I have a Pisa Euro dash with their VDO gauge package in my 88 Mera that I installed about 7 years ago. A couple of years ago my oil pressure and temperature gauges started acting strange . The oil pressure started indicating lower and the temperature went higher. I discovered they were affected by changing the dash light brightness level.
I did the google thing and the most common problem with gauges is a bad ground. I had the dash out to correct an AC airflow issue so I decided to address the gauge problem also. The gauge and dash light grounds were wired together in the harness that came with the gauge kit. There were butt connectors in the ground circuit at each point where a gauge or dash light was tied in which caused an increasing ground resistance with each butt connector. This must have caused some back feed current from the dash lights to affect the gauge readings. I separated the gauge grounds from the dash light grounds. I ran a single continuous ground wire for the gauges to the chassis ground point behind the radio and HVAC controls. I used crimp sleeves to make the connections for the individual gauges. My oil pressure and temperature gauges are stable now and my gas gauge is working better than it ever has.
