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Author Topic:   Intermittent fuel gage
Royboy posted 05-30-2006 09:27 AM ET (US)   Profile for Royboy   Send Email to Royboy  
Had a fuel gage scare while out trolling in my 1999 Outrage 17. Seemed like we were getting great mileage becasue the fuel gage hadn't really moved from 1/2 tank, untill I looked away for an hour or so of heavy fishing. When I looked back it was reading empty. We pulled lines and bolted out of there and on the way back to the dock the needle started bouncing bewteen zero and 1/2.

My question is: what is the proceedure for checking the gage and the sending unit. I have a multimeter and know how to use it, I'm just not too sure what I'm looking for here.

I was thinking that for the sending unit, I can look for a change in resistance while I have someone crank the tongue jack up or down (so that the fuel level moves while I'm checking). I could also pull the unit and physically move the float, but I'd rather not break the seal if I can avoid it.

Roy

Chuck Tribolet posted 05-31-2006 08:47 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
How much gas was actually in the tank? If it was actually
half full, you are looking for an intermittent open in the
gauge, wiring, or sender. It it was actually empty, it's
highly likely to be the sender (gauge usually fail stuck or
with a low reading, an open in the wiring will cause a 0 reading) or (VERY small probability) the wiring between the
gauge and sender is intermittently getting connected to 12V
which would cause a 0 reading.

First step is to reproduce the problem with multimeter in
hand. Fuel guages usually consist of a circuit from +12V through the guage then through the sender to ground. They
guage measures current -- low current = low fuel, high current
= high fuel. You may need to get on the water to do this,
but I'd start on the trailer by tapping and shaking. After
that I'd see what a voltmeter shows at the sensor side of
the gauge -- I'm not sure how much voltage swing there is
there -- it certainly won't be the full 0-12V (unless there's
an open between the gauge and +12V).

Second step is to inspect all the wiring at the gauge, sender,
and in between for anything that looks flaky. See if that
fixes (or least makes it impossible to reproduce) the
problem.


Chuck

Royboy posted 05-31-2006 08:57 AM ET (US)     Profile for Royboy  Send Email to Royboy     
Thanks Chuck,

When it started working on the way back in, it was bouncing between 1/4 and 1/2. I later put 21 gallons in her and it raised the needle to 3/4, so IF the gage is working, I must have had about a 1/4 tank. It's a 56 gallon tank.

I'd really like the peace of mind to know everything is working properly, and the components are not expensive so I don't mind chaning them out, but as a former aircraft mechanic, I abhor troubleshooting by changing out parts. Also, I'm not wild about towing with a full tank, so I'm well motivated to do the right thing.

I'll see what I can do with my multimeter, and then post the results.

Roy

Tom W Clark posted 05-31-2006 09:32 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
The problem is not a malfunction of the gauge but a problem of learning what the gauge is actually telling you.

Most Whaler fuel gauges do not read accurately unless the tank is level which it never is while the boat is in the water and less so while it is under way.

The bouncing you noted is caused by the fuel sloshing around while the boat is in motion. All a fuel gauge like yours can do is tell you the height of the float bobbing around on the surface of the fuel in the tank. Never try to interpret a fuel gauge accurately while underway!

Unless a fuel gauge sender unit is exactly in the center of the tank, the fuel level will vary depending on the attitude of the tank and even then only within the middle range of it capacity.

Factor in a tapered shape to the tank bottom and the progression from full to empty is even less linear.

For example, the fuel gauge for the 140 gallon tank in my own Whaler will stay stuck on "Full" until quite a bit of fuel is burned off. Once the needle is on "1/2" the tank is actually down to about 40 gallons. When it hits "1/4" I know I only have 15 gallons left and its time to find fuel NOW.

When you become familiar with the behavior of your fuel gauge you will find that is has great repeatable accuracy, if not absolute accuracy, and that is what counts.

Royboy posted 06-01-2006 01:17 PM ET (US)     Profile for Royboy  Send Email to Royboy     
No kidding, when the boat moves, the fuel does too?

I wasn't expecting dead accuracy out of this thing. I just want to know if I have a functioning fuel gage/sender. I pointed out that the needle was bouncing to illustrate that it was moving, as in "not stuck". I'm looking for technical information here, not an insult to my intelligence.

Roy

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