faulty fuel reading

[lpblasch]

When I fill the thing, and I KNOW it's full because I can see the gas in the neck, the gage will only read about 2/3 full. After driving for a while, it rises to about 3/4, but never higher.
From that point, it seems to stay on 3/4 for some time, then gradually drop as it should. And by the time it reaches the red "reserve" sector, it is quite accurate.
Any thoughts on what could be the problem?

[Dougster43]

I have the same situation and did a lot of troubleshooting on it - it's somewhere in another thread but in short it seems that there is not enough range of motion for the float arm of the replica sending units within the tank.

I took the float mechanism out of the tank, and verified that the fuel gauge reads full when the float is all the way up and empty at the bottom of the stroke. If I recall properly, there is roughly a 9" differential in height of the float between those 2 levels. The tank also measured 9" deep so I thought I could fix it by bending the float arm to the point that the gauge would read "full" when the float was pressing against the top of the tank. While this worked, and the tank did indeed read full with a full tank of fuel, I found that the gauge only reads slightly less than 1/4 when the tank is empty (and when I ran out of gas...)

I put it back the way it was so it will at least read the empty position correctly and I just live with the 2/3-3/4 reading on a full tank.

It seems that if the arm was shortened to allow for more range of swing that might fix the problem, but I decided that it is more trouble than it is worth and as long as it reads correctly at empty that is what matters.

I did take a suggestion from another Samba member (was it Mondshine?) and completely emptied the tank then put 2 gallons back in from a can to check the gauge reading. That way you know the gauge reading when you are getting low.

Good luck!!

[Joe 20]

Dougster, I had to laugh...I've done the same bending to the float in an old style toilet tank!
There has been talk before about these gauges being in-accurate. I agree, as long as it reads correctly when approaching empty it's ok. I generally never fill my tank much over 3/4 anyhow...even before the price of gas got so high.
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"Someday the world is gonna be populated by nothing but Volkswagens'" Corporal Klairvoy

[lpblasch]

Dougster-
Many thanks for the analysis. I am pretty sure it is accurate near empty because when it hits the red shaded area on the gage, I have about 1.5 gallons left, which seems about right.
I see Jbugs has a replacement float/sending unit for about $23 which they say fits our year Thing, so I might just try replacing my unit.

[Semper_Dad]

For what it's worth, I remember coming across a video explaining some issues with traditional "vibrators" or solid state "regulators" and the fuel gauge. Supposedly they are of different wattages and thus give different readings. I'll try an relocate the information. What really confuses thing is that the term "vibrator" is used for either.

OEM fuel gauge vibrator. (Photo from gallery)
Uses a bi-metallic contacts control voltage

Newer style fuel gauge voltage regulator. (Photo from gallery)
Uses a Zener diode to control voltage. Anybody have a spare to dissect?

EDIT: Found it:

Link


Calibration:

Link

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Walküre Restoration Thread: https://www.thesamba.com/vw/forum/viewtopic.php?t=737492

[mondshine]

Of all of the air-cooled VW's I have owned (many), none had a fuel gauge that read anything close to linear.
My fuel gauge stays above full for 50 miles. After another 50 miles, it reads 3/4 + a little. Then, ZOOM! Below 1/4 in about 75 more miles.

But anyway... There is some adjustability for the range of travel of the float arm.
It may be possible to bend the tab which limits the upward travel of the float arm so that the correct Ohm reading for a full tank (10-12 Ω) can be obtained.

If you were to measure out 2 gallons of gas, and add it to an empty tank, you would have some idea of where your gauge reads with 2 gallons left in the tank.
Then drive to a nearby gas station (that isn't crowded) and add 2 gallons at a time, pausing to make note of the gauge reading. Give it half-a-minute to stabilize.
Repeat this until the tank is full, and you will have a good idea of where 2, 4, 6, 8 gallons, and full reads on your gauge in your car.

On long road trips, I carry a 10L NATO gas can, in case I get careless between fuel stops .

Good luck, Mondshine

[Semper_Dad]

I use 150 miles max range when full as a guide when planning. I do carry several 2.5 litter jerry cans for trips (and insurance). IMHO, the 2.5s fit better and are easier to handle.

I picked up the 2.5 on Ebay. Belgium if I remember. Not Chinese.

The 81MM mortar box houses road spares, tools and liquids.

Usually everything covered by a tarp.

EDIT:
Found this additional video explaining the components of the fuel monitoring system. Uses a Super Beetle but everything is relevant except the sender.


Link

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Walküre Restoration Thread: https://www.thesamba.com/vw/forum/viewtopic.php?t=737492

[Dougster43]