CK5
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One Millionth Post on Fuel Gauge Not Reading Correctly - 1974 stock
- Thread starter Chief Brody
- Start date
78K5BlazerTJ
Registered Member
You may need to expand the funnel in your tank. Take the filter off and add some fuel hose so it can rest closer to the bottom of the tank...i doubt youll need to do this. Just a suggestion if nothing else works
rampage
3/4 ton status
Have you tried disconnecting the sending unit plug near the tank - with it disconnected the gauge should peg one way, and then short the sending unit wire (purple wire, at least on my ‘89) to ground should peg it in the other direction. This should tell you if the problem is the sending unit or gauge.
Disconnecting pegs to 3 o'clock ... nothing I've tried brings it back the other way. But it does fall on it's own from 1/4 to E just hooked up normally.
A new gauge is going in when I get it anyway, as I want the "correct" looking gauge in there...hopefully that solves the problem.
Now that I think about it, I don't think my fuel gauge has ever worked properly....
rampage
3/4 ton status
Just did a little research, 3 O’clock is with it open and shorted to bare metal on the frame is Empty. If you have some resistors or potentiometer in the 0-100 range you could connect that to the plug and see what the gauge reads. Also check the resistance reading of the sending unit to see what it reads.
78K5BlazerTJ
Registered Member
you can save yourself all this headache and just leave it.
Mine read 1/4 when full then 1/4 when it was 1/4 or dropping.
Fill up, track your miles.
I’ve read the guages were not meant to read exact, to prevent people from running out of gas. So when your gauge read empty there was still fuel in the tank. Furthermore it stayed on full longer than you would think it should.
We are all so used to “modern” fuel gauge reading and even those function roughly the same as the old ones to prevent what I wrote above..
Mine read 1/4 when full then 1/4 when it was 1/4 or dropping.
Fill up, track your miles.
I’ve read the guages were not meant to read exact, to prevent people from running out of gas. So when your gauge read empty there was still fuel in the tank. Furthermore it stayed on full longer than you would think it should.
We are all so used to “modern” fuel gauge reading and even those function roughly the same as the old ones to prevent what I wrote above..
A Mac tools test light (incandescent) plugged into disconected sender wire, will read 5/8th of a tank, works on all GM vehicles that era . Snap on light doesn't work not enough resistance.
I'm OCD...it has to work...doesn't have to be dead accurate, but it has to work...just like the clock in the dash...
I've heard this, particularly in regards to an old Cadillac setup, but I don't buy it for anything else.
The gauges are plenty accurate. They are all 0-90 ohm resistance (so same level of accuracy) and from what I've seen on the fuel and temp gauges, when setup properly, will be as accurate as they can be without being digital. GM spent time and money to make sure they were reading "correctly" (thus the various resistors) and they don't spend money if it's not important. But there is a lot to go wrong on the stock setup that can affect how well they work.
When talking about fuel tanks, it's important to remember that not many vehicles have tanks that are perfectly square or rectangular...on the K5 31 gallon tank for instance, the lower rear is angled, so there is less volume in the lower half of the tank than the top half. The float will drop faster as it gets closer to empty, even though your MPG remains the same. The taper isn't huge, but it's there.
78K5BlazerTJ
Registered Member
True enough. I do think accurate can be a bit relative. Relatively accurate enough to read full at full-ish then empty when getting close.
I get wanting it to read perfect as the OP wants, but ya cant have everything. I tracked my gauge issue down to the resistor and I linked plenty for him to read.
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78K5BlazerTJ
Registered Member
OP here is a step by step:
https://67-72chevytrucks.com/vboard/showthread.php?t=640615
Check the sender OHM reading if its correct its your resistor on the back of your gauge. Read this thread it will not get more detailed than this.
@Chief Brody
https://67-72chevytrucks.com/vboard/showthread.php?t=640615
Check the sender OHM reading if its correct its your resistor on the back of your gauge. Read this thread it will not get more detailed than this.
@Chief Brody
Thank you, that is a fantastic article on troubleshooting. The only thing I disagree with is checking the "sweep" of the sending unit FIRST. Replacing the gauge is not that big of a deal, and testing the resistor on the gauge is a good idea. I'm not a fan of dropping the gas tank if I don't have to...I'll pay for a new gauge as a trial so I don't have to do that, but I also stated that I currently don't have the correct gauge for my year Blazer...I want the correct looking gauge. I also never said I wanted perfect readings...I just want to know when I am getting to empty without keeping a log book. I rarely let any of my vehicles get below 1/4 tank of gas ever. I'd like the needle to be near or beyond the full mark when full and I'd like to see the fuel diminishing in my tank so I don't run out.
I hope my new gauge fixes it. But if not, then the tank will come down (I suppose you could cut a window in the bed looking right down on top of it).
I run out in the woods for long periods of time, and for me, knowing empty is empty, at least, is important. I'd prefer sweep to match the gauge F and E marks, but when E isn't E, and F isn't F, its difficult to know where your really sit in terms of fuel and range.
Especially not being a daily driver where I could crutch the problem by learning the relative position of the gauge.
Especially not being a daily driver where I could crutch the problem by learning the relative position of the gauge.
I had a custom dash in my '77 GMC the former owner made,similar to a stock car,made of aluminum and it had a Sun tach,Stewart-Warner oil,water temp,fuel and vaccum gauges in the factory location,and everything but one worked great--the fuel gauge ,it was evidently made to work on a Ford or Mopar,the "ohms" was wrong..
The gauge was very accurate,but with one catch--it read FULL when it was Empty,and vice versa!...I tried swapping the wires around and it threatened to go up in smoke--so I lived with it,but it took some doing to remember it was "backwards"..
My solution to this dilema was to peel off the little stick on letters that came on the label for a blank VCR tape,and I stuck an "E" over the "F and and "F" over the "E"...fixed !..
The gauge was very accurate,but with one catch--it read FULL when it was Empty,and vice versa!...I tried swapping the wires around and it threatened to go up in smoke--so I lived with it,but it took some doing to remember it was "backwards"..
My solution to this dilema was to peel off the little stick on letters that came on the label for a blank VCR tape,and I stuck an "E" over the "F and and "F" over the "E"...fixed !..
chulisohombre
1/2 ton status
Check the ground as well. There should be a ground coming from the sending unit to the rear crossmember. Then the other wire goes all the way to the gauge. My fuel gauge didn’t work either and it was because the ground had fallen off to the cross member. Also make sure the gauge cluster is properly grounded as well. I ran a new ground to the firewall and it fixed my other gauge issues. All the gauges were bouncing the needle with the blinkers running before that.
It's almost a guarantee that the gauge is going to fix it...I got a good feeling about it.
ummmmm NO
Known good gauge still reads 1/4 tank
Known good gauge still reads 1/4 tank
