# Fuel Gauge reads backward



## sailak (Apr 15, 2007)

I bought a new fuel sending unit since mine no longer functioned. Today while installing and testing I found that it reads backward -- when the float is "up" the gauge reads empty, when the float is "down" the gauge reads full.

I switched the wires on the sending unit, same results. I do not see anyway to swap the float arm around.

Any ideas?

The gauge is a VDO brand that I assume came with the boat in 1986. The new sending unit is a Moeller...part number 35772-10.

Thanks for your help.


----------



## I33 (Mar 5, 2007)

*sender and gauge work together*

I've seen this before. The sender and the gauge need to be matched. It sounds like your old VDO is not matched to the new sender. Swap one or the other, but get a matching set and you should be fine.


----------



## SecondWindNC (Dec 29, 2008)

Magic marker?

F --> --> --> E



(just kidding)


----------



## bljones (Oct 13, 2008)

You're sure you haven't wired it backward?


----------



## sailak (Apr 15, 2007)

Secondwind...don't think I didn't think about doing that. <G>

The gauge I didn't touch. The sending unit I tried reversing the wires but no effect on the gauge reading.


----------



## timangiel (Sep 8, 2006)

reversing the wires isn't going to do anything because it is basically a variable resistor and its resistance has nothing to do with polarity. as the arm is raised the resistance should decrease, allowing more current to flow causing the needle on the gauge to deflect farther and vise versa when it is lowered. its behaving as if it is installed upside down, but I can't imagine how that could be possible.


----------



## Pub911 (Oct 25, 2007)

Maybe you've capsized. Sorry, couldn't resist.


----------



## US27inKS (Feb 6, 2005)

Some senders are made to swap the arm to the other side, some are not. If you can't swap the arm, and you can't get a correct sender, cut the arm off and weld it on the other way. You may have to be a little inventive to get it done, but I'm sure it can be done. Just don't burn the sender.


----------



## wind_magic (Jun 6, 2006)

Did you buy a sender made for the southern hemisphere ? 

It sounds like you installed the fuel tank upside down. 


Seriously, I'd PM btrayfors, he probably has some simple op amp circuit that will reverse the voltage for you that won't require modification of the sender.


----------



## Stillraining (Jan 11, 2008)

Dang I hate my memory...I had this issue as well and I cant remember how I fixed it.. but I did..and I dont think it was taking it back but it might have been....Sorry


----------



## ampro (Feb 4, 2010)

Try reversing the wires to the fuel gage


----------



## ampro (Feb 4, 2010)

try reversing the wires to the fuel gage. That will reverse the current flow through the gage.


----------



## eherlihy (Jan 2, 2007)

The sender (in the tank) should be a variable resistor, which provides a path to ground. To test the sender, which you should have done before getting a new gauge, disconnect the wire from the sender, tape it to a known good ground, and turn on the ignition, or gauge circuit. If your boat conforms to ABYC, the sender to gauge wire should be pink. The other wire to the sender should be a ground, and should be black or yellow. You may also want to verify that this wire has a good connection to the boat's negative bus. (preform a resistance check to the engine block, it should read 0 Ohms) The gauge should read Full (in fact the needle should be pinned). If not, you have found *a *problem. If during this test the needle is pegged toward the Empty side of the gauge, I suspect that you may have connected +12V to the ground lead of the gauge, as ampro suggests above (twice).


----------



## scottyt (Jul 19, 2008)

the problem lies with the type of gauge and sender

vdo gauges work one reading a voltage, ie they put out 12 volts and when you add more resistance ( ie empty ) it is less voltage for the gauge to show. the other type is one that directly reads the resistance, a lower tank level will show a higher resistance.

vdo gauges also need as ground reference where a straight resistance gauge does not. what this means is if you have a loose wire or bad connection the gauge can read way off.


----------



## celenoglu (Dec 13, 2008)

Find the cables from the sender to the gauge. One should be the sender line and the other is connected either to positive or negative of the gauge. Change the positive to negatrive or negative to psitive. This might work.


----------



## Allanbc (Apr 19, 2007)

Think of it as a gauge that registers the amount of headspace in the fuel tank, not the amount of fuel!


----------



## northptsailor (Jul 7, 2005)

Get a vdo fuel sender and the problem goes away. The vdo sender for fuel and the vdo gauge work the same way i.e. the increase or decrease in resistance is matched to the needle in the gauge. I don't recall which way it is, but I do know that it is the opposite of the way their water gauge works. I know this because my boat came from Tartan with a fuel sender in the water tank. When I installed the proper non-cadmium plated water sender the problem was solved. The water sender is stainless and plastic and costs a lot more than the fuel unit so maybe they do them as opposites to pevent fuel senders going into water tanks. Although it did not work out that way with my boat.


----------



## eherlihy (Jan 2, 2007)

HERE is a link to the Moeller Marine Fuel Sender website - no such model as the 35772-10 (I suspect that you meant 0357_2_2-10).

Again, I suspect that you may have connected +12V to the ground lead of the gauge, and the ground to the +12V lead of the gauge.


----------



## eherlihy (Jan 2, 2007)

...Also, if you want to test the sender, you should see ~240 Ohms at E, and ~33 Ohms at F...


----------



## sailak (Apr 15, 2007)

Thanks for the info.

I replaced the sender, not the gauge.

Next time I'm at the boat I'll look at some of the ideas here.

Thanks again.


----------

