by Irish GTA » Wed Jun 26, 2019 9:30 am
Brian,
The tacho on mine didn't work when I bought my car. The signal is small and easily "lost" due to corroded terminal connections. I pulled all the white connector blocks apart & cleaned the terminals with contact cleaner, tightened them with a tweak with needle nose pliers and reassembled with silicone contact grease. The main culprits are big rectangular connector block top left of the instruments and one smaller one behind the instrument cluster.
Also check that the tacho needle is free to gently spin. Decades of dust accumulate in the tacho.
Since doing those things, the tacho on my car has worked consistently.
The fact that everything else on yours is working suggests that most connectors and earths are OK.
The fun starts with removing the steering wheel, then the fascia of the instrument binnacle which is brittle; so treat it gently. Arm yourself with M7 screws (Engine rocker cover, if you can't find them to buy) to remove the steering wheel, and electric wiring diagram, which will probably improve your French. Wiring diagrams change according to year. I have incomplete sets of diagrams from most years, in French, downloaded from internet. Whilst the whole lot is open, clean and apply silicone contact grease to all other connections accessible, plus consider changing any instrument bulbs (25-30p from motor factors).
Nigel