New leisure battery causing coolant cluster issues

[cristoffa]

Hi guys, wondering if anyone can help pinpoint the reason my 1983 1.9 auto petrol T3 is doing something slightly erratic for me and hopefully suggest a fix....
So, over the years I’ve had a couple of issues with the fuel and temperature gauges suddenly reading either very high or very low, and then breaking completely. I’ve replaced the voltage stabiliser a couple of times and this has fixed it.
However, a couple of weeks ago I decided to upgrade from an under passenger seat aux battery (100ah) to a “proper” leisure battery, which is an AGM 130ah Leoch. I extended the cabling using significantly wider gauge battery cable than the existing stuff, and housed the battery in a cupboard towards the rear on the drivers side, earthing the battery to the van having sanded down to bare metal.
When I started the van, the coolant temp needle went crazy as if it was boiling and the light started flashing, although the fuel gauge was correct. It did this a couple more times and then behaved normally. So, just to be safe I checked the coolant level (fine), disconnected, cleaned and reconnected the cable to the coolant sensor and disconnected and re connected the leisure battery. Having decided it was a gremlin and nothing serious I’ve then driven to Scotland where I still am, and it’s now taken to reading fairly normal on fuel and temp but with the coolant light flashing continuously at all times. I’ve read on the wiki page about the battery short circuiting and causing the light to flash, but the battery is enclosed and definitely not touching anything to cause it to short.
As I said, it’s all running fine and the battery is working well, it’s just really annoying! It doesn’t seem to make any difference if I’ve just been on ehu or not.
Any ideas as to the cause and how to fix it?
Thanks!
Chris

[Mocki]

Afternoon
I think what you have going on here is a coincidence......
Check and clean the main battery terminals and earth point to chassis , then check the engine bay earth points
( under the coil , top of LHS head , gearbox nose mount )