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Canadian Poncho Homepage -> Canadian Poncho -> General Beaumont Discussion -> Fuel gauge reading wrong....
Post Info TOPIC: Fuel gauge reading wrong....
LT1Caddy


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Fuel gauge reading wrong....
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Hi... This on a 66 Biscayne...Should be the same as 66 Poncho.   New tank, new gauge, checked the ground (cleaned to bare metal).

So I've been going through the car after restoration getting the bugs out. I had not tried to fill  the tank till last week, $100 (about 10 gal), because I was only running 10 miles into town. I know the tank is 20 USG. The gauge went to just "over" full. I've been just putting  $20-30 in it, so it was never much over 1/2 full. So I'm sure at $100, that would be be 10-12 CD gal.

Troubleshooting says if it goes "full or over full" usually the ground....It's good.

 

Any ideas....



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seventy2plus2


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Do you have another sending unit you can used just to test things out. You should be able to test things at the rear part of the trunk where the sending unit ties into the tail light harness. If I recall, the sending unit will go from 90 ohms to 0 ohms, I can't remember which is full / which is empty. Test a spare sending unit, then connect it to the tail light harness and see what the gauge says. The problem could be up in the gauge itself.

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4SPEED427


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I've read about issues with the repro sending units being the wrong resistance for the particular vehicle. It sure does sound like a bad sending unit though. And if it was a bad ground, you'd think it would have showed over full before adding any fuel. A bad ground makes it over full no matter how much fuel is in the tank.

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Canadian Poncho


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I agree with Carl. Sending unit looks to be the wrong spec. An Ohm meter placed between the sending unit feed wire and ground will give you a good idea.

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DonSSDD


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Ive seen reproduction sending units in the xframe years that were calibrated wrong for the gauge as well. I wonder if they stay off kilter proportionally? It may work fine on the lower end or close to fine?

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63 Parisienne sport coupe (The Big GTO), black, maroon interior, 409 4 speed; former owner of a 59 El Camino, 63 Corvette SWC, 62 Chev Bel Air SC.
1963- Pontiac top selling car in Canada

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ARCADIAN


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My gauge was reading full in my '65 Chevelle and I knew there was something wrong (as $20 worth of gas wasn't reading right). The previous owner had replaced the sending unit and tank and put a 90 OHM 3/8" unit in and it should have been a 30 Ohm 5/16" one. Once it was replaced the gauge is now reading correct. Cost for a new 30 OHM sending unit was $135 and about 1.5 hours time! Good to go!



-- Edited by ARCADIAN on Wednesday 20th of July 2022 07:33:05 AM

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LT1Caddy


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So I checked with the Ohm meter, from the power in to the ground and I have 54.4 ohms.

So I'm thinking I would not get 90 ohms unless the tank was full??

I thought I had the kept the old one, but I purged my excess "stock" last year in a cleanup...

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seventy2plus2


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I find your first post a bit confusing. Is the tank full, or you figure only about half? What is the gauge reading?

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LT1Caddy


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seventy2plus2 wrote:

I find your first post a bit confusing. Is the tank full, or you figure only about half? What is the gauge reading?

 Judging by what I spent, $100, that would be in around 10 gallons. The tank is 20 gal. I figure the tank is around 1/2 full. The gauge is showing a hair over 3/4 full. There  was I figure a little under a 1/4. So that should have taken me to around 3/4 full. When I noticed the gauge later in the day it was reading full...



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seventy2plus2


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I'm tapping out.

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LT1Caddy


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Trying to figure out if it's the gauge or the sender...

I found this info, wondering if it's correct... Disconnect power wire from sender, gauge SHOULD read full ??
Ground the power wire from sender, gauge SHOULD read empty ??

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DonSSDD


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Id suggest you add another say 5 gallons of gas, check ohms before and after or remove ground ground and see what it does before adding gas and if it goes to full or empty.

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63 Parisienne sport coupe (The Big GTO), black, maroon interior, 409 4 speed; former owner of a 59 El Camino, 63 Corvette SWC, 62 Chev Bel Air SC.
1963- Pontiac top selling car in Canada

Mahone Bay, NS Still not old enough to need an automatic
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LT1Caddy


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DonSSDD wrote:

Id suggest you add another say 5 gallons of gas, check ohms before and after or remove ground ground and see what it does before adding gas and if it goes to full or empty.

 I was thinking that, but then that's another 5 gallons I have to drain out to remove the sender.  So I'm trying to lessen the mess...LOL.

I'll try and check the gauge later to day to see if it is the gauge. I think I have found how to check the gauge on the internet to determine that.

But the tank and the sender are new, but in this day and age, you hope the product you buy works. ( it was installed early in 2021 so of course there is no warranty!!!)



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4SPEED427


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What if you drain 5 gallons and check it?

At least then you're headed the right way if you have to pull it out later.

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Take a jerry can with a couple gallons of gas and put it in trunk and go for a cruise and watch the gauge till you run out of gas. Then youll know if its working and if its not youll only have a couple gallons to deal with when you change the sending unit.

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beaucarlo


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i'm not sure on the dash trim on your car but i'll tell you what happened to my 

62 impala. the guage stopped working 1 day. after all the usual trouble shooting,

i found the dash guage wasn't grounding through the chrome facing on the dash 

itself.adding a separate ground wire to the back of the guage now it works perfect.



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bob lewis
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Hi Guys... All good ideas. I'm busy this weekend, can't get to the garage to check it.....

Thanks for all the help, stay posted.

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LT1Caddy


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Ok... so here's the story...So far

66 Chev fuel gauge.... "E" is at 10 oclock, "F" is at 2 oclock

So checking today, when the power is removed from the sender the guage reads EMPTY exactly (10 oclock)
when the power terminal is grounded, the sender reads PAST FULL..(3 oclock)

Question 1...should the needle read PAST the full mark?
Question 2...I'm thinking the sender is working, that the gauge may be the problem? ( reading past FULL, does that mean it has the wrong OHM sender)



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DonSSDD


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Lots of gas gauges go past full a ways when you fill up.

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63 Parisienne sport coupe (The Big GTO), black, maroon interior, 409 4 speed; former owner of a 59 El Camino, 63 Corvette SWC, 62 Chev Bel Air SC.
1963- Pontiac top selling car in Canada

Mahone Bay, NS Still not old enough to need an automatic
LT1Caddy


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DonSSDD wrote:

Lots of gas gauges go past full a ways when you fill up.

 This is WAYYYYY past.

Should be "F" at 2 oclock, but the pointer is at 3 oclock or just past....



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seventy2plus2


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When you have the sending unit disconnected, what is the resistance (in Ohms) from the sending unit wire to ground? Considering you seem to figure the tank is about half full, the resistance reading of the sending unit will tell us something.

On my 70, when I fill the tank, it reads WAY past full. It's been like that on every car I have ever had.

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Prefectca


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Many GM cars go way past full with the gauge disconnected at the fuel sender. Totally normal. Ground the wire it should go to empty. It sounds like the dash gauge is working properly.

Paul

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LT1Caddy


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seventy2plus2 wrote:

When you have the sending unit disconnected, what is the resistance (in Ohms) from the sending unit wire to ground? Considering you seem to figure the tank is about half full, the resistance reading of the sending unit will tell us something.

54.4 OHMS sender wire to ground



On my 70, when I fill the tank, it reads WAY past full. It's been like that on every car I have ever had.

 



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LT1Caddy


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Prefectca wrote:

Many GM cars go way past full with the gauge disconnected at the fuel sender. Totally normal. Ground the wire it should go to empty. It sounds like the dash gauge is working properly.

Paul

 Hi... NOT correct as I previously posted.  When the power is disconnected it will go to Empty, grounded it goes over full



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LT1Caddy


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So... I'm going to find the test procedure for the gauge and check that too.

In my estimation there should not be that much discrepancy in the full reading. If it is "correct" then how do you trust the empty reading????

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