Fuel Gauge sending unit issues.

[Trooper583]

Hey guys, here is this week's issue...I went last week to the gas station and filled the boat with fuel. I just ran it until the pump kicked off. I got in the water and my fuel gauge read 3/4 of a tank. I figured that maybe the nozzle had just stopped prematurely. I went back to the gas station this past weekend and this time I made sure that the tank was full. I made sure I could see the fuel in the filler neck before I left. Got to the lake, once again, reading 3/4 of a tank. at the end of the day I the gauge was showing 1/2 tank so I know that it was moving.

This morning I took the two wires loose from the sending unit on top of the tank. I was able to verify that they gauge itself is in fact not faulty by touching the center wire on the sending unit (red) to a ground. When I did this the needle pegged at full.

I took the sending unit out of the tank to inspect it. If found that it had a considerable amount of oxidation, corrosion, etc on the main mounting shank. I took the sending unit apart.( I did not change anything to do with the coil, arm, etc.) I basically only took the hardware apart. After giving everything a good cleaning, I reassembled the unit and tested it with a volt meter. According to my meter, the unit is fine. while doing the test, I could place my leads on the appropriate locations and as I moved the boom with the float up and down slowly, my meter would respond with nice smooth movement,mirroring the arm. I placed the complete assembly back into the tank and now I have nothing on the gauge. In fact, it looks as if it's not even hooked up. When the key is turned on the needle buries on the E side.

Is there something else I can check or is it time to put a new sending unit in. As always, thank a ton for your replies.

 

[Texasmark]

Re: Fuel Gauge sending unit issues.

I don't remember which way is max resistance ( E or F), but the circuit is a series meter and variable resistor and the more resistance the less the meter deflects. You may have answered the question for me when you said that you touched the center wire to gnd and got a FULL reading.

I don't know how long you did that but you could have burned out your gauge. I think there has to be a minimum of 30 ohms in series with the meter to get the F reading and not damage the meter.

You can turn your power on (ign key to on probably) and with the center wire at the tank disconnected, measure the loose end of it (that you just disconnected) to ground looking for 12v....if so, that says that you are getting power through half your circuit and if you had the remainder of the circuit (the tank resistive circuit) connected and operating properly you would get your reading.

So, if that's all fine, with the center wire still removed, ohm out your tank center terminal to the ground wire that connects to the metallic outer part of your sending unit, not the disc, the wire it'self which gets beyond the wire to metallic disc connection. You should measure some resistance, determined by where the float is. I think the range is something like 30 to 135 ohms for F to E. If the circuit is open then you did something to the tank unit after you measured it and during the reinstallation.

HTH,

Mark

 

[Trooper583]

Re: Fuel Gauge sending unit issues.

Thanks, I should have also mentioned that the sending unit in the tank is most likely 16 years old.(the boat is a 95) I'm going to take the sending unit out again in the a.m. and double check that I reassembled it correctly. I'm also going to leave the wires connected and manually move the float to see if that does anything. In regard to the reassembly, I've been thinking about it and it seems to me that if I had assembled it incorrectly, I would get a constant F reading. As always, I will continue to update this thread until the issue has been resolved so as to contribute to the community. I've learned a ton from 2 and 3 year old threads that the final info was posted on.

 

[Trooper583]

Re: Fuel Gauge sending unit issues.

Update: (Hopefully the last one)

I sat around last night thinking..."I wonder if I missed something when I reassembled the sending unit?" Well I went out this morning and removed the sending unit. I think sometimes it just helps to stare at the problem for a while. I left the red wire (main sending wire) attached to its post and then took the the ground wire and touched it to the down shaft. I forgot to mention that I had the key on. When I touched the ground wire to the down shaft, Poof! the gauge was working. Based on how much fuel I believe is in there it looks pretty accurate. I then placed the ground wire on the top mounting plate...nothing.

after further inspecting the sending unit once I removed it, I noticed that a rubber grommet had become bound up when I tightened the assembly down. When it bound up, the grommet worked it's way in between the contact point of the top mounting plate and the down shank thus insulating the two against each other. In short, I had lost a ground the sender when I reinstalled.

In summary, I believe that the original problem was the corrosion on the down shank and top mounting plate where they contact each other. When the problem started the gauge was working but reading low, I think this would be indicative of corrosion causing a weak ground.

 

[Texasmark]

Re: Fuel Gauge sending unit issues.

Update: (Hopefully the last one)



after further inspecting the sending unit once I removed it, I noticed that a rubber grommet had become bound up when I tightened the assembly down. When it bound up, the grommet worked it's way in between the contact point of the top mounting plate and the down shank thus insulating the two against each other. In short, I had lost a ground the sender when I reinstalled.

.

That's why I suggested that you ohm out to the ground wire terminal itself, not to the ground plate.

While I'm here, with the gas bouncing up and down, over time, the wiper will wear a grove in it and wear the wire into and either will result in an open circuit. Might be prudent to just replace the sending unit while you are there and then the problem is gone. Problem is, to get a sending unit that you can adjust properly to your tank....a daunting task for me anyway, and getting the right resistance so that float all the way down to float all the way up results in a F to E reading.

Good luck,

Mark

 

[Home Cookin']

Re: Fuel Gauge sending unit issues.

Back to the original "problem": boats on a trailer often do not fill to capacity b/c the hull on the trailer is not the same "level" as in the water. You can get an air pocket in the tank at the end that is not vented, too. It's not unusual. And gas guages are just for general ideas; they are never precise.

So before all the adventures, I'd say you didn't have a mechanical issue, just a boat thang.

Then you adopted government policy "if it ain't broken fix it until it is."

Then you got the dreaded mystery short, one of the most aggravating things that can happen, mechanically, to a boat. Especially when you put something back like you found it, no parts left over, and it doesn't work.

But now you know your sending unit intimately, and can fix yours and your friend's, and advise others on forums like this! So I agree with the suggestion to replace it now, or when it does finally give out, you will know what to do!

 

[Trooper583]

Re: Fuel Gauge sending unit issues.

Well, I forgot to add that it "appears" that I have the problem fixed. When I began the project the gauge was reading 1/4. This reading was on the trailer. When I got it working again, the at the same angle the gauge was reading between 1/2 and 3/4. I'll have to put it on the water before I can confirm I'm back in business. I think I'll see how much longer I can get out of this sending unit. At any rate, I'm glad that this issue popped up since I wasn't familiar with how this system was installed and worked until I got into it.