markvictor94
03.05.2021 21:19:29
Now let's see how the gauge works.
See full version: How Fuel Gauges Work
Now let's see how the gauge works.
The sending unit is located in the fuel tank of the car. It consists of a float, usually made of foam, connected to a thin, metal rod. The end of the rod is mounted to a variable resistor. A resistor is an electrical device that resists the flow of electricity. The more resistance there is, the less current will flow. In a fuel tank, the variable resistor consists of a strip of resistive material connected on one side to the ground. A wiper connected to the gauge slides along this strip of material, conducting the current from the gauge to the resistor. If the wiper is close to the grounded side of the strip, there is less resistive material in the path of the current, so the resistance is small. If the wiper is at the other end of the strip, there is more resistive material in the current's path, so the resistance is large. [links]
Something similar can happen when the float nears the bottom of the tank. Often, the range of motion does not extend to the very bottom, so the float can reach the bottom of its travel while there is still fuel in the tank. This is why, on most cars, the needle goes below empty and eventually stops moving while there is still gas left in the tank. [links]
This mechanism is one reason for the inaccuracy of fuel gauges. You may have noticed how your gauge tends to stay on full for quite a while after filling up. When your tank is full, the float is at its maximum raised position -- its upward movement is limited either by the rod it's connected to or by the top of the tank. This means that the float is submerged, and it won't start to sink until the fuel level drops to almost the bottom of the float. The needle on the gauge won't start to move until the float starts to sink.
Another possible cause of inaccuracy is the shape of the fuel tanks. Fuel tanks on cars today are made from plastic, molded to fit into very tight spaces on the cars. Often, the tank may be shaped to fit around pieces of the car body or frame. This means that when the float reaches the halfway point on the tank, there may be more or less than half of the fuel left in the tank, depending on its shape. [links]
When the float is near the top of the tank, the wiper on the variable resistor rests close to the grounded (negative) side, which means that the resistance is small and a relatively large amount of current passes through the sending unit back to the fuel gauge. As the level in the tank drops, the float sinks, the wiper moves, the resistance increases and the amount of current sent back to the gauge decreases.
The shell of the sender must be grounded, the filler is grounded and the center threaded post on the sender goes to the "S" terminal on the gauge. If +12 volts and ground are present on the gauge, the system should work. +12 volts and ground can be obtained from the same terminals on any other gauge (that works). here
To determine the location of the fuel tank, follow the fill hose. If the fill hose is at the back of the boat (which it usually is) the tank will also be in back. The sender is installed in the top of the tank but if it has been covered over with new decking, you need to find someone with an identical boat with hopefully an access port over the sender. You can then measure where it is, transfer those measurements to your boat and cut the access hole. here
Yeah when I got it the filler was not grounded. I quickly remedied that.
It might also help to know the path the original wiring would have taken from the sending unit to the gauge. I do have 2 unknown/disconnected wires that come up along with the fill hoses near the fill hole. One of these is black and the other is an off orange color similar but not the same color as the extra wire I spoke of near the gauge. I read no ground signal from either of these wires and no resistance across them..
Hi, new here and new boat owner. I have an 83 Ebbtide Catalina 18ft10 I think.
I haven't found yet how much fuel my tank holds. I have 2 gauges that dont work properly, Trim and Fuel. Trim always shows up as soon as ignition is on but right now im mainly interested in the Fuel gauge. In case it matters engine is 302 Mercruiser Alpha 1
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I don't know the layout on your boat Ed but when I had a Nitro 170DC I had to detach the strap that held the gas tank and pull the gas tank to put in a new sending unit. That is after I removed all three batteries and racks and unhooked the gas fill pipe scraped my knuckles free of skin and exhausted my sailor vocabulary then the screws holding the straps stripped out and had to be glassed in. Wonderful experience.:angry: more
Inspected the job a little more. IF the FLSU is bad, I'd have to remove the livewell to gain access to the tank. It looks like the livewell is glassed-in. It is not just bolted / strapped in as I hoped.
If the FLSU is a solid rod, I think it is literally impossible to replace. :blink: more
If you need to check the voltage I have used pieces from a coat hanger to extend the length of my testing prongs. I cut 2 pieces from the hanger about 12 inches long and used electrical tape to attach them to the negative and positive leads. Maybe that will help you reach the contacts with more ease? here