Super Beetle Fuel Gauge Fix

[l3illN]

I have been attempting to get my 1972 Super Beetle's fuel gauge working since acquiring the car two years ago. A set of aftermarket parts were in the car, but the sender and vibrator were fried. Like most people looking to fix this I ordered up a set of aftermarket parts, and found that they not only vary in quality and function but do not actually work when you put them in the car.

Searching led me to lots of posts which led me to Samba member Blue69Baja who I PM'ed for more info. A sampling of threads he contributed to can be found below. He asked in a PM for an update so here it is.

I started with a Dansk sender and an unknown gauge, like Blue69Baja says in the threads, the sender is not good. Then I bought a Mexican sender and it is better - but only if you use the adjustable voltage regulator as Blue69Baja describes. Even with this creative fix the system still isn't accurate from E to 1/1. The main reason I can find in my measurements is that the currently available senders and gauges have a linear response to the input (level of gas in tank, current in sender/gauge circuit) and the SB tank from E to 1/1 is not linear. I wanted to see if I could have an accurate system. I have some basic skills and time so I decided the best way to make it accurate is to break the link between the sender and the gauge.

To accomplish this I built a board centered around an Adafruit Trinket M0 processer board. The board uses a 6V voltage regulator for power, for the sender I have a simple voltage divider to determine the resistance. The gauge position is determined by a PWM signal fed from the Trinket to a MOSFET. I calibrate the system at five points; E (empty tank), R (1.1 gallons), 1/4 (3.6 gallons), 1/2 (6.1 gallons) and 1/1 (11.1 gallons). Calibration consists of reading the measured resistance of the sender and determining the PWM value for the gauge at each point. The Trinket has a USB interface so communicating with the device is easy (but complicated, moving gas around, changing values for the gauge).
Once I had the information I found that the system response was linear from E to 1/4, then differently linear from 1/4 to 1/1. The Trinket is programmed with a package called CircuitPython which is lightweight and easy to use (if you know these things, think easier than Arduino). The code that runs on the board reads the resistance of the sender, than uses one of two line fits to send a signal to set the gauge. It does this 10 times a second. The code starts to run when the ignition key is turned on.
Once I got the prototype board working I made a PCB that plugs right onto the back of the gauge. There are spade connections for the sender, power (same as the vibrator) and a ground. Pictures below.
I know this is total overkill, but I like projects like this and I now have a gauge that is accurate through the whole range.

Thanks to Blue69Baja.

https://www.thesamba.com/vw/forum/viewtopic.php?t=726138&highlight=
https://www.thesamba.com/vw/forum/viewtopic.php?t=795864&highlight=
https://www.thesamba.com/vw/forum/viewtopic.php?t=792209&highlight=
https://www.thesamba.com/vw/forum/viewtopic.php?t=791753&highlight=

[baldessariclan]

Nice job!! You ought to look in to replicating and marketing that unit, if it's feasible & cost-effective. Could be a potential total boon for those people stuck w/ having to rely on aftermarket fuel gauges, senders, vibrators etc.
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[vamram]

This is awesome. Am I understanding correctly that this is an add-on and the Blue69Baja's fix is a pre-req for yours?
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[Blue69Baja]

What a project! Your correct that even the Mex sender is a bit off but is the best.

All the vendors selling the aftermarket gauge, vibrator and sender are not interested in fixing this problem nor is the folks making them.
Sad!

Jim.
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[busdaddy]

Nice work!
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[heimlich]

Wow. Send me the design and I can look at having it manufactured.
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[l3illN]

VamRam

This is separate from the buck transformer fix that Blue69Baja worked out. The transformer is MUCH easier to implement and gives you a good reading (with the right parts) from 1/4 or 1/2 to E - where is really matters.

This is a separate way that, with my limited experience, gives proper readings through the whole tank. It also might be interesting to people with aftermarket or custom tanks.

[heimlich]

I like it for the programmability. This would solve a huge problem with aftermarket senders.
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Not all parts are made the same. NOS OE/OEM parts made mainly in West Germany, Early Germany, and Early Brazil are where VW produced the best quality parts and best fitting products.
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[l3illN]

heimlich,

I think the calibration process requires too much fiddly work right now (but maybe I am wrong, VW owners do a lot of fiddly stuff). BUT, I think you could have a product with this if you could bench calibrate it, which should work for standard tanks, you just need to know the height of the gas relative to the gauge for the levels marked on the gauge. You should be able to then sell a set of sender + gauge + board that would actually work when someone put it into their car.

PM me if you would like more details.

If anyone wants to help out with this by working through an installation I have a few more boards. Since this has only been put in one car I would need to see if the data is the same for other setups (super beetle, std, bus??).

[heimlich]

Tell me if I am wrong but the way I see it is you sell it and tell the person to empty their fuel tank. They then put X gallons in it - set it. Then X more gallons and set it, etc etc. Seems straight forward to me.

Can you put 4 buttons on the back? Each of those would correspond to empty, 1/4, 1/2, 3/4 and FULL.
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[l3illN]

It could be made to have an input to calibrate the sender (fuel level). Calibrating the gauge is more difficult as you need to adjust a value that regulates how much current goes to the gauge to achieve the temperature change that takes the gauge to the different marks. The response of the gauge is slow so you really couldn't have the value increase and get an input when the gauge is at he desired mark.
I have two gauges that have two difference resistances (27.1 and 30.6 ohms), these have a ratio of 1.3 across all the values. Unfortunately, this doesn't correspond to the ratio of the resistance. I think the resistance change is due to a change in the length of the bimetalic strip which indicates the gauges function a little different and the calibration could not be completed with a single resistance measurement - but maybe with a single current-value measurement rather than five.

[heimlich]

On the back of the gauge there is a little screw. You can turn that to make the gauge move very small amounts.
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[volksworld]

nice job...a couple years ago I also found the Dansk sender to be worthless and the no name mexican one worked...but I went through several used fuel gauges before I found one that played well with it...

[beetlenut]

Don't think that board would fit 73+ gauges since they have to come out the front of the dash. Would be nice to have a unit that would go in between the fuel sender's terminals, and the wires that go up into the back of the dash to the vibrator.
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[DUNGBTL]

I’m impressed with the creativity and ‘maker’ skills that went into this!

For the DUNGBTL build I was going to use vintage Porsche gauges. The tach and fuel gauge were areas of concern. The tach because of the Subaru EJ25 engine and the fuel gauge because…VW sending unit/Porsche fuel gauge compatibility.

Probably not a soln for folks here, however I’ve decided to use Speedhut programmable gauges to bypass these issues. From what I understand, the fuel gauge is programable to the fuel tank sending unit’s resistance (ohms) range, full to empty…which can be found by bench testing the vw sending unit.

The Bentley manual’s factory values are in ‘()’ in the pic above.

Thinking out loud here and probably not a cheap soln, however perhaps one could modify a Speedhut fuel gauge to work w/an oem vw gauge…using the electronics. Or contact Speedhut and work with them to get a hack.

My $0.02,
J.

[l3illN]

Hmm, Beetlenut, I don't know what the later models looks like. Do you have any pictures?
Dungbtl, this is likely the same sort of control. Do you know it the gauge can do a "non-linear" calibration?

[DUNGBTL]

[LeeFred]

This is also a common issue with 1st Generation Mustangs in that the aftermarket sending units are linear but the OEM units were non-linear due to the shape of the gas tanks.

As I read this thread, I saw the familiar words of linear, non-linear, gauge inaccuracy, etc. I don't know if this would work, but the work-around in the Mustang world is the MeterMatch Analog Gauge Converter. It works similar to what l3illn has created. It fits between the fuel sender and the gauge and the user calibrates it based on how much fuel is put in the tank. When I installed it on my Mustang, I had calibration points at E, 1/4, 1/2, 3/4 and F.

I believe it will work for the VWs because both systems (Mustang and VW) have 5V used for the gauges.

Here's a link to check it out:

https://www.technoversions.com/MeterMatch.html

[beetlenut]

[l3illN]

Beetlenut, the board isn't that much bigger than the vibrator it replaces. It is certainly not much taller, and it is 'flatter', but it is wider.