The above link to the PDF for Runtz instructions says that it is a "dash gauge voltage regulator" as Bartamos pointed out. It is not a dropping resistor. One Runtz regulator is used per gauge that requires voltage to operate. The ammeter is a current meter and does need one. Original AD truck water temperature and oil pressure gauges are mechanical. There have been postings of the Runtz failing and suggestions that they are caused by voltage surges when the starter solenoid is engaged. If this is true then, in my opinion, the Runtz needs to be redesigned to eliminate the affects of voltage surges.
A dropping resistor option can be used instead of the Runtz (connected the same way) but the voltage is not regulated so it may reduce the accuracy of gauge (more of a factor if the tank is empty but the gauge shows fuel remaining). Power resistors are typically used and they have a tolerance (resistance changes with temperature) of 5 percent. I have no idea of the best resistance value or the type resistor construction (carbon, ceramic etc) to use.
The best solution (my opinion) is to swap the gauge with one designed for 12 volts. It's not cheap and it requires (standing on your head) removal of the cluster and disassembling. The new gauge works with the same original 30 ohm fuel lever sensor.
Troubleshooting tips:
Runtz test:
With the ignition switch off, remove the Runtz from the fuel gauge post. Connect your voltmeter positive lead to the Runtz connector that you just removed from the post. To be safe I like to tape this voltmeter connection. Connect the negative meter lead to a good ground point. Turn the ignition switch on. You should read 6 volts +/- a small percentage provided the 12 volt input from the battery is normal (edit: the output voltage can be higher depending on which regular chip is used in the design).
Gauge test:
With the ignition switch off, remove the center wire from the gas tank sensor and tape it so that it doesn't touch ground. Turn the ignition switch on. The fuel gauge should read full. The center wire is not connected so this is maximum resistance as seen by the gauge. Also if the ground wire for the sensor is not making a good ground, it can have the same result as having an open wire or maximum resistance.
Fuel level sensor test:
To fully check the fuel level sensor it needs to be carefully removed from the tank. First thing, disconnect the battery. You will have gasoline fumes so cover or tape over the tank opening. The sensor gasket will need replacing afterwards so have one available. Remove the ground and center wires from the level sensor. With your voltmeter switched to ohms connect one lead of the voltmeter to the center connector and the other to the metal base of the sensor. The fuel level sensor reads a very low to zero resistance when the arm is down (tank empty). With the arm raised to maximum, the resistance is 30 ohms +/- a small percentage. As the arm is raised and lowered, the resistance should change uniformly. If it is erratic or no change, the sensor is bad.
Last edited by buoymaker; 05/14/2021 2:47 PM.
