Gas Gauge Dilemma-Part Deux
February 3rd, 2013, 02:17 PM
#1
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Join Date: Jun 2008
Location: Seguin, TX.
Posts: 92
Gas Gauge Dilemma-Part Deux
About two months ago, I submitted my “car ain’t running” sadstory https://classicoldsmobile.com/forums/442/56308-puzzled-any-suggestions.htmlWell I have been narrowing possibilities down and decided to drop the gas tank.When I drained the tank, I only got ˝ gallon out of it, though my gauge said I wasfull. A quick rap on the tank verified the emptiness inside. I thought thiswould be a quick fix, so I took out the tank and removed the sending unit. Thepick- up sock had disintegrated, but the inside of the tank was clean.Installed a new sock and gathered the necessary new fuel hoses forinstallation. For grins, I grounded thesending unit wire and gauge still reads full. I put my old trusty analog ohmmeter to the sending unit, and the readings are close to what the factory shopmanual said they should be. Hooked up sending unit wire to the sending unit andgrounded it to my tail light stud, took the unit from empty to full, and still,no movement at the gauge. I read 6 volts at the negative side of the gauge withthe ignition key on, but could not get a reading at the sending unit wire atall. I have visually check the wiring where I could see the loom, even underthe rear seat. Is my gauge toast or do I have another problem? Also, if I haveto pull this thing out to have it repaired, is there a way to disconnect thewires from the pins without destroying the pins? Is there anyone that sells theprinted circuits on the back of the housing? Any help would be appreciated.
February 4th, 2013, 07:48 AM
#2
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Join Date: Oct 2011
Location: Chelmsford, Mass
Posts: 604
A broken fuel Gauge wire means open circuit and your fuel gauge should go Beyond Full.
If the gauge wire is shorted, your fuel gauge will go to below Empty.
The same theory applies to your fuel sender unit. You should replace your sender unit every 10-20 years. Do not bother to fix it; just replace the whole sender unit.
In your memo, you indicated, you found the Fuel gauge wire at the tank and checked and found 6 Volts.
And then you shorted the fuel gauge wire and the gauge needle did not move.
It should have gone from beyond-Full to below-Empty with a short.
It seems like one of your problems is in the dash board. It could possibly be a bad fuel gauge.
I performed similar tests on my CS last year, Short = Empty, and Open = Full.
I had 2 problems, a Bad Sender unit (which I did replace) and a loose/poor ground in my instrument panel.
Suggestions: Read the link below and clean all of the grounding straps in your instrument cluster. Try to get your gauge to work without the sender unit (Short = Empty / Open = Full+). Once this is working, you can then debug the sender unit.
https://classicoldsmobile.com/forums...s-issuses.html
If the gauge wire is shorted, your fuel gauge will go to below Empty.
The same theory applies to your fuel sender unit. You should replace your sender unit every 10-20 years. Do not bother to fix it; just replace the whole sender unit.
In your memo, you indicated, you found the Fuel gauge wire at the tank and checked and found 6 Volts.
And then you shorted the fuel gauge wire and the gauge needle did not move.
It should have gone from beyond-Full to below-Empty with a short.
It seems like one of your problems is in the dash board. It could possibly be a bad fuel gauge.
I performed similar tests on my CS last year, Short = Empty, and Open = Full.
I had 2 problems, a Bad Sender unit (which I did replace) and a loose/poor ground in my instrument panel.
Suggestions: Read the link below and clean all of the grounding straps in your instrument cluster. Try to get your gauge to work without the sender unit (Short = Empty / Open = Full+). Once this is working, you can then debug the sender unit.
https://classicoldsmobile.com/forums...s-issuses.html
February 4th, 2013, 08:45 PM
#3
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Thread Starter
Join Date: Jun 2008
Location: Seguin, TX.
Posts: 92
Miles71- Thank you for your input. The thing that is stumping me is I read 6 volts at the negative side of the gauge with the ignition key on, but could not get a reading at the sending unit wire at all. Would the gauge being pegged at full have anything to do with not getting a reading of any kind at the fuel sender wire at the tank?
February 5th, 2013, 05:48 AM
#4
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Join Date: Oct 2011
Location: Chelmsford, Mass
Posts: 604
Hi Louis:
I do not remember what the Open voltage of the Fuel sending wire should be. Maybe I’ll check mine later. I would assume that reading the open wire at the tank connector near at the rear bumper should be a Positive?
The most common fuel gauge problem is the Over-Full indication. This is usually caused by a bad Tank sender or a damaged/corroded/Broken fuel gauge wire. My CS had the Over Full problem like yours.
When I tested My 71 CS last spring, I disconnected the Tank wire near the bumper. Checked for voltage from the Gauge (can’t remember the actual voltage, Key ON), then shorted the tank wire to Ground with alligator clips.
I then checked the Gauge and the needed moved from Over-Full to Below-Empty with a temporary direct short.
This confirmed, I had a working fuel Gauge.
It also confirmed I had a bad sender unit. I had the shop replace the Sender.
All worked good for a few weeks until a loose ground on my dash board caused the Fuel Gauge to change readings depending on the status of the Head lights. I fixed the dash ground tab issue and it has worked perfectly since.
Your description of the Negative 6 volts does not sound right. Maybe it’s just terminology. At the fuel tank sensor wire connector, unplug the wire. One side will go to your Gauge. The other side will go into the tank and sender Unit.
Make sure you have the correct wire and know which end goes where.
While unplugged, the Tank Side of the wire will have Zero volts.
The Gauge side of the wire will have 6-12 volts.
Connect the Positive Lead from your DVM to the Un-plugged Fuel Sensor Wire. Connect the Black/Ground lead from the DVM to the Metal bumper. Now check the voltage with Key on. You should see a Positive voltage between 6-12 vdc.
Since the Fuel Gauge wire is currently disconnected, your Gauge will be indicating Over-Full.
Now short the wire to the Bumper with clips and wait a few seconds.
The Gauge will be fooled into thinking your tank is empty and the needle should move towards Empty.
If you do not see the Gauge move towards Empty while the Gauge wire is shorted then you have the following issues:
You are testing the wrong wire.
Your fuel Gauge is broken.
I do not remember what the Open voltage of the Fuel sending wire should be. Maybe I’ll check mine later. I would assume that reading the open wire at the tank connector near at the rear bumper should be a Positive?
The most common fuel gauge problem is the Over-Full indication. This is usually caused by a bad Tank sender or a damaged/corroded/Broken fuel gauge wire. My CS had the Over Full problem like yours.
When I tested My 71 CS last spring, I disconnected the Tank wire near the bumper. Checked for voltage from the Gauge (can’t remember the actual voltage, Key ON), then shorted the tank wire to Ground with alligator clips.
I then checked the Gauge and the needed moved from Over-Full to Below-Empty with a temporary direct short.
This confirmed, I had a working fuel Gauge.
It also confirmed I had a bad sender unit. I had the shop replace the Sender.
All worked good for a few weeks until a loose ground on my dash board caused the Fuel Gauge to change readings depending on the status of the Head lights. I fixed the dash ground tab issue and it has worked perfectly since.
Your description of the Negative 6 volts does not sound right. Maybe it’s just terminology. At the fuel tank sensor wire connector, unplug the wire. One side will go to your Gauge. The other side will go into the tank and sender Unit.
Make sure you have the correct wire and know which end goes where.
While unplugged, the Tank Side of the wire will have Zero volts.
The Gauge side of the wire will have 6-12 volts.
Connect the Positive Lead from your DVM to the Un-plugged Fuel Sensor Wire. Connect the Black/Ground lead from the DVM to the Metal bumper. Now check the voltage with Key on. You should see a Positive voltage between 6-12 vdc.
Since the Fuel Gauge wire is currently disconnected, your Gauge will be indicating Over-Full.
Now short the wire to the Bumper with clips and wait a few seconds.
The Gauge will be fooled into thinking your tank is empty and the needle should move towards Empty.
If you do not see the Gauge move towards Empty while the Gauge wire is shorted then you have the following issues:
You are testing the wrong wire.
Your fuel Gauge is broken.
February 7th, 2013, 05:56 PM
#6
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Join Date: Jun 2009
Location: southeastern Michigan
Posts: 14,262
It should got TO empty and stop there. Empty is zero ohms, which corresponds to an empty tank, and you can't have less resistance than zero.
It goes ABOVE full at open circuit because the Full reading is 90 ohms resistance at the sending unit, while an open circuit is, of course, infinite ohms, which is much more than 90.
I disagree completely. Sending units can last a very long time, and there is no reason to replace one until it fails. It's not like they fail and you don't notice it. When it fails, it's obvious, and then you go through the trouble of replacing it. I have a 40 year old wagon with its original sending unit. When and if it ever fails, I'll go through the trouble, and it is considerable trouble in this car as the gas tank must be removed, and doing so requires loosening and dropping the rear axle on one side. In some cars, the sending unit can be gotten at relatively easily. In both of my old cars, replacing it requires removing the gas tank, which is why I don't do it unless a sending unit has failed.
It goes ABOVE full at open circuit because the Full reading is 90 ohms resistance at the sending unit, while an open circuit is, of course, infinite ohms, which is much more than 90.
You should replace your sender unit every 10-20 years.
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