Does the fuel gauge also read incorrectly? If so suspect the voltage stabilizer behind the instrument panel. I changed a few of them in the 1980s when I worked in a Vauxhall dealer. If not check the temperature sender wire for chafing to earth. If there is no chafe to earth on the wire then the temperature gauge itself is most likely faulty, and I have changed them too.

Removing the instrument panel on these is very easy. Remove two screws on the upper side of the bezel and remove it. Then remove the single screw at the top and middle of the instrument panel. Pull it out and disconnect the speedo cable and the multiplug. A word of warning though. Disconnect the battery before you do anything. The dash frame is metal and has sharp edges. It's easy to scrape the printed circuit against it and short circuit it. If you do it will partly melt and be ruined.

Edited by Railroad. on 11/10/2016 at 16:30