Coolant Temperature Gauge
WARNING: This page does not describe the selected car, but rather 6 other vehicles, including the 1991 GMC Pickup, 1991 GMC Cab & Chassis, 1991 GMC C3500 HD, 1991 Chevrolet Pickup, and 1991 Chevrolet Cab & Chassis. However, it is still accessible from the selected car via links, so may be relevant.
Coolant temperature gauge measures electrical current resistance from coolant temperature sending unit, mounted in cylinder head. Sending unit has a resistance of 55 ohms at 260°F (127°C) and 1400 ohms at 100°F (38°C). As coolant temperature increases, current flow through sending unit to gauge increases due to less resistance, causing gauge pointer to move toward HOT.