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Gas tank sending unit

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amcfitz View Drop Down
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    Posted: Dec/02/2022 at 3:48pm
Hi everyone. I have a 1975 AMC Hornet Sportabout. Changed the gas tank because the old one was damaged. We have tried two different sending units. They work as they should when we are holding them in our hands. The ground is connected and we are touching the unit to the tank. When we installed them both and add ten gallons of gas. The gauge will come off the peg to empty and stays there. We have tried this with both units, same results. The new tank is galvanized. Could that be an issue. We are stumped and looking for help. Thanks
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JD View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote JD Quote  Post ReplyReply Direct Link To This Post Posted: Dec/02/2022 at 8:16pm
the ground wire from the sender to the body making a good connection? 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amcfitz Quote  Post ReplyReply Direct Link To This Post Posted: Dec/04/2022 at 6:02pm
I am pretty sure it is otherwise it wouldn’t work outside the tank.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote 6PakBee Quote  Post ReplyReply Direct Link To This Post Posted: Dec/04/2022 at 9:13pm
If they work with the sender removed and manually moving the float, the problem isn't electrical.  This is assuming when you have them outside of the tank that you have the sensor wire connected and a good ground.  You could also take an ohmmeter and read the sensor with the float against the lower stop and when installed in the tank with the 10 gallons.  I would bet you have the same resistance.  I have no experience with AMC but the '67 Mopar B-body repop senders have the same problem.  I am convinced that the float lever isn't clocked accurately with the rheostat shaft.  That is a project for this summer.  I am going to cut an old tank apart and see just where the float ends up at 0%, 50%, and 100% tank level.
Roger Gazur
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1970 Sonic Silver auto AMX

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JD View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote JD Quote  Post ReplyReply Direct Link To This Post Posted: Dec/04/2022 at 9:23pm
Float not floating? 

Edited by JD - Dec/04/2022 at 9:25pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bigbad69 Quote  Post ReplyReply Direct Link To This Post Posted: Dec/06/2022 at 11:05am
If you have a good ground from the float assembly to the frame of the car, then I would say the problem is as 6PakBee describes, or the float doesn't, as JD said.
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amcfitz View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amcfitz Quote  Post ReplyReply Direct Link To This Post Posted: Dec/06/2022 at 6:18pm
Well we have a good ground otherwise it would not work outside of the tank. I just checked with a Ohm meter the original unit and it is working as it should. We have tested the float and it does float. We are stumped and it sounds like you are all stumped also. The original sender in the old tank worked fine. I think I am going to back with that and see what happens.
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JD View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote JD Quote  Post ReplyReply Direct Link To This Post Posted: Dec/06/2022 at 6:37pm
With the sender in the tank and 1/2  full of fuel with your 10 gallons the sender should read about 30 ohms from the sender stud to the ground tab on the sender. Full would read about 10 ohms and empty about 73 ohms at the sender . A quick way to test the gauge part for function is to have the key on and touch the sender lead at the tank to ground very briefly while someone watches the fuel gauge it should head straight for the peg at full. 

Edited by JD - Dec/06/2022 at 7:07pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote JD Quote  Post ReplyReply Direct Link To This Post Posted: Dec/06/2022 at 7:15pm
Maybe inspect the new sender real good where the little wiper finger goes along the rheostat winds of wire on the flat board.The wiper finger may be loosing contact running off the wire wraps in the area where the finger is pointing at the level of fuel thats in your tank. New parts from China sometimes aren't as high quality as the originals and need some adjustment. In this situation it would appear that its functional at each end of full and empty but having a hiccup in the middle going unnoticed until its in the tank. 

Edited by JD - Dec/06/2022 at 7:19pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 6PakBee Quote  Post ReplyReply Direct Link To This Post Posted: Dec/07/2022 at 10:13am
Originally posted by amcfitz amcfitz wrote:

Well we have a good ground otherwise it would not work outside of the tank. I just checked with a Ohm meter the original unit and it is working as it should. We have tested the float and it does float. We are stumped and it sounds like you are all stumped also. The original sender in the old tank worked fine. I think I am going to back with that and see what happens.


I wouldn't say we're stumped, it's just that the answer is not straightforward. You say the sensor works when you have it in your hand.  I take that to mean when the float rod is on the lower stop the gauge reads empty and when its on the upper stop the tank reads full.  That tells me the electrical part is working fine.  You then say the float, floats.  That tells me the mechanical part is working fine.  If it does not read correctly in the tank, the float position for the tank level is incorrect.  It's a calibration issue.  Simple as that.
Roger Gazur
1969 'B' Scheme SC/Rambler
1970 RWB 4-spd Machine
1970 Sonic Silver auto AMX

All project cars.

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