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mondshine
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PostPosted: Fri Nov 11, 2022 3:06 pm    Post subject: MeterMatch Reply with quote

Here is something that I posted on my local VW club's facebook page.
I won't complete this project until next spring, when I fill the Thing's fuel tank with fresh gas, but it might interest some people here.

Yes, he is crazy!

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Here’s a little project that I have considered for years.
The non-linear behavior of the fuel gauges on every ACVW that I have ever owned was always mildly annoying, but never enough to make me try to fix it.
I am convinced that this characteristic of the fuel gauge operation is based on the irregular shape of the fuel tank, where the bottom half of the tank holds considerably less volume than the top half.
So, with some time to kill this winter, I took on a project with an electronic device called a MeterMatch.
The MeterMatch interpolates output from resistive senders, and outputs a calibrated resistance averaged between four set points.
To start, I set up a “test rig” with a power supply, a fuel sender from my old 1302, and the fuel gauge from my old 181.
The 1302 sender reads 79Ω with the float at its lowest point, and 9Ω at the upper limit.
I expect to see some variation from these readings when the actual “in the car” sender and gauge are in substituted, but this “test rig” is a good starting point.
The testing went just as expected, so I am ready to do the actual installation.

So anyway, my plan is to do the actual “in the car” installation and calibration next spring, when I am ready to fill the fuel tank with fresh gas.
First, I will drain the fuel tank. My Thing’s fuel tank is said to hold 40L. Making my measurements in metric will keep the arithmetic simple.
Next, I will add measured amounts of fuel for each set point.
My four set points will be 5L, 10L, 20L, and 40L.
I will pour in 5L of fuel, and using the MeterMatch, set the fuel gauge to read “top of the red”.
Then pour in another 5L, and use the MeterMatch to set the fuel gauge to read 1/4 tank.
Another 10L for 1/2 tank, and another 20L for full.
The MeterMatch unit has a provision for a low set point alarm, which I intend to set at 7.5L.
The alarm provision will flash an existing LED indicator already in place on the dashboard.
As an explanation of the wiring diagram…
My Accusump has an additional oil pressure sender which provides a path to ground when engine oil pressure is being provided by the Accusump (rather than by the oil pump). There is also a piezo buzzer which will sound under the same conditions. System voltage is sent to the piezo buzzer through a low voltage cutoff device, so that the buzzer can only sound when the engine is running (and therefor, system voltage is at or above 13.2V). All of this is already in place (before the MeterMatch installation).
The MeterMatch device can be set to provide a pulsing (or steady if desired) path to ground for a low fuel warning light. My intention is to set the MeterMatch device to pulse the Accusump warning light when the fuel level drops to ~7.5L (2 Gal.). The diode will prevent the piezo buzzer from sounding with the "low fuel" warning.
So, in use, when the Accusump warning light is flashing, it indicates low fuel.
When the Accusump warning light is on steady, and the piezo buzzer is sounding, it indicates that engine oil pressure is being provided by the Accusump (rather than by the oil pump).

Just 133 days to go until the first day of spring 2023, and Springtime is Thingtime!

Here's the manufacturer's video:

Link


Good luck, Mondshine
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Joe 20
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PostPosted: Fri Nov 11, 2022 8:27 pm    Post subject: Re: MeterMatch Reply with quote

you're not crazy...just way smarter than me!
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Semper_Dad
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PostPosted: Sat Nov 12, 2022 9:34 am    Post subject: Re: MeterMatch Reply with quote

Joe 20 wrote:
you're not crazy...just way smarter than me!


...than us. Laughing
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KAmes
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PostPosted: Sun Nov 13, 2022 9:15 pm    Post subject: Re: MeterMatch Reply with quote

And here I've been pondering leaving the fuel gauge out of my dune buggy project altogether. Anxious
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SilverThing Premium Member
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PostPosted: Sun Dec 04, 2022 5:27 pm    Post subject: Re: MeterMatch Reply with quote

I'm interested to see how that works.

My VW gas gauges have generally been pretty accurate in terms of half, quarter, and reserve. I've only had issues with my 1973 Thing when it has suddenly stopped going below half. Both times this happened I ran out of gas on the same block as a gas station, thankfully.
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mondshine
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PostPosted: Sun Dec 04, 2022 6:41 pm    Post subject: Re: MeterMatch Reply with quote

As I understand it...
The voltage to the fuel gauge is un-regulated.
System voltage can range from 12.6V with a fully charged battery and engine off, to ~14V with the engine running (depending on other loads; lights, wipers, etc.).
The stock fuel gauge vibrator is not a true voltage regulator, but rather a thermally actuated switching device which toggles system voltage to the gauge on-off-on-off two or three times per second.
So, over time, the average voltage to the gauge is about 5V. Later model (1980’s water cooled) VW’s used a solid state regulator chip to power a very similar type of fuel gauge (and water temp gauge) at 5V.
The gauge itself is also thermally actuated; where a heating element deflects a bi-metallic strip to move the needle.
All of this serves as sort of a damping mechanism so that the gauge needle isn’t flopping around on bumpy roads with a partially filled fuel tank.

On the ground side, the fuel sender provides a path to ground with varying resistance based on the fuel level; from about 80Ω empty to about 10Ω full.
The Ohm readings that I observed from a (1302) fuel sender indicate that over distance of float travel, the resistance is fairly linear, but the float travel in inches does not reflect fuel volume, based on the shape of the fuel tank.
In other words, with an oddly shaped fuel tank, fuel level and fuel volume are not congruous, in that one inch of float travel at the top of the tank might represent two gallons of fuel, but that same inch of float travel at the bottom of the tank might represent less than a gallon.


So, I have good reason to believe that this MeterMatch thing will allow me to calibrate the fuel gauge based on fuel volume, rather than by fuel level.

Meanwhile, I have sketched a revised wiring diagram which includes a 5VDC regulated power supply to the fuel gauge. There's some extra stuff in the diagram, but it should be understandable.
Image may have been reduced in size. Click image to view fullscreen.


Not much more to add; it will be at least four months until it's time for fresh gas and top-down motoring.

All of this, just to solve a problem which is only mildly annoying, but it's good clean fun.
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KAmes
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PostPosted: Mon Dec 05, 2022 8:20 am    Post subject: Re: MeterMatch Reply with quote

So are you planning to use the mentioned later vw 5 volt regulator or something else? Despite my earlier comment I'm also interested in seeing how this turns out, I can think of several uses for such a thing.
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mondshine
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PostPosted: Mon Dec 05, 2022 8:36 am    Post subject: Re: MeterMatch Reply with quote

Kevin-
This is the buck converter I intend to use:
https://www.amazon.com/dp/B07P663XJV?psc=1&ref=ppx_yo2ov_dt_b_product_details

It's about the same size as the standard vibrator, and could be mounted on the back of the speedometer case like the original.

I probably could use a smaller capacity converter (it would have been physically smaller too), but this one worked nicely on my "test rig".

These inexpensive buck converters are more efficient than a single regulator chip (like VW used in the '80's), so heat from the chip is not an issue.

Another potentially useful item is the low voltage cutoff gizmo.
This is an easy and inexpensive way to power a device only when the engine is actually running.

Anyway, I will post again when this project is finished.
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