After years having to live with an erratic coolant temperature and fuel gauge, I decided to do something about it. And it worked.

Here is how to fix the gauges.

1) Remove the insrument panel and dismount the back (black) panel. This will take about 15 to 20 minutes.

2) Dismount the whole PCB by making it loose from the clips at the side.

3) You will now see that the coolant and fuel gauges have three thin pins coming out. These pins slide in the connectors when the PCB is mounted back in place.

4) Looking carefully at these pins, you will see that around 1/3 of the height from the top, they have a tiny black deposit. These deposits are at the same height on all pins. The rest is all shiny like new. It is at these dots that the pins are grasped by the connectors (point contact). At these points, the pins have corroded.

5) Carefully remove these black deposits. I have used a very fine abrasive paper (sandpaper), afterwards clean them with a solvent.

6) Reasemble everything again. They will work again like new and the coolant temperature gauge needle should settle again in the middle.

This clarifies why the gauges behave normally again after that the panel is given a thump. It's all about bad contacts.

7) When I removed the panel I also found out that some of the warning light bulbs (these are soldered on the PCB) had come loose. This clarifies why, for exapmle, the oil pressure warning light wasn't working. The soldering was literally broken. I soldered them again and the lights are now working nicely again.

The whole job took about 1 to 1,5 hrs.

Good luck. Sorry, I had not time to take any photos but please feel free to ask if you have any queries.

Joseph

Thanks Joseph, I have a fuel gauge that even though it always gives a reading, sometimes it varies quite a bit between a 1/4 tank and half a tank. Do you think this would be the same issue?

Yes I'm sure. My fuel gauge had the same symptoms.

There's nothing queer with the gauges themselves, it's the contacts that go astray.

Yesterday and today I started and used the car a couple of times and by now I would have normally got plenty of different readings on both gauges.

Bot now they're both stable and giving uniform readings throughout.

Great, I'll strip the dash when the weather warms up a bit, thanks for the tips.

In the meantime we're a week further and the gauges are (still) working perfectly.