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Fuel gauge sending unit woe's

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  #1  
Old 03-08-2018, 03:49 PM
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Fuel gauge sending unit woe's

I'm stumped here. I installed a new poly tank and sending unit months ago. The gauge read accurately for a while then wouldn't go above 3/8. I checked wires and connections, all fine. I connected the connector to another sending unit, and operated it by hand while someone else watched the gauge, and it worked fine. So last week I dropped the tank, and inspected it with a bore scope, nothing inside it. I installed a new sending unit. It worked fine for a few days, then right back 3/8 reading at full tank.

Just now, I ran the connector from the forward tank to the rear tank. Same inaccurate reading on gauge. I traced the wires, re-did the ground. I again connected to the old sending unit, operated it by hand, an it works fine. Deductive reasoning tells me that the float is not moving, but I don't see how that could be.

Anyone, HELP!
 
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Old 03-08-2018, 05:38 PM
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since you used a poly tank, did you run a good ground wire to the sending unit?
 
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Old 03-08-2018, 06:13 PM
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Willowbilly, I thought of that but figured since the connector is grounded, the tank shouldn't need an additional ground. The forward tank is poly and the sending unit has no ground. HOWEVER, since you've said, I'm gonna do it.
 
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Old 03-08-2018, 10:46 PM
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The constant voltage regulator (behind instrument panel) may be getting ready to drop dead
 
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Old 03-08-2018, 11:36 PM
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67xr7kat, yep I thought of that too. I have a spare and will try that if grounding sending unit doesn't work. Thank you both for the advice.

One thing I haven't tried is to bore scope the tank while it's installed and sending unit is installed. I think I'll run the truck til it should have about 1/2 tank, then stick the borescope down the filler and look at the float on the sender.
 
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Old 03-09-2018, 08:14 AM
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Well, grounding the sending unit didn't work. I figured it was redundant since once the connector is attached, the sending unit is grounded. But it was certainly worth a try. I can't find my spare instrument voltage regulator, so I'll order another. The spare was a takeoff anyway.
 
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Old 03-10-2018, 10:06 AM
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Instrument panel voltage regulator swap didn't change anything. A buddy borrowed my borescope so I'll get it back then check then float while in the tank.
 
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Old 03-10-2018, 01:23 PM
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Good luck. Since it worked before and is now acting up. the ground was worth a shot, but the regulator would have effected the other gauges as well. But at least you have a new one.
Just don't throw away the old good one yet!

Since "new" stuff is often crap, the new sending unit may very well have stopped working, or is just setting in to it's new normal. Well worth the trouble to take it out I think. Big PITA usually, but having a working gauge is priceless.

Good luck again.
Oh, and if you haven't already figured it out, while the sender is out you should test it on the circuit while it's still out. That way you can make any mods/adjustments needed. Same would go for any new ones you might have to get, if this one turns out to have died an early death.
I learned long ago to mess with them while they're still out, because my Brand new Ford units (bought new from Ford in about '78 or so) read exactly the same as my old ones. Found out that tweaking the originals both at their stops and their float arms corrected the gauge reading and they stayed that way for the next 20 years or so until I got an aftermarket rear tank. Front tank kept on working until it got removed about 10 years later.
Sold them all still working to someone new.

Paul
 
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Old 03-10-2018, 03:29 PM
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Paul,

Thank you. I did test the new, new sending unit before installing. And you're right, that's a lesson you shouldn't have to learn the hard way.. Most of what I know was learned the hard way. Thanks again!
 
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Old 03-11-2018, 01:27 PM
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So the new sender pushed the gauge through it's entire range while still out? Then it sounds to me like the stops don't need any tweaking then, but the float arms still do so they sit in the fuel properly.

Hmm, I wonder... Are you able to verify that the tank is actually filling up all the way? I don't think I've ever heard of anyone with one of our trucks not being able to, but have had it happen often enough on the older trucks to still remember it being a problem.
Which tank again? What year and model truck? Mine has bigger tanks than many from the factory (29 and 19 I think?) but I remember seeing many different capacities listed over the years.

Paul
 
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Old 03-11-2018, 03:19 PM
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assuming nothing is physically blocking the arm from moving all the way up, any chance your float has a hole in it? when you pulled it out of the tank, it could have warmed up enough to force the gas out...when you put it back in it worked until the float got enough gas inside of it to not be able to float again?
 
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Old 03-11-2018, 04:45 PM
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1Ton, yep I tested it before installing it. And yes sir, I've confirmed tank is filling by pump readout, and by driving the tank down to empty. Weird ain't it.

Dlbruch, I thought of that when I bought the new one. That's why I went with a Spectre which has a solid float, instead of the copper barrel type. I have seen a carburetor float sink because of a leak at the solder seams before.

This is truly frustrating. I didn't mess with it this weekend because I was installing an Eaton Truetrac in the rear end. I'll have my borescope back tomorrow and will be able to look at the float while inside the tank. If that's not the issue I may just throw a fit and rewire from the gauge back.

Thank you all for helping!
 
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Old 03-11-2018, 10:41 PM
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Re: sending unit woes

Hey Rangerxxx...,
I think before doing a rewire, I would get a spool of 14/16 ga. wire, get as close as possible to the back of the gas gauge, bypass the existing wire and run a temporary feed to the sender secured with zip ties and drive it to see if the problem reoccurrs. Just sayin.
 
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Old 03-11-2018, 10:42 PM
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Sorry, duplicate post
 

Last edited by 67xr7kat; 03-11-2018 at 10:43 PM. Reason: duplicate
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Old 03-12-2018, 08:02 AM
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67xr7kat, yep, good call. I'm sure not gonna do more wiring than I need to.
 
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