Fuel tank sending unit?

[PhilPB]

I think I have a bad sending unit on one tank. I have the fuel gauge on the flybridge which is connected to a three-way toggle switch. One position/tank stopped reading.
These are my unscientific steps, which I presume were right to point to a bad sending unit:
- Disconnected wires separately on switch to see which one wasn't working
- Checked connection on wires and they seem ok
- Replaced switch and same results
- Connected seperate (ran wire from engine room to flybridge) Sender wire from sending unit using aligator clips to switch and same results. Also at this step I ran new/tester ground wire from generator battery
- Cleaned connections and same results
- Tapped on sending unit and same results

I think this points to a bad sending unit and need replacement.

Haven't removed as I don’t want open hole in tank and its a pita to access.

Dimensions as to the external height of tank is 12.5" inches. Diameter of top of unit is 2.5"- ish.

Not sure of the mfg of current unit.

Ideas what to get and where?

 

[PhilPB]

I'm surprised there are no identifying marks on it

 

[Barking Sands]

Usually you can just connect the two wires at the sending unit and the gauge should go full scale full. That tests the gauge and all wiring in between and you can safely assume its the sender. Both of your sender wire terminals appear to be isolated. So it would seem one goes direct to ground. I would check that one for a good ground with a meter too.

You will have to verify compatibility but these KUS senders have magnetic reed switches, which IMO are very reliable. They should be standard ohm range and the linked one is 12 inches. Other sizes available every .5 inch. If it were me I would by a gauge and two senders (because you are using a toggle to read 2 senders on one gauge?) and change everything over. Probably be about $150 for all.

https://amzn.to/45dti2G

There are other brands that are slightly less that appear to be the same. I bought a Rupse gauge and sender for about $38 for both X2 and recently installed them and they have been working great. There are details on sender size and hole pattern (SAE) in these listings.

https://amzn.to/3tgEnCM

 

[PhilPB]

Usually you can just connect the two wires at the sending unit and the gauge should go full scale full. That tests the gauge and all wiring in between and you can safely assume its the sender. Both of your sender wire terminals appear to be isolated. So it would seem one goes direct to ground. I would check that one for a good ground with a meter too.

You will have to verify compatibility but these KUS senders have magnetic reed switches, which IMO are very reliable. They should be standard ohm range and the linked one is 12 inches. Other sizes available every .5 inch. If it were me I would by a gauge and two senders (because you are using a toggle to read 2 senders on one gauge?) and change everything over. Probably be about $150 for all.

https://amzn.to/45dti2G

There are other brands that are slightly less that appear to be the same. I bought a Rupse gauge and sender for about $38 for both X2 and recently installed them and they have been working great. There are details on sender size and hole pattern (SAE) in these listings.

https://amzn.to/3tgEnCM

One wire does go to ground. I isolated that by disconnecting the ground wire and directly connected the sender connector to the ground on my generator battery via alligator clip jumper. I then connected a wire directly from the sender terminal to the switch as to isolate all original wiring but still had no reading.

 

[rslifkin]

I second the KUS sensors. I've got them in my fuel and water tanks and they've performed perfectly. Much more consistent readings for fuel level than the original swing arm float sensors.

 

[Barking Sands]

One wire does go to ground. I isolated that by disconnecting the ground wire and directly connected the sender connector to the ground on my generator battery via alligator clip jumper. I then connected a wire directly from the sender terminal to the switch as to isolate all original wiring but still had no reading.

Did you try just connecting the two wires at the sender together? Or sender wire direct to your grounded alligator clip? You can also use a meter to take a reading right at the sender...set on ohms and read what you get. Should be somewhere between 240-30 ohms. It its open the sender is bad.

You can also check for a good ground between that terminal (the greenish one..lol.)and you battery ground

 

[PhilPB]

Thanks. If I understand you correctly, if I cross the two terminal and the gauge goes to full, the sender is bad?

I also misstated that I connected the sender to a new ground, I just created a new ground by going directly to ground on the nearest battery from the groung terminal on the sender.

If I do get new senders and gauge my only hope is that the senders have the same diameter and bolt pattern as well as the gauge being the same diameter.

 

[Barking Sands]

Thanks. If I understand you correctly, if I cross the two terminal and the gauge goes to full, the sender is bad?

I also misstated that I connected the sender to a new ground, I just created a new ground by going directly to ground on the nearest battery from the groung terminal on the sender.

If I do get new senders and gauge my only hope is that the senders have the same diameter and bolt pattern as well as the gauge being the same diameter.

The Kus brand has good dimensional drawings. Yes..generally connecting the positive sender wire to the ground terminal is a great start point. If the gauge goes full scale then you know the gauge, ground and sender wires are all good. If it doesnt go full scale then you have more work to do. As always...some installations could be different..so take everything I say with a grain of salt and doible check with any resources you might have. Such as find the gauge manufacturer and look at any onlines resources. Boat manuals you may have, etc.

 

[PhilPB]

The Kus brand has good dimensional drawings. Yes..generally connecting the positive sender wire to the ground terminal is a great start point. If the gauge goes full scale then you know the gauge, ground and sender wires are all good. If it doesnt go full scale then you have more work to do. As always...some installations could be different..so take everything I say with a grain of salt and doible check with any resources you might have. Such as find the gauge manufacturer and look at any onlines resources. Boat manuals you may have, etc.

Thanks for putting it plainly for me!
I did cross the terminals and the gauge went full scale. Now to figure out which replacement will work. There is no documentation for the unit, or imprint on it to identify

 

[PhilPB]

So I had a brainstorm. Look at the working sender on the other tank.

 

[Barking Sands]

Thanks for putting it plainly for me!
I did cross the terminals and the gauge went full scale. Now to figure out which replacement will work. There is no documentation for the unit, or imprint on it to identify

Thats great news. Glad to hear it.

 

[PhilPB]

Now that I know what needs to be replaced, seems like it should be a seamless task. Unfortunately, I cannot locate a duplicate part to match the working one. Can't find the manufacturer (Marpac) website or phone number. There are many marine parts with that mfg, but no contact info for them. The closest unit I can find that has the seemingly same dimensions is a Seachoice unit. There is a Marpac sender that is similar to mine in looks, but lacks specifications.

 

[PhilPB]

This is funny, albeit a bit stupid. I received a new sending unit via amazon (Seachoice, as it was the only unit that actually had dimensions of the flange and screw holes). I needed to change the ground connector to a spade connector, which I did...twice). Anyways, I tried to be smart and used jumpers from the boat wiring to the new unit and checked my tank gauge on the flybridge (only station with gauge) as I changed position of the float to simulate Full, Empty, Half. All positions indicated the new sender was reading correctly.
My old sender, before it stopped working, was reading about midpoint between 1/2 & 3/4 full. Of course I used that as a valid data point. I installed the new sender and the tank is now reading 7/8 full. That must be wrong I thought, so I disconnected and repositioned the sender so the float was pointing to a different position and tightened the screws on the flange and reconnected the wires, still read 7/8. SOMETHING MUST BE WRONG! I went back and disconnected the wires, unscrewed the flange and repositioned the sender 180 degrees, then only hand tightened 3 of the screws (I'm working smarter now). The stupid gauge was still reading 7/8 , so now I removed the sender and thought that maybe I did have 7/8 fuel so I broke a paint stick (yes it was clean), broke it in half and worked my way back under the floor to "dip" the tank. Well, now I'm sure I'll prove that I probably need to do some major work like removing the tank since I always think the worse case scenario so there is no surprise. So, the stick hit diesel within a few inches and based on that new data point the new sender is actually giving the correct reading! Why I was sure that the old and broken sender just had to be right, I have no idea.
I reinstalled the new sender and wires and looked at the gauge reading 7/8 full and decided I just had more fuel in that tank than I thought.
Time to take it out and break the next thing!

 

[Barking Sands]

Thats like finding a $20 bill in your pocket. Good deal. Glad you got it all sorted out.

 

[PhilPB]

Thats like finding a $20 bill in your pocket. Good deal. Glad you got it all sorted out.

You and me both. Thanks for the help!

 

[csternot]

I think I have a bad sending unit on one tank. I have the fuel gauge on the flybridge which is connected to a three-way toggle switch. One position/tank stopped reading.
These are my unscientific steps, which I presume were right to point to a bad sending unit:
- Disconnected wires separately on switch to see which one wasn't working
- Checked connection on wires and they seem ok
- Replaced switch and same results
- Connected seperate (ran wire from engine room to flybridge) Sender wire from sending unit using aligator clips to switch and same results. Also at this step I ran new/tester ground wire from generator battery
- Cleaned connections and same results
- Tapped on sending unit and same results

I think this points to a bad sending unit and need replacement.

Haven't removed as I don’t want open hole in tank and its a pita to access.

Dimensions as to the external height of tank is 12.5" inches. Diameter of top of unit is 2.5"- ish.

Not sure of the mfg of current unit.

Ideas what to get and where?

Look into KUS marine gauges and sending units. Very common in boats, and easy do it yourself equipment. From your photos, it looks like the standard top mount for their sending units as well.
Available in multiple lengths for work in different size tanks.