Silvertip
Supreme Mariner
- Joined
- Sep 22, 2003
- Messages
- 28,771
The word "naked" normally gets attention so now that I have your attention -- here is a little info on how a typical fuel tank sending unit operates. There seems to be a lot of issues with these and since I had to figure out why mine didn't work (brand new boat I might add) I thought I would share this information.
Shown below is a fuel tank sender that has been removed from the bracket and round mounting plate that you see when you look at the top of the tank. The float arm is also removed but we can work around that. The assembly you see is riveted to the bracket and is also the ground connection which is what was bad on this unit. The rivet was not set tight and hence, no ground. The float arm is attached to the plastic pivot that moves the two tabs that sweep back and forth across the resistance element.
In this picture, a multi tester (ohm meter) is connected to the "sender" terminal which sticks through the metal plate at the top of the fuel tank. As mentioned, the ground terminal is the rivet that passes through the hole in the tab at the bottom of the assembly which makes contact with the bracket which connects to the round mounting plate and on which is a spade type ground connection.
In the picture below the ohm meter is reading 29.5 ohms which indicates to the gauge that the tank is full. Most senders operate in a 33 ohm (full), 103 ohms (half full) and 240 ohms (empty) range. In this position the float arm would be full up travel and the two slider tabs would be at the bottom of their travel.
The long and short of this discussion is to merely indicate that when troubleshooting a fuel gauge problem, if the gauge moves at all, it is rarely the problem. So go to the sender, disconnect the sender terminal and measure resistance. If infinity (open circuit), repair or replace the sender. If you know roughly how much fuel is in the tank (empty, half, or full) the 33, 103 and 240 ohm readings should show that. Keep in mind resistance can vary by 10 ohms or so. Therefore, don't junk the sender as these are made really cheaply. You can also remove the sender and operate the arm manually to verify the sender doesn't have any dead spots. Hope this helps -- boat safely.
Shown below is a fuel tank sender that has been removed from the bracket and round mounting plate that you see when you look at the top of the tank. The float arm is also removed but we can work around that. The assembly you see is riveted to the bracket and is also the ground connection which is what was bad on this unit. The rivet was not set tight and hence, no ground. The float arm is attached to the plastic pivot that moves the two tabs that sweep back and forth across the resistance element.
In this picture, a multi tester (ohm meter) is connected to the "sender" terminal which sticks through the metal plate at the top of the fuel tank. As mentioned, the ground terminal is the rivet that passes through the hole in the tab at the bottom of the assembly which makes contact with the bracket which connects to the round mounting plate and on which is a spade type ground connection.
In the picture below the ohm meter is reading 29.5 ohms which indicates to the gauge that the tank is full. Most senders operate in a 33 ohm (full), 103 ohms (half full) and 240 ohms (empty) range. In this position the float arm would be full up travel and the two slider tabs would be at the bottom of their travel.
The long and short of this discussion is to merely indicate that when troubleshooting a fuel gauge problem, if the gauge moves at all, it is rarely the problem. So go to the sender, disconnect the sender terminal and measure resistance. If infinity (open circuit), repair or replace the sender. If you know roughly how much fuel is in the tank (empty, half, or full) the 33, 103 and 240 ohm readings should show that. Keep in mind resistance can vary by 10 ohms or so. Therefore, don't junk the sender as these are made really cheaply. You can also remove the sender and operate the arm manually to verify the sender doesn't have any dead spots. Hope this helps -- boat safely.