fuel gage and key switch

[redfish1940]

my fuel gage and trim switch dont work. sender looks good and getting power to it. I put a new gage in does the same thing, nothing.
When I turn the key on the gage jumps a tiny bit.Shows empty.
could I have a bad key switch? everything elese works.150 2005 merc.
Anybody? thanks.

 

[Silvertip]

Re: fuel gage and key switch

If everything else works and the motor starts -- the key switch is obviously NOT the problem. The fact that you measured voltage (regardless what it was) at the sender does not mean it is good. And replacing stuff because you "think" it is the problem is a waste of money as you found out. Had you done a search on fuel gauge problem (or similar words) would have found diagrams and troubleshooting techniques. But since you didn't, here it is again. The gauge needs +12 volts and ground (key on). Check it with a meter. Mementarily short the "S" terminal on the gauge to ground. This should peg the meter to full. If not, the gauge is bad or you didn't check +12 volts and ground. Disconnect the wire from the "S" terminal and measure resistance to ground. If the tank is full or nearly full the resistance should read about 33 ohms (give or take a little). If the tank is half full you should see about 109 ohms. If it is empty or nearly empty it should read about 240 ohms. If resistance is not correct or you don't get a reading at all, repeat the test at the sender itself. If resistance is ok at the sender then the wire between the sender and the gauge is bad. If the resistance readins show open circuit or short circuit then the sender is bad. Remember, the metal shell of the sender needs to be grounded or this system will not work. Here is a diagram of the system.