Replacing Fuel Tank Sending Unit - Difficult?

I have a 1997 Chaparral deckboat (since new) with a 66 gallon gas tank. The sending unit seems to be bad, the gauge is stuck at 3/4 full.

Is there anything difficult or tricky about replacing these sending units? Can I use a generic or universal sending unit or do I need a specific kind? Can anyone comment on which brands of sending units might be of better quality and last longer? I notice that one sending unit maker offers a reed switch rather than a float, is this a better unit than a float unit?

Thank you!

Re: Replacing Fuel Tank Sending Unit - Difficult?

The reed type sending units are more expensive but a lot of users prefer them. They will probably be a little better in telling you the level of fuel than a float style as the float will be resting on the bottom showing E when there is still some fuel and show full for a while before the level drops enough for the float to start floating. Floats can also drop off the arm if the clip comes loose. That being said, I opted to go cheap recently when I replaced mine with a universal Moeller float style sending unit that works for tanks 4"to 12" (they have other sizes).

Disconnect the battery, put the cigarettes away, and working in a well ventillated area (outside) clean the top of the tank near the sending unit and remove the 5 screws holding the sending unit to the top of the tank. Don't drop anything in the hole - I suggest stuffing a rag in the hole. Pay attention to the orientation of the sending unit and the direction the float arm is facing. Make sure your new sending unit comes with a gasket before you start the project.

Measure down from the top of the sending unit to the bottom of the tank. The Moeller one had a table on the back of the package for how long to cut the arm depending on the depth of your tank. Do not cut it shorter than this length or as it approaches empty, it could get jammed vertically on the bottom of the tank (ask me how I know...). You should aslo measure down to the gas level as it will be a rough sanitary check for the amount of fuel you have.

The 5 holes are not equally spaced, but off by a couple degrees; if it is not lined up correctly, you will not be able to get all 5 screws in. Align the gasket holes with the screw holes on the tank and mark both with a sharpie. Repeat with the gasket and the new sending unit. My sending unit came with two sets of screws - #10-32 and #10-24, make sure you separate them and figure out which ones work for your sending unit. They also included gasketed washers for under the screw heads. The float arm can also be adjusted in 90 degree increments so it does not rub up against teh side of the tank - knowing the correct hole pattern and the original sending unit float orientation will make this step easy. The rest is as simple as putting everything back together. If you can't get all 5 screws in, you probably have the orientation wrong so take it apart and re-try.

Good Luck!

Re: Replacing Fuel Tank Sending Unit - Difficult?

louiefl said:

The 5 holes are not equally spaced, but off by a couple degrees; if it is not lined up correctly, you will not be able to get all 5 screws in.

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I never have understood why they're like that. When I was rebuilding my Checkmate I replaced the sender when I had the fuel tank out and drove myself crazy trying to get the flange to line up with the gasket to line up with the mounting holes. There is one and only one way that everything is going to line up.

I sprang for the sender with the reed switches and like it pretty well, it doesn't bounce around while under way nearly as much as standard senders. One drawback is that there are only certain lengths available, so unless you get lucky and your tank exactly matches one of the stock lengths there will still be some fuel left when the gauge shows "E". In my case I've got 5 or 6 gallons left when my gauge first hits "E" (of course that's not all bad). Another thing is that there are only enough reed switches in the mechanism to give a resolution of 1/8, in other words as you're filling it the gauge sits on 1/8 then jumps up to 1/4. Keep filling and it jumps from 1/4 to 3/8 then from 3/8 to 1/2, there's no in between.

Re: Replacing Fuel Tank Sending Unit - Difficult?

360 degrees / 5 holes is 72 degrees apart if they were equally spaced. If the first hole is located at 12:00, #2 and #3 are 72 degrees clockwise / counterclockwise of the first one. Holes #4 and #5 are 68 degrees from those two. That leaves 80 degrees between holes #4 and #5. Not sure why it is like that. My gasket also had a smaller hole randomly tossed in for confusion (I'm sure it is some sort of orientation hole from a previous application).

When I bought my boat, I immediately smelled gas when I lifted one of the deck plates and the PO said "its a boat". I saw the sending unit was covered in silicone, and saw some gas staining at the unit, so I figured that the gasket was bad. Turns out he could only get 4 of the 5 screws in (because it was incorrectly positioned) and then hit it with sealant.

Post 4 for pics, I've since replaced the old unit in the pics with the new Moeller I bought:
http://www.classicaquasport.com/phpbb3/viewtopic.php?f=1&t=9236

http://www.wemausa.com/tank-sensors/details_SSS_SSL_tank_sensors.htm

This would have been my second choice - they sell them in 1/2" increments.

Re: Replacing Fuel Tank Sending Unit - Difficult?

louiefl said:

...and work in a well ventilated area (outside)

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Well ventilated means the air is moving!
If the wind isn't blowing a good breeze, a floor fan will help to blow the fumes overboard.

Also be sure the tank is nowhere near full when you open it up or you will have a major spill.

Re: Replacing Fuel Tank Sending Unit - Difficult?

I like the idea of the WEMA reed unit. My only concern is measuring the tank depth so I get the correct length sending unit.

Will I be able to remove the old sending unit with the gasket intact so I can reseal the opening with the old sender while I wait for the new sender to arrive? The boat stays on a lift in a boathouse. If not what are my alternatives and will the screw hole threads stand up to the extra in and out cycle?

Thank you!

Re: Replacing Fuel Tank Sending Unit - Difficult?

The gasket was some sort of rubber and remained in one piece and not stuck to either the tank or the sending unit, but no idea how old it was. If you have a West Marine nearby, they stock them for less than $6. Judging on the material, it doesn't look like a natural cork product that will fall apart, but possibly worst case it would have a compression set and not seal 100% - just don't start out with a full tank as mentioned above. My tank was plastic with metal inserts and after screwing around with mine, I've probably had the sending unit on and off 8 times with no ill effects. I'd douse them with PB Blaster or Kroil before attempting to remove them anyway.

Re: Replacing Fuel Tank Sending Unit - Difficult?

metriccrescentwrench said:

Another thing is that there are only enough reed switches in the mechanism to give a resolution of 1/8, in other words as you're filling it the gauge sits on 1/8 then jumps up to 1/4. Keep filling and it jumps from 1/4 to 3/8 then from 3/8 to 1/2, there's no in between.

Click to expand...

Reed type senders use a single switch. Reed switches are finitely variable by design so I don't understand how this is possible. You have something else going on.

Dangit said:

I like the idea of the WEMA reed unit. My only concern is measuring the tank depth so I get the correct length sending unit. Will I be able to remove the old sending unit with the gasket intact so I can reseal the opening with the old sender while I wait for the new sender to arrive? .

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No problem there. Mine sat for 10 days until I got around to following up on the repair. Just removed the old sender and use a tape measure or yard stick to measure from the bottom of the tank. Order the proper length according to the manufactures recomendation. Generally, plastic tanks require a sender 1" shorter than your measured height and metals tanks 3/4" shorter than your measured height

Re: Replacing Fuel Tank Sending Unit - Difficult?

Dangit said:

I like the idea of the WEMA reed unit. My only concern is measuring the tank depth so I get the correct length sending unit.

Will I be able to remove the old sending unit with the gasket intact so I can reseal the opening with the old sender while I wait for the new sender to arrive? The boat stays on a lift in a boathouse. If not what are my alternatives and will the screw hole threads stand up to the extra in and out cycle?

Thank you!

Click to expand...

Next to or near the sending unit on the tank should be a manufacturer's data plate. It will have the tank capacity, model and maybe some other stuff. Contact the tank manufacturer and get the sending unit specific for your tank.

Re: Replacing Fuel Tank Sending Unit - Difficult?

I used a cheap Sunpro float-style unit from Auto Zone in mine and so far it works fine. Of course, my boat is an older 'glass runabout with a 21 gal tank. For a nicer boat, I'd probably spring for a nicer, more accurate sender. As for the mounting holes being staggered, automotive manufacturers started doing that a long time ago to control which way the float arm points inside the tank. While boat tanks are usually square or rectangular, automotive tanks can be virtually any shape. If the float arm was pointing the wrong way, it would hit the tank wall before it traveled through its entire range of motion. Since this has become more or less standardized over the years, it's no suprise it carried over to the marine industry when they began using automotive-style fuel level senders.

Re: Replacing Fuel Tank Sending Unit - Difficult?

iboats sells a solid state adjustable unit. I don't know much about it, except it's expensive.

http://www.iboats.com/Teleflex-Soli...4596201--session_id.894831596--view_id.886253

Re: Replacing Fuel Tank Sending Unit - Difficult?

jdlough said:

iboats sells a solid state adjustable unit. I don't know much about it, except it's expensive.

http://www.iboats.com/Teleflex-Soli...4596201--session_id.894831596--view_id.886253

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Did some research on this. I think if you're going to keep your boat long term, this is the way to go. I like the fact that it will dampen the sloshing of your fuel while under way, so you have a better indicator of how much fuel you have. Plus it's adjustable. I need one and I think this is it!

Re: Replacing Fuel Tank Sending Unit - Difficult?

Dangit said:

I have a 1997 Chaparral deckboat (since new) with a 66 gallon gas tank. The sending unit seems to be bad, the gauge is stuck at 3/4 full.

Is there anything difficult or tricky about replacing these sending units? Can I use a generic or universal sending unit or do I need a specific kind? Can anyone comment on which brands of sending units might be of better quality and last longer? I notice that one sending unit maker offers a reed switch rather than a float, is this a better unit than a float unit?

Thank you!

Click to expand...

There is nothing tricky about them. It's a rheostat connected to a float. 33 ohms at bottom of travel and 240 ohms at top. If you have access to it, it's an easy do.

If you're fitting a universal one, match the float arm length at bottom of travel to the original. Face the same way as the one you remove (which should be toward the center of the tank) and you should find the mounting holes lining up properly. They will line up only in one orientation.

Brand shouldn't matter much. These are commodities. Most probably come from two or three factories.

Just did mine two days ago and I'm pleased with its operation.