Fuel level sending unit test

[skim61]

Does anyone know how to test the fuel level sending unit on a fixed fuel tank? I'm not sure if the boat make/model is necessary, but mine is a Chris Craft Amerosport 284.

 

[ufm82]

Re: Fuel level sending unit test

There should be an access hatch over the sending unit. Pop the hatch out and check the wiring to the unit. If you pull the unit out you can physically move the float arm and see if the gauge responds. A DVOM will give you the readings if you hook it up in series and set the range to ohms. Check to see that you have power at the gauge first of course.

 

[Fireman431]

Re: Fuel level sending unit test

A quick test, after determining you have 12v + & -, is to ground the (usually) pink wire from the sending unit to the ground screw. Make sure the key is in the 'on' position. If the gauge pegs past full, then your gauge is good and the problem will probably lie with the sending unit.

 

[skim61]

Re: Fuel level sending unit test

Thanks for the info, guys. Question: can I "pop" the sending unit out with gas in the tank? Both gauges are reading, one shows half tank, the other quarter tank. But, I have yet to see them drop while the motors are running. Total tank(s) capacity is 125 gals. This is why I am questioning the readings.

ufm32: can you explain connecting the DOVM in series?

Foreman: how do you determine 12v + or - ?

 

[RandyJ]

Re: Fuel level sending unit test

All the ones I've worked on have 5 bolts holding them to the top of the tank. As long as it's not packed full of gas it's fairly simple to remove the sending unit.... assuming you can get to it without major surgery

 

[Texasmark]

Re: Fuel level sending unit test

The fuel gauge and a variable resistor (rheostat) in the fuel tank make up a series circuit operated by 12v usually from the ignition switch ( to keep it from draining the battery when not in use). Resistance in the tank varies from somewhere around 35 ohms to 135 ohms and system OEM's caution against short testing as the meter can be damaged by the slug of unresisted current, they say even for a short "short" test.

I don't remember which is which, i.e. 35 ohms full or vice versa. Doesn't matter as you can figure it out while testing.

The only series testing would be with power off using your ohmmeter and moving the float from one limit to the other while observing the resistance reading.

Problem with fuel gauges is the inability to accurately position the float as a function of how much fuel is in the tank and how much reserve fuel you want in your tank when the gauge reads empty.

The center screw is the 12v lead and the disc is the - or ground. If you get them reversed, the current reverses hence the needle reverses and could be damaged.

If you are getting a different reading with the key on vs the engine running you are getting some interference or your alternator is putting out it's recharge voltage which is roughly 14.5v rather than 12.75 so the current will be more and for a given position (fuel level) , you will read differently. The position of the needle is strictly how much current runs through it.

HTH,

Mark

 

[Silvertip]

Re: Fuel level sending unit test

The vast majority of U.S. fuel tank senders have a resistance range of 30 - 240 ohms. 30 ohms = full, 240 ohms = empty and approximately 109 ohms at 1/2. You do not need to remove the sender or even access it to measure the resistance. Disconnect the pinp wire from the "S" (send) terminal on the back of the gauge. Using your multitester (ohm meter) meausre the resistance from the pink wire to ground. If you know roughly how much fuel is in the tank, you can determine what the approximate resistance reading should be. If you don't know and want a more precise check, then you need to pull the sender and move the arm through its travel while watching the meter range from 30 - 240 ohms. 10% error in the readings can be expected and would be considered normal.

Another check is to make sure the floats actually float. If they are fuel soaked and sink they cannot make the gauge register correctly. If the float arms are misadjusted readings can be wrong. You cannot make the gauge read spot on when full, empty and anywhere in between. So calibrate the sender so the gauge reads empty with an empty tank. You know when the tank is full because -- well -- you filled it.

Ensure the senders are the correct length for the depth of the tank. Too short they read empty before the tank is empty.

 

[Fireman431]

Re: Fuel level sending unit test

Fireman: how do you determine 12v + or - ?

A test light or multi meter is the easiest way. With a test light, hook the clamp to the battery 12v- and touch the sharp tip to the center "S" terminal (key on). The handle should light. Hook the clamp to the battery 12v+ and touch the tip to the outside grounding screw. Same light. The multimeter will do this easier as you don't need access to the battery, you can ge tthe readings from the terminals at the sending unit directly and verify that it's getting proper voltage.

 

[skim61]

Re: Fuel level sending unit test

Just an update--on the shakedown cruise last weekend, the fuel gauges definitely dropped (a little fast if you ask me!) as we were cruising, went back up after refueling, and then dropped again about the same amount on the way back to home dock. I'm guessing they're working OK. Thanks for all the help.