from the tach and you should just be able to pull the cable out of the housing from the driver's seat. Give it a good wipe down with solvent to get any old grease off, then as you insert it coat it liberally with the graphited speedometer lube. This fixed the needle vibration issues I was having on my GTV.

The other thing that can happen is that if you don't get the cable & cable housing seated properly on the back of the tach, the housing can press on the spinning bit in the tachometer and cause similar problems (I realize I'm not explaining this well.) So when you reinstall the cable, make sure the metal end of the cable housing is fully and properly inserted into the tach body before you tighten down the nut.

If these don't fix it then it's likely something internal to the tach and requires professional repair.
 

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Discussion Starter #7
I removed the centre cable and lubricated that as you guys suggested. Test drive shortly.

I get why binding or friction would cause the tachometer needle to move jerkily, but why would it get driven beyond the redline?
 
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