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One Millionth Post on Fuel Gauge Not Reading Correctly - 1974 stock

Chief Brody

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My gas gauge is stuck on 1/4 tank when full.

How do I know? I filled it up today, 19 gallons of pure Ethanol free gasoline.

Gas gauge didn't move. It will move lower...not higher...wait that not true.

I did figure 8's in an empty lot today after filling it up. The gauge made it to 1/2 full...and then went limp to 1/4 again.

This is driving me crazy...
 
The rheostat on the sending unit that operates the gauge is not much more than a bare wire coil wrapped around an insulator,and the float has a "wiper" contact that slides up & down across the wire coil and changes the resistance,which makes the gauge to towards full or empty..

I've read about the rheostats getting gummy from ethanol and preventing a good contact between the wiper and wires on the coil..
Cheveron Techron additive claims it can dissolve that varnish and restore the fuel gauge's ability to read the level correctly--but its a crap shoot in my opinion..but it might work if thats the cause..

I've also read about many aftermarket sending units crapping out rapidly,mostly the ones with the sending unit built into the electric fuel pump assembly..

Here's some hints on fuel gauge woes..

Troubleshooting Your Chevy Truck Gas Gauge Failure - Chevy Trucks Blog
 
I've always wondered how can there be an
electrical rheostat inside a gas tank and it not explode???

I mean all electrical contacts ( the wiper coil)
at some point have to arc or spark as it moves....

Is it due to lack of oxygen in the tank?
Or the flash point of gas?

:dunno::dunno::dunno:

I know diesel is more forgiving,, I used to throw lit cigarettes into puddles of diesel all the time..

ok....back on track.
 
I've always wondered how can there be an
electrical rheostat inside a gas tank and it not explode???

I mean all electrical contacts ( the wiper coil)
at some point have to arc or spark as it moves....

Is it due to lack of oxygen in the tank?
Or the flash point of gas?

:dunno::dunno::dunno:

I know diesel is more forgiving,, I used to throw lit cigarettes into puddles of diesel all the time..

ok....back on track.

You need the correct ratio of oxygen to gas. You can throw a lit match inside a full tank of gas and it’ll just go out (although I don’t recommend doing it for legal reasons :whistle:). Haven’t you watched how hard the Mythbusters had to work to make some explosions happen?
 
Time to pull the sending unit and take a look. As said, probably varnish which can be cleaned off with contact cleaner. Radio Shack use to sell it as ‘Volume knob cleaner’ or maybe even carb cleaner would work. Also check to make sure the wiper on the rheostat is making good contact throughout its movement. You may need to bend it a little. A ohmmeter would be handy to check the resistance while you move the float straight up and down. Make sure not to put any lateral pressure on it so you simulate what it would do with gas raising and lowering it.
 
You need the correct ratio of oxygen to gas. You can throw a lit match inside a full tank of gas and it’ll just go out (although I don’t recommend doing it for legal reasons :whistle:). Haven’t you watched how hard the Mythbusters had to work to make some explosions happen?

I figured it had to be something like that...
but then again,, there was TWA flight 800...

Still freaks me out that they fuel pumps in gas tanks too...
 
^ ditto for me also!--hard to believe an electric motor can be sealed well enough to keep gas out yet still run and have sparks at the armature,brushes,etc,and not have it get hot enough to set the gas on fire--I know they submerge the pump to keep it cool,and primed,but what happens when your down to 1/8th of a tank ?....one seal leaks,then what..I would think today's oxogenated fuels would have enough O2 in them alone to at least pop off an explosion in the tank..

But I also agree we tend to think cars blow up like firecrackers after seeing them do so in many movies--as noted above,its a freak thing to have something blow up,even when your intending to do so,Mythbusters proved that..still doesn't mean we can feel 100% "safe" though either..looking back at how many cars we cut apart with torches at the junkyard,there was only a few incidents of any troubles,most caused by being in a rush or getting too complacent,not being careful enough..

Not only does it suck to replace an in the tank electric fuel pump,I always have an uneasy feeling about any electric motor running in something liquid,especially flammables--even a submersible 110V utility pump can electrocute you if its not properly grounded if you touched the water..

I have used a old GM in tank fuel pump as a fuel transfer pump ,like when I needed to add diesel fuel to my house oil tank when I was unable to buy the minimum amount of gallons to get a delivery,I was getting diesel at the pump and adding 5-10 gallons every day--it sucked trying to pour 5 gallon cans into the tank ,especially the outside filler,you must stand on a ladder and spill some ,hold the fuel can still till its empty--no fun when its snowy..

--I took a plug out of the top of the tank and used that GM pump to suck the diesel into the big house tank,it takes only a minute or so to empty a 5 gallon can,but every time I've used it I cringe and feel ready to run if I see a flash..:eek:..It is just stuffed in a 4 foot length of heater hose on the inlet side of the pump,and a 3/8" hose on the output,I put a universal fuel filter in that line..

I've only been brave enough to use it on gasoline to transfer gas to one of my tractors from a gas can or drain the tanks for storage,I'm not sure I'd trust that setup on a car or truck sized gas tank..
Diesel has a higher flash point,gas can go whoof a lot faster in warm weather..

I have seen what a metal gas tank on an older small engine can do when it catches fire--one Briggs & Stratton engine a friend had had a weepy tank and it caught fire when it backfired..we tried to put it out,but the pressure built up in the tank,flaming gas came squirting out of the vent hole on the tank,then we decided it was time to RUN..

The thing burned about 30 seconds before it went "pop"...shards of metal from the tank went by us,we could hear them buzzing when they flew past us :eek:..the engine burnt badly enough to ruin the ignition coil,recoil starter.and some aluminum fins on the cylinder were blown off,and some were melted from heat..When we removed the remains of the gas tank,the side facing the engine was what ruptured first,and it caved in the block there and cracked it..!
 
Read about upper explosive and lower explosive limits....
 
I saw this on eBay and snagged it. NOS in box:

DSC_0100-1.jpg DSC_0101-1.jpg DSC_0102-1.jpg
 
I had an 83 GMC pick-up where the gauge read 1/4 when the tank was full, and slowly dropped as the fuel was consumed to read empty when it was empty. I replaced the gauge in the cluster, and that was all it took to fix the problem.
 
I had an 83 GMC pick-up where the gauge read 1/4 when the tank was full, and slowly dropped as the fuel was consumed to read empty when it was empty. I replaced the gauge in the cluster, and that was all it took to fix the problem.

Well........crap.........I got one of those I pulled from a parts yard that I could test with....it never occured to me that my gauge could be bad. When I did the ground test the gauge went full on 3-O'Clock

DSC_0106-1.jpg DSC_0107-2.jpg
 
If you are careful, you can hook that gauge up without digging into the cluster. Positive, ground, and sending unit wire.
 
Easier to try than the sender unit...

In my case the truck had two tanks, and the gauge behaved the same no matter which tank was selected. That convinced me it was probably a gauge issue, and it was. I've had more than a few bad factory gauges over the years, enough that I usually chuck another gauge in as my first diagnosis.
 
The fuel gauge in my pickup pegs out at 3 o'clock and stays there all the time when the ignition is on..

I was able to get the sending unit plug off (being a stepside bed,it was difficult but possible),and I tried plugging in a known good sending unit I had and grounded it,turned on the ignition,and I moved the float up & down,the gauge responded accurately ,so that means the sending unit I swapped into the fuel tank from the original was junk,I am not happy I failed to test it FIRST..just my luck it wasn't the wiring or gauge,now the tank has to come off again..

This truck had dual tanks and all the switches and wiring was butchered and or missing when I bought it..
I was able to get the right side tank to work with the fuel gauge,and I ran it off that tank until it rotted thru--then I got a good used tank and swapped it for the old drivers side tank that was junk from day one..

I extended the wires from the right side sending unit to that side rather than try and figure out why the drivers side wires wouldn't make the gauge work,since all the dual tank wiring under the dash is hacked--I do remember having to jump two wires on the selector switch to get the gauge to work on the right side tank..

I think dual tanks are a pain,the setup they used was over complicated and prone to failure..I never drive far enough to empty one tank,so I have no use for 2 of them when I cant afford to fill just one up!..
 
There is a resistor on the back of your guage that can go bad, its held in by to screws but three are back there.

My old 83’ read 1/4 when full. Changed fuel sending unit, lines etc. nothing worked. I sold it before fixing it. Also the site selling the stock resistor closed. I forget what OHM resistor you need.

https://67-72chevytrucks.com/vboard/showthread.php?t=640615

Maybe this thread will help?
http://67-72chevytrucks.com/vboard/attachment.php?attachmentid=895550&d=1332518885


And here:

http://forum.73-87chevytrucks.com/smforum/index.php?topic=25339.msg211241#msg211241
 
The color of the resistor indicates the resistance value.

You can remove it, and use a multimeter to make sure it's still "resistive" and not open.
 
I realized that that fuel gauge is not for my year Blazer...I went ahead and ordered a new one from Classic parts...sometimes hard to troubleshoot with 44 year old parts.
Could be that mine does or does not require the resistor on the gauge.
 
Looking at that fuel sending unit, there isn't a lot to go wrong...so I am taking the advice above and going to try the dash gauge replacement before I drop the tank. I know that it floats because it goes up to 1/4 and down, and with my test of shaking the tank with hard figure 8's in the parking lot it floated up to 1/2 tank. My Blazer has had the wrong fuel gauge in it the whole time...it has a newer model...not sure which year, but after looking at all the resellers, the 73-75 have a different gauge.
That said, I don't know if the tank and sending unit are 1974....hard to know these things on a truck that old...who changed what over the years.
 
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