How do you troubleshoot 1984 Vanagon low-reading temp gauge that has been getting progressively worse. It used to read dead center at LED but now barely moves at all. What's up?

1. Instrument Voltage Regulator

(small three pronged, behind the temp gauge) 2. Thermostat 3. Fuel tank sensor I have actuall had this happen in two of the four Vanagons I have owned. The Instrument Voltage Regulator solved it for $20