Fuel Gauge Issues

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Jesse Martin

Jesse Martin

Title Don't need no stinkin title!
Joined
May 28, 2017
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Location
Corona CA
On my 72 Scamp, I'm having issues with the fuel gauge. The issue is that the gauge will only go up to around 1/8 of a tank while there is a little over 10 gallons. The Sending unit is brand new from OER and Is putting out around 40 Ohms. I tried a different dash I had laying around and the fuel gauge worked so I swapped that one out and then bam the same problem but now on both gauges, so then I suspected it may be the Voltage limiter and that it fried the gauge, so new gauge and limiter and it is still having the same problem but now the gauge goes to the correct level then drops back down to 1/8 tank. What could be causing this? Here is a video of the issue.

 
Dash isn't grounded properly. run a wire from the metal housing to one of the studs that the steering column mounts to
 
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Are you saying you measured the sender resistance with that amount of fuel in the tank and it is 40 ohms? That is not correct.

Here is an aftermarket copy of the factory gauge tester. It is nothing more than a 3 position switch with 3 resistors, added by someone else in yellow. That is empty, 1/2, and full scale for either temp, fuel, or oil (rally) gauges

c-3826-jpg-jpg.jpg


With about 1/2 tank of fuel, then, you should show somewhere in the range of 23 ohms

Does your temp gauge work OK? That would indicate the IVR is working

There are several problems that can occur with gauges.........

1--Power is fed to the cluster so the pin at the connector could be loose/ corroded

2--(As mentioned, the senders must be grounded AND THE CLUSTER must be grounded. The were originally grounded through the mounting screws,----a poor deal---add a ground pigtail to the cluster and bolt to the dash column mount)

3--The IVR can be bad/ out of calibration. Again if the temp gauge works............

4--The IVR contacts "socket" in the PC board can be not making contact with the PC board traces

5--The studs/ nuts on the gauge connections might be loose at the pc board and not making contact

6--The gauge itself might be out of cal or have other problems

7--There might be a bad connection in the wiring harness. The gauge sender wire goes through the left kickpanel connector (on the way to the rear) as well as the sender connector could be loosed/ corroded/ intermittent

8--I know you say you checked, but just to be complete, the sender might not be grounded or MIGHT HAVE AN INTERMITTENT ground
 
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Thanks for the info, I thought half tank would be 72-10÷2=32 ohms. I guess I'll pull the sender and check that.
 
Do you have a ground from tank/sender to body?
I doubt the sender is the issue. My bet is the ivr and / or ground some where or bad connection. My reasoning is the guage rising than falling = an increase in resistance after key turned
 
Does sound like a ground issue. Have you taken the wire off the sender and grounded it with the ignition on? It should go to full and stay there.
 
Yeah when the wire is grounded it shoots straight to full. As for dash ground I have a power probe which is connected to the battery that allows me to ground whatever it touches. I think it is the sender, I had hoped that getting a $100 one from Classic industries would be right but I guess I'll have to mess with it to get it reading correctly l.
 
Sorry to say but the re-pops are pretty much not accurate. As long as electrically your system is ok, you can bend the float arm, lengthen it, shorten it , do what ever and it might be close but not correct like the OEM sender was.
 
Jesse Martin said:
Thanks for the info, I thought half tank would be 72-10÷2=32 ohms. I guess I'll pull the sender and check that.
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That is incorrect too. The range is 80-10. Calculator says half is 45 but... that's not how they work. I'll have to get a bit technical,,, The tank is not symmetrical so half of its volume is not half of the senders travel. As the nichrome winding inside a thermal gauge heats up, it's per inch/total resistance changes. This is why approx' 73 ohms will get up to the empty hash mark, then approx' 34 ohms signal is approx' 1/4, approx' 23 ohms signal is approx' half. Approx' 14 ohms is approx' 3/4. approx' 10 ohms is full. see how 1 ohm wouldn't make much difference in the cooler end of the range but that same 1 ohm would make noticeable difference in the hot end of the range? Anyway...
The aftermarket senders are linear ( like the calculator says ) so they do not work correctly. The meter match module suggested above does help correct it
Because the gauge needle goes up then falls back again, is see 2 separate problems. I want to ignore all the above for now and ask,,, New mechanical limiter or new solid state regulator?
A faulty mechanical limiter can produce the needle swings you have.
 
It's a new mechanical limiter, a reproduction of the original. It seems to only do the jumping with the new gauge both of the others just go straight to 1/8 tank but those to move much slower than the new one. I'm thinking Dana67Dart is right and it's just the sending unit. I ended up taking apart the original sender and cleaning all of the contacts and it began to work. What I may do is swap the sending unit and float over to the newer 3/8 pickup.
 
Jesse Martin said:
It's a new mechanical limiter, a reproduction of the original. It seems to only do the jumping with the new gauge both of the others just go straight to 1/8 tank but those to move much slower than the new one. I'm thinking Dana67Dart is right and it's just the sending unit. I ended up taking apart the original sender and cleaning all of the contacts and it began to work. What I may do is swap the sending unit and float over to the newer 3/8 pickup.
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We have seen threads/reports concerning those aftermarket reproduction limiters. Those didn't work right either. Regardless, they just can't compare with a solid state regulator. Anywhere between 9 and 35 volts in gets a steady 5 volts out. No pulse, no variance, no needle swings.
So... After warming engine, temp gauge to proper reading, ignition switch off, a minute for needles to return home, then switch on again. If this produces a similar needle swing in temp gauge instead of the fuel gauge, or both needles swing, you'll know where the fault is. Good luck with it.
 
How about the ground strap at the sender. Just put a new gas tank in today and noticed that both my cars were missing the sender ground strap. google it
 
TF360 said:
How about the ground strap at the sender. Just put a new gas tank in today and noticed that both my cars were missing the sender ground strap. google it
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Yeah, that was the first thing I thought of, I solder a wire onto the pickup and used a tech screw to attach it to the body of the car.
 
Well, the gauge is now working. I decided to get the meter match and it fixed the problem, but because of my luck whenever working with electrical something happened and it burned up one of the traces on the board.
20181211_090318.jpg

I used a leg off of a resistor I had laying around from testing and used that as a jumper between the two points where the trace had burned away. Even though the power side of the board is now working the sender side of it is broken, so I had an idea of just tieing the output and input back together and to just use the board as an 8v power supply to the gauge and boom now it works perfectly.
20181210_165403 (1).jpg

So Redfish is right and It was the new mechanical voltage limiter. The Meter-Match board is now acting as a solid state limiter putting out a steady +8V and the gauge works perfectly. Even though the board is partially broken it still works as a limiter, so I'm going to keep it there. The biggest thing from this it that I bought a brand new voltage limiter from classic industries and it doesn't work, I wonder how many people this has affected, and if I had just received a correctly working part this problem would have been solved weeks ago.
 
8 volts is too much. The gauges should read a little high and may live a shorter life too.
 
Yup. There are projects "all over the internet" using a 5V regulator to build your own. While those has their own problems, there is a tremendous difference between 5 and 8V The originals pulsed like a flasher and provided about a 50% duty cycle, so they were +/- about 6V "equivalent" to DC (50% of 12)
 
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