Fuel Gauge Accuracy

[Max Welton]

Having solved some of the more serious problems, I'm dealing with some small stuff.

My fuel gauge is inaccurate. Today I gassed up when the needle was half way between R and 1/4 and it only took 5 gallons.

This is what it reads when full (minus a 20 mile ride home).

I'm used to going by miles traveled since my MPG is known as ~25. But I would like to make the gauge more accurate. Any Ideas?

1967(ish) type-3 Fastback.

Max
_________________
1967 Type-3 Fastback
Under the Knife https://shoptalkforums.com/viewtopic.php?f=24&t=151582
Home Stretch https://www.thesamba.com/vw/forum/viewtopic.php?t=767836

[KTPhil]

Start with grounds.

The stock gauge grounding is via the two spring arms that hold it in the pod, and this is not reliable. Consider running an additional (try to use brown) ground wire from the gauge body to the behind-dash ground tabs.

Next ground fault is the sender, which grounds through a mounting screw, and through the tank to the mounting tabs, also unreliable. Add an eyelet lug under one of the sender mounting screws and run the wire to a nearby body ground.

And tighten the female connectors (maybe replace them with a new, factory-style double crimp connector, or at least a properly-soldered one). Add heat shrink to protect them from shorts.

You can check for ground faults with a grounded temporary jumper wire, and see if that moves the needle.

Last resort is a new sender, but some of these are junk, too.

Gauges seem pretty bulletproof.
_________________
Current Fleet:
- '71 Fastback
- '69 Westfalia
Retired:
- '67 Beetle
- '65 Beetle (x2)
- '65 Bus
- '71 Squareback

[Max Welton]

I've never liked the idea of relying on clips and stuff for grounding. So as I've been reassembling the car, everything has gotten an actual ground wire. That's gauge pods, brake/turn/parking lights ... and the sender on the fuel tank.

Now, the sender is actually a variable resistor, right? Does more resistance mean a fuller tank or more empty?

Max
_________________
1967 Type-3 Fastback
Under the Knife https://shoptalkforums.com/viewtopic.php?f=24&t=151582
Home Stretch https://www.thesamba.com/vw/forum/viewtopic.php?t=767836

[sctbrd]

About 90 ohms empty and 0 ohms full. The float reduces the resistance of the wire in the float- as it rises, so less resistance as you fill it.

https://www.thesamba.com/vw/forum/viewtopic.php?p=8792057

[gt1953]

My fuel gauge reads full for some time then starts to drop. I did remove the sending unit when i got the vehicle, it sat for ten yrs. If you do remove the sending unit be careful up dissassembly. The wire are fragile. Agree with the gounding of the sendng unit. I spent a lot of tme repairing every ground connection i could find. I used a star washer on the ones I was able to locate. No electrical issues.
_________________
Volkswagen: We tune what we drive.
Numbers Matching VW's are getting harder to find. Source out the most Stock vehicle and keep that way. You will be glad you did.

72 type 1
72 Squareback
({59 Euro bug, 62, 63, 67, 68, 69, 73 type ones 68 & 69 type two, 68 Ghia all sold})

[Max Welton]

The grounds on this car are as good as I could make them. Wiring harness is new.

I suppose I could try another sender and see if I get lucky. I'll test mine with an ohm-meter first.

Max
_________________
1967 Type-3 Fastback
Under the Knife https://shoptalkforums.com/viewtopic.php?f=24&t=151582
Home Stretch https://www.thesamba.com/vw/forum/viewtopic.php?t=767836

[Zwitterkafer]

Grounds are crucial of course. But when grounding is poor, the readings tend to be erratic, before the eventual total failure. If your Full reading is consistently low but at the same spot, it could be that the float itself is sticking. If all else fails, remove the sender and turn it end for end a number of times, listening to the float as it slides along the resistor wires, and hits a stop at either end with a small clunk. If that does not happen, carefully open to inspect and find out why.
_________________
"Criticism comes easier than Craftsmanship"
- Zeuxius, 400 BC

[Bobnotch]

[gt1953]

Jim Adney rebuilt my speedometer, works great as does the trip meter. I wuld recomend him.
_________________
Volkswagen: We tune what we drive.
Numbers Matching VW's are getting harder to find. Source out the most Stock vehicle and keep that way. You will be glad you did.

72 type 1
72 Squareback
({59 Euro bug, 62, 63, 67, 68, 69, 73 type ones 68 & 69 type two, 68 Ghia all sold})

[Max Welton]

[sctbrd]

Nearly everywhere on fuel tank level info, resistance of the sender is the most talked about value. Makes sense, but...
Question about voltage coming out of the gauge to the sender:

What should the voltage read coming OUT of the gauge (to the sender)?

With 6 volts at the inlet of the gauge, I get 2.3 volts out to the sender.
With the gauge reading 1/4 tank, but I have about 1/2 tank in it (checked with sender out). Havent been able to fill it up all the way yet to check readings at FULL- Ill probably take the sender out and check its operation in a container.

Operation of the systems seems straight forward, especially after reading a bus thread:
https://www.thesamba.com/vw/forum/viewtopic.php?t=627043

The gauge itself has 63 ohm resistance across the G and + terminals, and 23 ohms across a coil of wire at one side of the gauge.
So, with that I would NOT expect 6 vdc at the sender connection.

With my 60 year old original wiring, would it be necessary to add a resistance in the circuit to calibrate the gauge to read correctly?
(yes, new wire would be best )
Or am I missing something?

Additional interesting info- found that the gauge itself will go to FULL with a 1.2 vdc battery connected across it, when it is removed.

[mdege]

This is the theory behind the gas gauge:

https://en.wikipedia.org/wiki/Wheatstone_bridge

measuring the Voltage across the sender does not mean much if you don't know the resistances of the other three resistors.

The good thing about this circuit is, that the supply voltage does not really matter. The instrument shows the difference between two voltage dividers which both are supplied by the same source. This also shows why good grounds are essential for the system to work right. Both grounds should have the same potential (voltage level). and that will only happen if you have clean grounds.
_________________
- Michael

1965 21F: Restoration of a former '65 firetruck
1963 Typ3 Notchback project
1988 Multivan Magnum 112i
1984 Standard: My son wanted a bug for his first car

[sctbrd]

Wheatstone bridge, that is buried in the back dust of my brain...
It's been a long time, 1988 Navy Electrician's school. Makes sense. Thanks.
I also came across this today, and with my actual readings and the voltage drops, it looks like my gauge is working fine.
I may still add a resistor to fine-tune the pointer.

[Bobnotch]

[Bobnotch]

Something else to keep in mind, your gauge might not be reading the bottom end correctly, but the top end is right. I've ran into that on other cars, where the top of the gauge goes further than the bottom (it drops quickly from half tank), but takes forever to get to half tank. Some cars are different too, and work the opposite way, meaning the top burns off quickly.
_________________
Bob 65 Notch S with Sunroof
71 Notch ...aka Krunchy; build pics here;
http://www.thesamba.com/vw/forum/viewtopic.php?t=249390 -been busy working
64 T-34 Ghia...aka Wolfie, under construction... http://www.thesamba.com/vw/forum/viewtopic.php?t=412120

[Max Welton]

Thanks Bob. I do carry a 1 gallon gas can and am using a hydramat pickup which is really good at scavenging that last little bit of fuel. When the weather cooperates (thunder bumpers today), I'll run the tank dry and know for sure what my range is. As long as I'm within 25 miles of a gas station (one gallon), I'm good.

This vid may give an idea of how much capacity I'm giving up.


Link


Max
_________________
1967 Type-3 Fastback
Under the Knife https://shoptalkforums.com/viewtopic.php?f=24&t=151582
Home Stretch https://www.thesamba.com/vw/forum/viewtopic.php?t=767836

[Bobnotch]

[Max Welton]

I guess we'll see. The mat is very light dry and winds up saturated with fuel so it isn't really displacing its entire volume. I took it out to change the mount a couple weeks ago and was surprised at the weight difference dry vs wet.

Btw, I replaced the gauge illumination bulbs with leds. I can see now. Maybe a little too white though.

So far I only have 535 miles in my log with an average of 26.99 mpg. Mixed driving including a trip to Boulder last week and one up to Littleton before that. Current tank has 192 miles and still a fair bit to go. I had the needle substantially lower than as shown and it only took 7.067 gallons.

Max
_________________
1967 Type-3 Fastback
Under the Knife https://shoptalkforums.com/viewtopic.php?f=24&t=151582
Home Stretch https://www.thesamba.com/vw/forum/viewtopic.php?t=767836

[Max Welton]

Something else I might try is getting the sender to sit lower in the tank. I could make a new gasket out of a sheet of thin nitrile. It seems to off more than that would change but who knows.

Max
_________________
1967 Type-3 Fastback
Under the Knife https://shoptalkforums.com/viewtopic.php?f=24&t=151582
Home Stretch https://www.thesamba.com/vw/forum/viewtopic.php?t=767836

[Bobnotch]