Is your servo gauge inaccurate? Is it no longer working at all? This is a common problem on older boats, but is easy to fix. The first step is to determine whether the problem is with the servo gauge or the sending unit. The test for this is straightforward. First, check that the servo gauge is receiving 12 volts of power. Turn on the engine’s ignition and probe with a multimeter between the ground and the positive terminal on the back of the servo gauge; it should be marked with a “+” or an “I.” If there is no voltage then the fault is in the ignition circuit—and the servo gauge is probably good. If there are 12 volts at the servo gauge, either the sender, the servo gauge or its wiring is the culprit, so you need to proceed to the next step.
With power running to the servo gauge, disconnect the sending wire; it will be marked with an “S” at the back of the servo gauge. Once the wire is disconnected, the servo gauge should jump to its highest possible reading. If this is the case then the servo gauge is good and you can proceed to the next step. If the servo gauge does not reach its maximum reading, it is faulty and must be replaced.
Another test is to jump a wire or a screwdriver across the sending pin to the ground pin on the back of the servo gauge. If there is no ground pin, use a longer wire and jump the sending pin to the engine block. When you do this, the servo gauge should go to its lowest reading. If it does, it is working properly.
If the servo gauge is good, the next step is to check the other system components, as either the wire running to the sender or the sender itself must be faulty. To check the wire, disconnect it from both the sender and the “S” pin on back of the servo gauge. Set your multimeter to the Ohms scale and check the resistance within the wire. If there is no resistance (as close to zero Ohms as possible), the circuit is good and the sender is faulty. In most cases, the sender and the servo gauge need to be matched to the resistance in the sender’s rheostat, so to be completely sure you are getting accurate readings, replace both the sender and the servo gauge. Several companies provide pre-packaged “ready-to-go” installation kits.
How Tank Sensors Work
Most sensors have a mechanical floating arm and a rheostat. When the arm is all the way down, in the “empty” position, the resistance in the circuit to the servo gauge is near zero. As the arm rises, resistance in the circuit also rises to around 200 Ohms. This resistance is what moves the needle on the servo gauge.
Often a problem occurs when the sending unit’s floating arm becomes inoperative. On older units the floats may be made of cork. Over time these floats can lose buoyancy or even sink altogether, causing the servo gauge to indicate that the tank is constantly empty.
Another common problem results when the rheostat doesn’t transmit the correct electrical current to the servo gauge, even though the floating arm is moving up and down properly. In this case, both the sender and the servo gauge need to be replaced.
This article comes from sail edit released