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Fuel Tank Sending Unit inop when installed
#1411134 Mon May 24 2021 03:19 AM
Joined: Apr 2009
Posts: 13
G
'Bolter
I purchased a new sending unit for the in cab fuel tank. It bench tests correctly. When I install it, it appears to not work. With the sending unit wire not attached and a full tank of gasoline I get the following. Continuity between the sending unit post and the tank-cab-negative side of the battery. I believe I should be getting about 28 ohms of resistance, just as I saw when I bench tested the new unit. I do not have a ground strap installed on the sending unit. The readings I am getting is that all the sending unit post and body are at the same potential as the truck frame and negative side of the battery. Perhaps I am missing something, it is odd to me that a unit bench tests correctly 0-30 ohms as I move the float up and down, but does not work when installed. The float works, I can see that when I place it in the tank.

Re: Fuel Tank Sending Unit inop when installed
gallatin #1411162 Mon May 24 2021 01:02 PM
Joined: Mar 2014
Posts: 139
G
'Bolter
I had a similar problem. It turned out the sending unit was grounding out on the gas tank. I discovered this by accident when I loosened the screws holding the sending unit and it started working correctly. It's been a few years, so I don't remember exactly how I fixed it, but basically just move the sending unit around to ensure it's not grounding out. There's enough "slop" in the screws to allow it to touch the tank when tightened down securely.


Greg Brown
'49 Chevy 3100
Re: Fuel Tank Sending Unit inop when installed
gallatin #1411167 Mon May 24 2021 01:35 PM
Joined: Feb 2019
Posts: 1,969
P
AD Addict
Your in cab tank sending unit needs go be grounded to the cab in order to work properly. In addition, you cab needs to be grounded the to frame and the engine. I would check your grounds.

Looking back at you past posts, you have a ‘48 with a newer cab and had both a frame mounted tank and cab mounted tank. Do you still have both tanks? If so, do you have gauges for both tanks?

Last edited by Phak1; Mon May 24 2021 02:23 PM.

Phil

1952 Chevrolet 3100
Project Journals
‘59 235 & hydraulic lifters
“Three on the Tree” & 4:11 torque tube
12v w/ Alternator
Re: Fuel Tank Sending Unit inop when installed
Greg Brown #1411185 Mon May 24 2021 04:39 PM
Joined: Apr 2009
Posts: 13
G
'Bolter
Greg,

Thanks for your thoughts. Yes, this is the issue I have. Without any screws securing the unit to the tank the sending unit works correctly. When I put one screw in the resistance to ground decreases, with 2 screws holding the unit to the tank, the unit is totally grounded. I have used the slop in the mounting screws to move the unit, but have not been able to get out of the situation. Will relook the situation, it is puzzling.

Re: Fuel Tank Sending Unit inop when installed
Phak1 #1411187 Mon May 24 2021 04:50 PM
Joined: Apr 2009
Posts: 13
G
'Bolter
Phil,

Thanks for your thoughts. The problem is that the sending unit is grounded to the cab. That is to say the body of the sending unit as well as the post that the sending unit wire lands on are both grounded to the tank, cab, and negative side of the battery. When the unit is not installed (none of the 5 screws in place) in the tank it works correctly. A puzzling situation.

At anyrate; Yeah my truck is a 1948. It has been in the family since 1979 & had two tanks when we got it. Looking at things, my best guess (emphasis on "best" and "guess") is that the tank under the bed is stock and the tank in the cab was added by someone. The fuel gauge reads the tank under the bed. The cab tank was never metered. I am adding that function and currently have it hooked up to the fuel gauge. There is a valve near the passenger's feet that selects which tank is in use.

I sorta like the 2 tank system. Have never seen another Advanced Design Truck that has 2 tanks.

Joe

Re: Fuel Tank Sending Unit inop when installed
gallatin #1411190 Mon May 24 2021 05:23 PM
Joined: May 2005
Posts: 8,245
B
Sir Searchalot
As Phil indicates, the sender/flange is SUPPOSED TO BE GROUNDED. The flange screws are SUPPOSED TO GROUND THE FLANGE of the sender to the tank.
The center tap is isolated and is NOT SUPPOSED TO BE GROUNDED. It's wire goes to the gauge. Many times the flange does not get good ground due to the gasket on the flange and rubber pads on the tank, rubber body mounts, rubber motor mounts......all trying to circumvent the ground path and have to be "jumped" with braided straps (which must be viable). There are many connections along the way to get corroded or broken. Many times you need to add a wire under one of the flange screws to the frame or body or battery.

If the center tap, or it's wire, touch ground, the gauge will not read properly.

I assume the gauge works with under bed tank/sender. I assume that both tank senders and gauge are 0-30 Ohms.

Gas gauge/sending is just about the number 1 electrical issue on Stovebolt. Like most electrical issues, it's usually the ground path. It's a long trouble invested path.

Yes, I believe the tank was under bed in 1948.


Watch out for careful drivers!!!
I'm away on an ego trip. Will be back on Feb 30.
I'm not an Auto Mechanic, but I play one on TV.
I charge $0.02 for every opinion and I take Paypal.
Plan B is always better than plan A, by definition.
Sometimes no nonsense makes sense, in a sense.
You can't teach a new dog old tricks.
Honk if you're Amish


Re: Fuel Tank Sending Unit inop when installed
gallatin #1411253 Tue May 25 2021 02:37 AM
Joined: Apr 2009
Posts: 13
G
'Bolter
Greg,

Your experience was the ticket. After bending the sending unit some, the fuel gauge system is working!

So I took the sending unit out of the tank & tested it again by putting a ground to the negative terminal on the battery and simply moved the float arm with the unit laying on the seat. The fuel gauge went up & down just it should. When I put the sending unit back in the tank and put a couple screws in, the frame of the sending unit and the terminal of the sending unit had continuity. Same old story. Sooo with nothing to lose I took the unit out of the tank and physically bent it. I bent the conductor that runs from the rheostat to the sending unit terminal so that there was no way it could touch the vertical arm that holds the rheostat/float. Then I bent the float arm so that, when installed, it would be away from the side of the tank. When I installed the bent sending unit......Bingo! it works.

Re: Fuel Tank Sending Unit inop when installed
gallatin #1411254 Tue May 25 2021 02:41 AM
Joined: Apr 2009
Posts: 13
G
'Bolter
I appreciate people's replies to my issue. I can see why Bartamos says gas gauge is the #1 electrical issue on Stovebolt.

A take away question I have is; Do people have recommendations on good manufacturers or styles of sending units to purchase?

Re: Fuel Tank Sending Unit inop when installed
gallatin #1411274 Tue May 25 2021 12:20 PM
Joined: Feb 2019
Posts: 1,969
P
AD Addict
At the present time, it seems most of the available sending units are all made in China and most likely in the same factory. One of our fellow ‘bolters Jon G, is working on fixing the issues. Look at these threads.

https://www.stovebolt.com/ubbthread...project-the-start-of-it.html#Post1396986

https://www.stovebolt.com/ubbthread...r-project-stage-2-of-it.html#Post1397294

https://www.stovebolt.com/ubbthread...-project-the-final-word.html#Post1401881


Phil

1952 Chevrolet 3100
Project Journals
‘59 235 & hydraulic lifters
“Three on the Tree” & 4:11 torque tube
12v w/ Alternator
Re: Fuel Tank Sending Unit inop when installed
gallatin #1411301 Tue May 25 2021 02:55 PM
Joined: Apr 2009
Posts: 13
G
'Bolter
Phil,

Thanks for the info on manufacturers. disappointing, but believable.

Speaking of believable, that guy Jon is into it! Alot of R&D by him. After my recent experience with sending units, I would pay for a unit that is of high quality!

Re: Fuel Tank Sending Unit inop when installed
gallatin #1411302 Tue May 25 2021 03:16 PM
Joined: Mar 2014
Posts: 2,364
J
'Bolter
Thanks Phil,
The link to stage 2 shows the best of all sending units I've been able to make. It is the most stable, the most simple, the most accurate and will be extremely long-lasting. But due to the increase in the cost of brass and resistance wire lately it can't be made economically and therefore I'll have to wait to start making these for sale.

And please...anyone who is buying a sender unit, consider the 0 - 30 ohm units sold on eBay by a seller whose ID is renbusan. I've said for years these are the best of all available options and they are far superior to the ones being sold by the usual suspects of vendors out there. Recently a friend asked "why are these better?" They're better for a few reasons. One, the body is made of nylon and the circuit board of fiberglass. Two, the wiping surface is flat and vertical. That is infinitely better than the semi-circle arc type, but is more difficult and costly to make and with the arc somebody making senders only needs to come up with one. "One size fits all?" Well, no it actually doesn't. It leads quickly to inaccuracy and failure. Three, the wiping surface is made of tiny metal pieces, rounded on each end which are bonded to the fiberglass and which begin set into a poured-in-place resistive compound. Very accurate and a clever design. Four, it is made in Taiwan and it was designed by somebody who knew exactly what they were doing. Take a look at the image below. Increase the size if you can and you'll see what I'm talking about when you look at the metal fingers. This is the unit:

https://www.ebay.com/itm/261122347319?hash=item3ccc1ad137:g:FBgAAOSwpPVgBuwZ&vxp=mtr

And the image of it is below...

Attached Files
IMG_8455.JPG (88.84 KB, 50 downloads)

Jon

1952 1/2 ton with 1959 235
T5 with 3.07 rear end
Re: Fuel Tank Sending Unit inop when installed
gallatin #1411645 Thu May 27 2021 10:24 PM
Joined: May 2005
Posts: 8,245
B
Sir Searchalot
.................and Ethanol has no effect on the nylon or fiberglass.


Watch out for careful drivers!!!
I'm away on an ego trip. Will be back on Feb 30.
I'm not an Auto Mechanic, but I play one on TV.
I charge $0.02 for every opinion and I take Paypal.
Plan B is always better than plan A, by definition.
Sometimes no nonsense makes sense, in a sense.
You can't teach a new dog old tricks.
Honk if you're Amish


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