Have you cleaned the ground where the negative battery cable connects to the frame? Clean the bolt, the hole it screws into and the cable. When I purchased my 5210 the coolant needle jump around and this is how I fixed it.
Both fuel and coolant sensors are variable resistor (potentiometer). There output is measured in ohms.
Resistance of potentiometer on gauge sensing circuit controls the fuel level shown on gauge. As float moves up or down with changes in fuel level, electrical resistance of potentiometer changes accordingly. When tank is full, resistance is low and gauge needle moves to full position. When tank is empty, resistance is high, and gauge needle moves to empty position. Potentiometer resistance between
these two positions varies in direct proportion to changes in fuel level. This allows gauge to accurately
display quantity of fuel in tank regardless of its level.
Temperature sender is a variable resistor that responds to changes in coolant temperature. Low
coolant temperatures cause high resistance. High temperatures cause low resistance.
Fuel full low resistance/ empty high resistance
Cool high temp low resistance / low temp high resistance
In both cases you are increasing the resistance as you stated.
Here are the steps I would take disconnect either the fuel or coolant sensor wire at the sensor. Take an ohm reading with the engine at idle. Then increase the rpms and take another reading. Note the color of the wire and the number on the wire. Plug the sensor back in. Now remove the plug from the back of the cluster and repeat the ohm test. Using the ground for the cluster and grounding to the frame/engine.
Let us know what you find out.
