Fuel gauge problem

[LST46]

I’m helping a friend diagnose a problem with the fuel gauge on his 74 Dart. When he fills it up the gauge goes to full but drops like a rock when he drives it. When it hits empty the tank will only hold about 5 gallons and it’s full again and the gauge is back to reading full. We tried a new sending unit and it does the exact same thing. I know the fuel sender for the 60’s cars would read about 10 ohms when full and about 73 ohms when empty and that’s what our sending unit reads. Is the sender the same on the newer A-body’s or did they change the ohmic value? If so my only guess at this point is the fuel gauge in the dash is screwy. Am I missing anything?

 

[Dana67Dart]

That description is exactly what ALL aftermarket senders do. Only an OEM sender or a meter match will fix it

Short answer... The tank is not a rectangle and the gauge requires a curve. 10 is full, 23 is 1/2, 73 is empty. The OEM senders are calibrated for the shape of the tank AND the curve for the gauge.

After market senders are linear , ALL OF THEM

 

[67Dart273]

There is a device called "metermatch" and several knock offs of that same thing, an adjustable electronic box that scales the readings to correct them. If you do a search here, there are a number of articles on the terrible state of affairs of "repop" fuel senders. The originals are not linear, nor are the gauges. The replacements are likely more linear, and simply not accurate to the gauge.

 

[Mike69cuda]

I installed a meter match and it works great. I built a calibration box to set the points on the meter match so you don’t have to remove and add gas to calibrate it. Can loan it to you if you decide to go that way.

 

[LST46]

That description is exactly what ALL aftermarket senders do. Only an OEM sender or a meter match will fix it

Short answer... The tank is not a rectangle and the gauge requires a curve. 10 is full, 23 is 1/2, 73 is empty. The OEM senders are calibrated for the shape of the tank AND the curve for the gauge.

After market senders are linear , ALL OF THEM

Ah… I didn’t think of that. When I tested it with my ohm meter I noticed it was linear and thought that was correct. Thanks

 

[LST46]

There is a device called "metermatch" and several knock offs of that same thing, an adjustable electronic box that scales the readings to correct them. If you do a search here, there are a number of articles on the terrible state of affairs of "repop" fuel senders. The originals are not linear, nor are the gauges. The replacements are likely more linear, and simply not accurate to the gauge.

Now that you mention that I remember hearing about it a few years back but it slipped through my aging memory. Thanks

 

[LST46]

I installed a meter match and it works great. I built a calibration box to set the points on the meter match so you don’t have to remove and add gas to calibrate it. Can loan it to you if you decide to go that way.

Thanks for the offer Mike. Rather than doing the two way shipping of borrowing yours and returning it if you’d happen to have a schematic for your calibration box I could build one for myself. I’m an electronics tech and have a whole basement full of parts

 

[Mike69cuda]

Thanks for the offer Mike. Rather than doing the two way shipping of borrowing yours and returning it if you’d happen to have a schematic for your calibration box I could build one for myself. I’m an electronics tech and have a whole basement full of parts

It is basically just a box with resistors in it to simulate the aftermarket sender. I will send you the info.

 

[Mike69cuda]

The resistor values are on the face of the box. These are the values I used to set the 4 points on the meter match.

I believe that you also need to have a solid state voltage regulator as well to get the meter match to work.

 

[67Dart273]

But how do ya know that your box matches each or every one of the repop senders? Or do you just use it for initial and then re-check by filling the tank in steps?

 

[Mike69cuda]

My theory is the repop senders are “relatively” consistent. Production tolerances should be good enough. I am not aiming for perfection. I figure that if I am within a gallon or two, that is good enough. I got the values off a chart made by Dana67dart if I remember correctly.

The proper way would probably be to drain the tank and add gas, but that is a real pain in the butt on many levels, so I took the lazy man’s way out.

I run the car to a half tank and then put gas in it and see if it takes a half tank, forgot exactly what that is as it has been a few years, but it was within about a gallon or so at different fill levels I tried.

 

[Dana67Dart]

I run the car to a half tank and then put gas in it and see if it takes a half tank, forgot exactly what that is as it has been a few years, but it was within about a gallon or so at different fill levels I tried

18 gallon tank

0 = empty
4.5 = 1/4
9= 1/2
13.5 = 3/4
18 = full

16 gallon tank
0 = empty
4 = 1/4
8= 1/2
12= 3/4
16 = full


At 12-16 mpg that's not allot of driving time from 1/4 to empty.

Also some pickups might not be in the optimal position. Mine has about 1.5 to 2 gallons in it when the sender sucks air.

 

[67Dart273]

AND NOW yet ANOTHER annoying story from the old days. Whenever it was that the "gas crunch" got going, 72, 73? It got pretty annoying in San Diego, stationed at NAS Miramar, BUT I got screwed / stuck on shore patrol in downtown San Diego, run by a JARHEAD, and he insisted on 12 hour shifts with no breaks for staff, we'd work 2-3 days 0500--1700, then get 2-3 days off and work 2 -3 days the opposing shift. I never enjoyed a good healthy crap or sleep all the several months I was there.

BUT WORSE was the fuel situation. The base filling station at Miramar was ALWAYS busy with a long line, and I still worked part time out there at the Auto Hobby Shop.

The commercial stations, you'd be down in the freeway cut, and you'd see a giant whatever, Shell/ Chevron/ Texaco, sign all lit up and round and round, so you'd take the exit and not ONLY were they out of gas, but the station was all locked up WITH THE SIGN STILL LIT UP.

Anyhow, I bought a brand new Johnson/Evinrude / OMC fuel tank, with the mounting kit, and screwed it down and strapped it down in the trunk. Modified the vent with a hose "overboard." Installed a second fuel pump on a toggle and plumed it into the tank. So when the main got low, I'd just turn on the spare pump and pump the spare tank into the main. I could tell "it was done" because this was a loud old pulser. The main tank had one of those great big solid state pumps, I don't remember the name.

THERE WERE TWO OR THREE TIMES in those days, when I managed to pull into a working station, and BOTH FUEL PUMPS WERE rattling for air!!! I never had to push that thing, but it HAD to be close!!!

 

[LST46]

The resistor values are on the face of the box. These are the values I used to set the 4 points on the meter match.

I believe that you also need to have a solid state voltage regulator as well to get the meter match to work.

View attachment 1716441918

View attachment 1716441919

Thanks Mike. That should be easy enough to build

 

[LST46]

AND NOW yet ANOTHER annoying story from the old days. Whenever it was that the "gas crunch" got going, 72, 73? It got pretty annoying in San Diego, stationed at NAS Miramar, BUT I got screwed / stuck on shore patrol in downtown San Diego, run by a JARHEAD, and he insisted on 12 hour shifts with no breaks for staff, we'd work 2-3 days 0500--1700, then get 2-3 days off and work 2 -3 days the opposing shift. I never enjoyed a good healthy crap or sleep all the several months I was there.

BUT WORSE was the fuel situation. The base filling station at Miramar was ALWAYS busy with a long line, and I still worked part time out there at the Auto Hobby Shop.

The commercial stations, you'd be down in the freeway cut, and you'd see a giant whatever, Shell/ Chevron/ Texaco, sign all lit up and round and round, so you'd take the exit and not ONLY were they out of gas, but the station was all locked up WITH THE SIGN STILL LIT UP.

Anyhow, I bought a brand new Johnson/Evinrude / OMC fuel tank, with the mounting kit, and screwed it down and strapped it down in the trunk. Modified the vent with a hose "overboard." Installed a second fuel pump on a toggle and plumed it into the tank. So when the main got low, I'd just turn on the spare pump and pump the spare tank into the main. I could tell "it was done" because this was a loud old pulser. The main tank had one of those great big solid state pumps, I don't remember the name.

THERE WERE TWO OR THREE TIMES in those days, when I managed to pull into a working station, and BOTH FUEL PUMPS WERE rattling for air!!! I never had to push that thing, but it HAD to be close!!!

lol… when I was 16 I bought a 66 Buick that not only the gas gauge didn’t work on but neither did the trip odometer so it wasn’t easy to keep track of the fuel level and being a kid I never had enough money to keep it full. After running out of gas twice and having to walk to get gas I put my old 10 speed and a gas can in the trunk so when I ran out of gas getting to the gas station and back was a lot faster

 

[LST46]

I appreciate the input from everyone. You all have a great day