Reproduction Parts for 1916-1964 Chevrolet Passenger Cars & 1918-1987 Chevrolet & GMC Trucks


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RLackey Offline OP
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What OHM’s reading at HALF FULL should I see, when reading the top of the sending unit terminal directly? Still 6v. Using 0-30 ohm sender.
Randy

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Should be a steady progression between ~0 at empty and ~30 at full. I've never specifically tested at half way but it's linear so I assume it would be ~15 ohms.


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RLackey Offline OP
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I am probably trying to “reason this out” by giving it too much thought. But my gas gauge (which has been tested with the new sender, progresses and declines as it should when moving the sender arm, measures correctly on the gauge outside of the truck. Testing the OHMS on both tank sender post and at the end of the sender wire match.

Gauge moves to E and F as it should when sender wire is disconnected (sender wire test).

This is a new 18 gallon tank.

When the tank is empty, the gauge reads E. (1.7 OHMS)
At 9 gallons, gauge reads halfway between E and 1/2. (8.1 OHMS)
At 18 gallons, gauge reads halfway between 1/2 and F. (17 OHMS)

The new tank sender unit has a clean separate ground wire to the chassis.
The instrument cluster has a clean separate ground wire to the chassis.

What I am trying to figure out is why the gauge is registering less gas than is actually inside the tank.

One final note, the new sender unit came from The Filling Station and was preset for my truck. The float arm is adjustable.

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This is just a guess but the float arm may need to be bent down a tad. If the sender is showing the correct resistance at both limits it seems reasonable to me that it's not reaching it's upper limit when the tank is full. Bending the float arm down a bit should make the wiper travel higher on the windings in the full position.


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RLackey Offline OP
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UPDATE
I have 14.7 OHMs at the fuel sender post (installed inside tank half full with gas). That’s good.
Connected sender wire to sender. Have 14.7OHMs at wire’s end connector, which goes to gauge. That’s good.
When I connect the sender wire to the gauge, OHMs reading on gauge sender wire post reads 7.1
Problem with the fuel gauge or could this gauge be a prior replacement, even set up for 12 volts? Note, truck is still a 6v truck.
Thoughts…

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That sounds like a universal sender that is set wrong. You need to find the instructions and follow them. An incorrect float arm length or incorrect depth adjustment can cause that. Chevs of the 40s has a how to video explaining how to adjust it.

Last edited by old216; 06/17/22 07:12 PM.

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RLackey Offline OP
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Thanks for the suggestion. The Filling Station preset the sending unit based on tank depth per their instructions. I double checked it upon receipt. It was correct.

My main question is why the sending unit ohms decrease from 14.7 to 7.1 at the point of connection when the sending wire is physically connected to the fuel gauge posts and an ohm reading is taken from that post with not power connected to the gauge. Why would the resistance drop?

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If I am reading this right...

The reason you get a lower ohm reading when you connect the sender wire to the gauge, is that there is an "operating coil" in the gauge connected between the terminal you attached the sender wire to and the ground.
So you are now actually testing the resistance ohms in the operating coil in the gauge.

Hope that helps.


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I bought a sending unit from TFS several years ago. When it came, it was in a Dennis Carpenter box. I knew that Dennis Carpenter reproduced a quality line of Ford reproduction parts but was not aware that he was in the Chevy restoration parts business.
I called Steve at TFS and he said that since the Ford sender was made for the same OHM range, it would work OK in a Chevy. I never got it to work right and ultimately replaced it with a sender from Jim Carpenter. It doesn't work right either so I assume I have other issues.
If the Ford gauge has the same range of operation 0-30 ohms, as I understand, It seems that the depth of the tank would come into play somewhere.
This is not a criticism of the Ford gauge from TFS since I clearly have something going on that precluded me from getting an accurate reading at the dash gauge.
If the sender needs more room to swing to make use of the entire 0-30 range, shortening the arm would probably be the only solution.

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Everything I'm seeing here suggests Tiny is exactly right.

When you added 1/2 tank of gas (9 gallons), your gauge reading raised by 1/2 tank (from 1/4 to now reading 3/4).
That is exactly what it should do.

But... your gauge is reading 1/4 tank lower than it should.
So as Tiny said, you maybe just need to relocate the position of the float with a little bending?


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Does your sending unit have a brass or cork float?

I agree that a sending unit and fuel gauge are nowhere near as linear as you would like. Consider that the float arm moves in an arc. Yet the fuel level in a rectangular tank is relatively linear. Matching a circular motion to a liner motion is tricky.

I also agree that when you try to read the resistance of a sending unit that is connected to the gauge it includes the resistance of the gauge.


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One more thing.

You said you got your sender from The Filling Station, but the float arm is adjustable.
I believe the correct sender does NOT have an adjustable arm, and should be this one:
https://store.fillingstation.com/de...946_TRUCK_FUEL_SENDING_UNIT_030_OHM.html

This one is listed as a replacement for GM part number 1516287, which should be the original part for your truck.


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RLackey Offline OP
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Thank you. I suspected that, but wanted to hear some else say it without being lead to say so.

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RLackey Offline OP
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RESOLVED!
Thanks to all that contributed their ideas.
All were taken into account.

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So what fixed it? What turned out to be the problem in case someone else has this issue?


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RLackey Offline OP
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Cannot say specifically, but after reading everyone’s posts, I went to bed last night to sleep on it. Got up at 4:30am and went straight to the garage. Started to go thru every one of these steps in succession, beginning at the tank.

1. Removed the sender and visually checked the float for proper movement. With the tank HALF FULLof gasoline (pre measured), reinserted sender into tank.
2. Added a secondary ground wire to chassis attached directly to sender. Tested for OHMs. 19 at sender post.
3. Then proceeded to check on sender wire to gauge. Found a splice under the chassis from sometime in the past. Cut out the old splice. Cut a new section of wire from sender to wiring harness and re-spliced with a new weatherproof connector. Tested for OHMs at the end of the sender wire harness, where connection is made to gauge. 19 OHMs.
4. Took a tap/die and cleaned the threads on both the gauge sender post and the nuts that attach sender wire to that gauge post. Attached sender wire to gauge.
5. Added a new ground wire screwed into gauge mounting screw, terminating the other end directly to a dash/chassis support bar.
6. Turned battery connection back on and ignition switch on. Gauge read approximately 1/2 full tank.
7. Removed all gas from tank. Gauge read EMPTY.
8. Added gas back to tank, 4 gallons at a time. After each fill, checked gauge. Each time, gauge read 1/4, 1/2, 3/4 and FULL respectively.

So, there you go. I finished late this Saturday morning. I took my time, no rush, no interruptions and enjoyed the morning breezes while lying on my back on the floor and in the cab.

It’s simple mechanical stuff, but I do not know which one(s)of the above actions made a difference. In the end, I have a working fuel gauge (accurate).
Randy

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Isn't it a great feeling when you win a battle? Even if sometimes you're not sure which "weapon" won it for you. laugh


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Glad you were able to get things working as they should.

Your work and results show the value of paying attention to details.


Rusty

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