Welcome to the brotherhood of maintenance
and repair, it will teach you patience, or how to spend your money
, depending on the outcome.
Any circuit has to have a path out and a path back for the current
to travel over in order to drive a gauge. If a sensor has only one wire, it is relying on the tank, or engine
to be the "ground" or return path. If you have three or four wires between the gage and the sensor, you may have a powered transmitter or even a digital sensor, which is harder to troubleshoot.
For two wire systems, you always have the option of adding an additional wire from the instrument to the sensor, rather than relying on the tank to be the ground path. Ground wires or bonding wire often suffer from corrosion
. Since they are hidden or not obvious, they don't receive any TLC when systems are cleaned up. If you have a bad or broken ground, you will not have a reliable gage.
When I got my boat, it had a stud on the engine
that had a lot of ground wires all coming to that stud. It seemed as if every system's ground came back to that single point. This is not good. It is hard to troubleshoot and you have half of each system on a dirty and vibrating piece of equipment
. I am a fan of one or more ground bus bars which provides many screw terminals along a common bar. This way, each ground wire can be individually tightened down and labeled. The bar ties all the grounds together and makes testing the grounds easy.
A multimeter can diagnose your system. You may need some extension wires and aligator clips to extend the reach of the multimeter's test leads, but if you measure resistance between the guage's ground terminal and the sensor housing it should read very close to zero ohms. If it reads fairly high, you have a corroded ground system. This could be the ground wire turning to green powder, or corrosion
at the mounting terminal on the tank, guage or somewhere in-between. Corroded crimps on the wire terminations can also pose a problem. If the multimeter reads infinite resistance, the ground is non-existent. Then you need to either clean the existing ground system, or install a new wire.
on a boat is not like wiring in a house. You should consider inspection
of your wiring part of your annual maintenance
, with wires being replaced as needed. My advice when replacing wiring or installing new systems is to take the time to remove the old wiring. There is nothing more frustrating than trying to diagnose a problem with 20 odd wires in the bundle that aren't being used any longer.
Bad crimps on the wire ends are very common. They don't normally reveal themselves initially, but since the wiring is not fully compressed by the ferrule, corrosion and intermittent connections can start inside the fitting. This only gets worse over time, from continued corrosion and the small amount of arcing as the circuit makes and breaks. Testing the wire ends can be hard. If you move the wire, you may re-establish the connection momentarily and get a good reading.
If it is a critical system, and I got a bad reading, I start with a visual inspection
. If I don't like what I see, I just replace the wires.
If the crimp looks suspect, but the wire looks fine, I cut off the wire end and crimp on a new one.
If when I cut off the old wire end, the wire itself looks green, I replace the wire immediately. Green wire is stiff and brittle. It can turn to powder inside the insulation
jacket. Depending on what the circuit does, this can be dangerous.
Your multimeter can also be used to test the sensor itself. Most two wire sensors either vary the resistance or capacitance that the circuit sees. If you put your multimeter across the sensor contacts (when it is not wired into the system) you can make a determination as to whether the sensor is good or bad. Most maintenance manuals
will tell you what results to expect.