Fuel gauge/sending unit troubleshooting

S

snal

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C Dory Year
1994
C Dory Model
22 Angler
Vessel Name
Thelma Lou
I'm trying to figure out why my fuel gauge isn't working (hasn't worked since we purchased the boat). Seems to be wired correctly, and when I turn the starboard ignition on...the gauge goes from empty...to a false reading of 1/4 tank (no matter how much fuel is aboard)....if I turn on the port ignition...the gauge slams to far below empty. There is a switch, which rests in a neutral position, and is a momentary switch which is supposed to allow port "OR" starboard tank readings. All of the above happens when the tank switch is at neutral, and momentary port/starboard checks make no difference in the gauge readings.
The switch seems to check out fine....so is there a way to check the gauge and sending units?
 
It sounds like a faulty fuel sending unit as you suspected. You would want to pull the unit from the tank, turn the key to give it power, and raise/lower the float on the unit itself. Have someone in the cabin check to see if the gauge rises and lowers. I suspect you will find it does not.

Now...the fuel slamming to below empty is suspect....when I had this problem on a former boat it was due to a short in the system (nothing to do with the fuel sending unit). I had a ground wire that came disconnected.

With that caveat, and know how unreliable FSU's are, I would start with start.
 
Snal,

You can check whether its a gauge problem or a sender problem without removing the sender from the tank.

Buy a new sender unit and hook that up to the wiring at the tank. Now, move the float up and down. If the gauge reacts properly, replace the sender. If the gauge readings are not correct with the new sender, suspect the gauge.

You can also buy a new gauge to perform a similar test at the dash end of the wiring.

If neither of these tests are conclusive, I would suspect the wiring.
 
Remember the current going through the sender rheostat and back to the gauge is Negative not Positive! Positive goes only to the gauge. No sparky near fuel tank !
Marc
 
You will get a spark if you arc an negitative wire, just as much as if you arc or monentary touch a positive wire if the circuit is engergized. Since the only potential open part of this fuel guage circuit is close to ground--the chance of a spark is minimal--BUT, if there is a short in the guage, there could be a spark at the sender or tank. So disconnect the power from the entire circuit before trouble shooting the sending uint!

Many of the fuel sender problems are poor ground--both at the guage, the tank, the wire or to the negative bus bar--so check all of these areas first.

The guage is grounded--and there is a potometer or rheostat--which measures curent resistance. Generally the resistance thru this pot should be 250 to 33 ohms. This resistance can be read with your digital volt meter.

Fiara has a diagram on trouble shooting:
http://faria-instruments.com/site_manua ... ApxVII.pdf

The installation of Fiara fuel sensors is at:

http://faria-instruments.com/site_manuals/IS0075G.pdf

There is an excellent article on trouble shooting sending units at:
http://www.diy-boat.com/index.php?optio ... &Itemid=49
 
thataway":2yvbzsdc said:
You will get a spark if you arc an negitative wire, just as much as if you arc or monentary touch a positive wire if the circuit is engergized. Since the only potential open part of this fuel guage circuit is close to ground--the chance of a spark is minimal--BUT, if there is a short in the guage, there could be a spark at the sender or tank. So disconnect the power from the entire circuit before trouble shooting the sending uint!

Many of the fuel sender problems are poor ground--both at the guage, the tank, the wire or to the negative bus bar--so check all of these areas first.

The guage is grounded--and there is a potometer or rheostat--which measures curent resistance. Generally the resistance thru this pot should be 250 to 33 ohms. This resistance can be read with your digital volt meter.

Fiara has a diagram on trouble shooting:
http://faria-instruments.com/site_manua ... ApxVII.pdf

The installation of Fiara fuel sensors is at:

http://faria-instruments.com/site_manuals/IS0075G.pdf

There is an excellent article on trouble shooting sending units at:
http://www.diy-boat.com/index.php?optio ... &Itemid=49
Click to expand...

Ain't that Variable Resistor a potentiometer? :mrgreen: :beer
 
Thanks for all of the advice!
 
Pulled both sending units...both suffered from a broken wire inside. One was completely loose...the other held by a single strand. New units are on the way.
Tanks were quite clean for being 13 years old.
 
Glad to hear you got to the bottom of it. Fuel gauges are nice but FSU's are so unreliable. I'm tempted to explore a way to remove the FSU's and have a cap where I can access and put in a stick.

Connect the new FSU's to the wires and test them (as I described previously) before you install and seal them into the tank.
 
matt_unique":scxx5sb5 said:
Glad to hear you got to the bottom of it. Fuel gauges are nice but FSU's are so unreliable. I'm tempted to explore a way to remove the FSU's and have a cap where I can access and put in a stick.

Connect the new FSU's to the wires and test them (as I described previously) before you install and seal them into the tank.
Click to expand...

Thanks for your help Matt...I'm going back with the original style units ($20ea.)...the sliding models were $58ea.

BPS has new 19gal tanks (w/sending units) for $119...almost went that way so I could "see" how much fuel is left....my tanks are so stained/yellowed that it's difficult to see the fuel level.
 
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