My tacho stopped working some time ago and I was advised to change the cable as these can give problems. So I changed the cable but still no reading. I removed the tacho and tried it with a spare cable direct into the distributor and hey presto it worked, so then I put it loosely back into the dash and it was still working. Once it was all screwed back together it had stopped working. I have repeated this palaver 3 times now with the same result.
It has occured to me that the inner cable might be too short as there is not a lot of the "squared" end of the inner cable protruding from t