Thread: OT- Gas gauge on boat reading "full" with empty tank

120 gallon tank I thought was full. Gas was at least 4 years old so I had two 55 gallon drums to receive the old gas but it only filled up one of them before running dry. Curiously the gauge did move a little...from pegged all the way to just a hair below the "full" mark.

I confirmed the tank is empty with dowel rod however. Checked for corrosion at terminals on send unit and gauge and all check out ok with ohmmeter. So, which is more likely to be kaput, the send unit or the gauge ?

(Probably the send unit since it will be a major PITA to replace....idiots that built the boat got the hatch cover such that one would have to be 'Rubberman' to get to it, much less pull it out...probably have to jigsaw out the floor to receive a whole nuther larger hatch cover !)

Perhaps remove the gage and measure voltage to the rear terminals as you manually move the float thru its range...if you can bend some sort of coathanger to toggle it from the filler port. (good time for 2 or 3 helpers )

If the voltage is moving thru some sort of range (wild guessing 2-6 volts, but really no clue what that should be, though) then you can have some confidence in the float/sender.

It sounds like you have a short to ground somewhere or a stuck float. It has been a few years but I am pretty sure that your sender resistance should be 90 ohms empty and 0 ohms full. Measure ohms at sender wire on back of gauge to tank ground. Or wherever you can access the signal wire.

Forget checking resistance or voltage. A quick and easy check to isolate the problem is to remove the wire from the center terminal on the tank sensor and ground it to the tank. With the ignition switch on, when you ground the tank sensor, the gauge should jump from empty to full. If the gauge does this, then the problem is the sensor, if not, then the problem is with the gauge or it's wiring. Tank level sensors are more commonly the culprit as the motion of the vessel in the water constanly changing the the liquid level in the tank shortens their life. You also need to insure that there is no corrosion where the sensor is screwed to the top of the tank. The fuel gauge simply measures resistance across the fuel sensor,(there is no voltage involved in the fuel gauge at the tank) so a little bit of salt or corrosion around the center terminal of the sensor will cause problems. If you need to remove the sensor from the tank top, the five screws that hold the sensor to the tank top have one screw offset ever so slightly so it only goes in one way, a witness mark with a sharpie on the sensor, tank, and gasket makes installation a breeze. Also be aware that the mounting screws have nylon or rubber isolation washers that need to be inplace. If you find that you have to replace the tank sensor, you might consider using the new Reed switch type sending units as they have proven to be pretty much bullet proof. One other thing, some boat builders ground the tank sensor to the boat's 12VDC ground and use this to complete the circuit back to the gauge. The better builders run an independant ground wire from the tank sensor to the gauge, the independant ground can eliminate many problems down the road.

At the gauge, remove the wire on terminal 'S'. Switch on power, gauge should read empty.
If so - defective wiring to sensor or defective sensor.
Diesel or petrol?
It is a good idea to keep fuel tanks filled as much as possible to prevent condensation in the tank, only in your case, opps! Dam water and electrics.

Gauge testing:
At the back of the gauge, remove the wire going to terminal 'S'. Place a link between terminal 'S' and 'G' or 'GND' terminal. Switch on power, gauge should read full.


The above conventions are for Americian type instruments. If European, the results will be opposite.

Mark

Good advice, above, but make sure "ground" is "ground." At least some gas gauges are a "bridge" circuit, not a simple current meter. This means that

one terminal must have power

2nd terminal must go to sender, which MUST be grounded

case of meter must be ground, or third terminal for that purpose.

As above, short sender lead to case, and make sure both case and sender terminal are at ground.

I'd think first thing is check the wiring, both to gause ground and tank ground. Short sender to ground, and if gauge doesn't react, check all wiring, or check gauge separately, at the gauge, as above.

IF the "simple circuit is akin to automotive types, Full = open. Empty = short.

If your certain the tank is 120gal and is empty, when refilling, do it in stages, say 30 gal at a time.
Switch on power at 1st 30gal, check gauge - does it read 1/4 full, repeat in 30gal stages.
Not all boats have cube shaped tanks as a lot follow hull profile and give dodgey gauge readings.

Mark

Not much more to add here, since the main points of diagnostics have been covered. I can say however, there are two different standards of tank sender and tank gauge in general marine use. This is important to know, because the gauge on each type will flip the opposite way.

The most common "US" standard, is the 240-30ohm system, with the sender measuring 240ohm when empty, and 30 ohms when full. Grounding the sender input on the gauge should result in Full reading.

The other type is the European 0-90ohm standard used by most VDO brand gauges. Sender measures 0 ohms when empty, and 90 ohms when full. Grounding the sender input in this case makes the gauge read Empty.

Most likely stuck float, little chance of accessing to move float. Best path remove sender, free up or replace (I'd replace it) easiest path fill 3/4 and take a nice bumpy ride, not freed, replace now.

Re accessing float, yes, impossible without jigsawing out for a new larger hatch cover. If anyone here happens to know anyone drawing or supervising the contruction of 23 foot center console boats at Sea Pro in 1998, please get me their name so I can assemble a firing squad of some sort

There may be several ways to solve this problem:

The Tank Tender only requires a 1/8" NPT hole tapped into the top of the fuel tank for its pickup tube.

Alternatively, if you have a little more room, you could install one of the vertical float style senders from Wema USA, or one of the vertical "no moving parts" electronic senders from Centroid.

Note: since none of these devices have float arms sticking out, it doesn't matter where you install them, so long as they can reach the full depth of the tank.

gnorbury, believe it or not, I used to manufacture what I called the Tank Monitor for boats. Of course I didn't actually make any of it, but got my custom guages from Beede and send units from Centroid. All incorported with a custom panel and switches for monitoring up to 12 tanks.

Having said that, I gradually became disillusioned with the Centroid send units. The no moving parts capacitance system sounds good in theory but the sad reality is I had so many send unit failures months and years later that I was resigned to ending the product line. (sales were slow also, otherwise I would have been more inspired to invent an improved send unit if possible)

The pneumatic Tank Tender, IMHO, is a joke and it astounds me how popular that thing is. If you need details on why I dislike it so, say the word and I will explain further.

The Wema I've never used, but my memory of it is positive...probably a good send unit.

This is on a boat I have for sale, so to be honest all I care about in this case is the cheapest thing that will work, so I'll probably see if I can just fix the one that's already in there, and if not, replace it with one of the usual suspects.

And if you're now wondering why someone who used to make fuel gauge systems (plus water and waste monitoring) doesn't know all the ins and outs of troubleshooting a fuel sender, it's because my Centroid send units were voltage based rather than resistance based and I'm not all that familiar with the "standard" send unit systems. Plus it was many years ago, and memory fades.

Can recommend the the wema unit.
Fitted one to an ex RNLI lifeboat, tidy unit.

Mark

bdx, I saw one of your lifeboats in real life on my last trip to the England and Ireland back in 1995 and was very impressed....those are some *serious* looking boats !

Milacron

That one could be either a Severn or a Trent Class with 2 x 1200bhp cat's.
The RNLI stuff is pretty hot. Hard to believe its actually a charity.

Mark

Is there enough clear deck space around/above it to use a hole saw above the sender or is it too close to a coaming or some other obstruction? What I've done in some cases is to punch a hole large enough to put in one of those PYHI inspection ports. It's a quick, clean way to close up an unwanted hole in a deck.

Chris, it really couldn't be in a worse position. If you can imagine an eight inch round plastic inspection port, open and looking directly down at the tank, the OD of the send unit flange just barely intersects with the OD of the inspection hole.

So, it's just far enough away from the inspection hole that there is no way to angle the send unit out even if you could somehow get to all 5 screws, and just close enough to the hole that there is no way to put in a seperate second inspection port.

(there is about 3 inch clearance between the top of the send unit and the underside of the deck)

As I said, find me who designed this thing so I can line up the firing squad...and no last cigarette !