Reading / Calibrating late Bay fuel gauge

[SGKent]

Does anyone have a late bay (1976 or later) with the 211-957-063B 0003 gauge still intact with ALL original factory installed parts - sender, vibrator and gauge like the one below?

If so do you know how many gallons each line represents?

The owner's manual says that the tank holds 14.6 gallons, with the reserve being 1 gallon, but the way the lines are, it appears that reserve is about 2 gallons.

Using the bottom line as empty, then the marks would be:
1/1 = 14.6 gallons
1/2 = 7.30 gallons
1/4 = 3.65 gallons
Reserve ?
0/0 = 0 gallons

Red (Reserve) should be a little more than 1/8 (= 1.83) gallons. Book says 1 gallon but the gauge appears to be about 2 gallons so best guess is the E line leaves 2 gallons and the book is wrong.

However if one approaches the gauge as the tank has a total of 12.6 gallons plus 2 gallons reserve = 14.6 total, then the math changes for each line.

If someone has a totally original stock tank, sender, gauge and vibrator, then we could compare against their numbers.

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[busdaddy]

Do consider the guys who designed that were thinking in metric, I'll bet the divisions make more sense when not converted to gallons.

When I have doubts I drain all the gas and measure it as I put it back in noting what the gauge says every couple gallons.
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[SGKent]

[Buenos Diaz]

[buckswilde]

I thought late bays are 15.9 gallons. Pretty sure that's true of 78 and 79 at least.

[buckswilde]

I just looked at all of the manuals posted in the technical section for late bays and it says 14.6, but I swear my manual for my 79 says 15.9 gallons. I'll double-check once I can find it. Now I think my brain is getting old.

[buckswilde]

Okay, I'm getting this from the Bently Offical Service Manual - All years are 15.8 gallons total, including reserve.

[bsairhead]

[SGKent]

I actually looked in the 77 manual that I have here at the desk. Below is also a jpg from the manual online here. It reads 14.6 US Gal with Reserve being 1 gallon of the total capacity. Since we can't buy senders that actually read right we have to calibrate them ourselves. Worst case I can put in 2 gallons and see where it reads, then add 4 more and see if it is about 1/2.

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"Most people don't know what they're doing, and a lot of them are really good at it." - George Carlin

[richparker]

Early bays are 15.9 gallons.
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[busdaddy]

Here's a rabbit hole for you: https://www.thesamba.com/vw/forum/viewtopic.php?t=711130&highlight=calibrate
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[my59]

Wow, I have to admire the desire for accuracy and the time expended.

Given the 59 beetle came with a decades old notebook with mileage and gallons added, when I got the bus I simply instituted the same process. This was before I knew that the bus gauge reads empty after the first 60 miles or so.

SGkent, rock on!
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my59: Well son, my grandfather died before I got to drive it, so does that answer your question?
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[SGKent]

[telford dorr]

If you really really really really want an accurate fuel gauge, there's a way to do it (but it's not trivial...)

Use a microcontroller (like an AtTiny85).

This requires that the fuel sender and gas gauge indicator are repeatable (always giving the same resistance reading for a given fuel level, and indicating the same fuel level for a given amount of gauge current). These values must be mathematically "well-defined".

You create a circuit to pass a fixed current through the fuel sender, and measure the resulting voltage with the microcontroller A/D converter. You power the gas gauge with a fixed voltage and drive it with the Pulse Width Modulated output of the microcontroller. You select the sender current to use the maximum range of the A/D converter, and the gauge voltage to use the maximum range of the PWM drive.

In software, you use a method known as piece-wise linear approximation to calculate the PWM output for any given reading from the A/D converter. This is done by finding the voltage readings for, say, 9 tank levels (empty, 1/8, 1/4, 3/8, 1/2, 5/8, 3/4, 7/8, and full. You find the pulse width needed to make the gas gauge read these levels, and put them in a look-up table. For any reading other than these fixed points, you assume that the value is linear between the known points, and so is the PWM drive. The error is usually insignificant.

Using this method, you can interface ANY gas gauge to ANY fuel sender. Also works for temperature and pressure gauges. It's how you make retrofit engines work with old gauges in old cars.

Takes a bit of work to do, but it works!

FWIW
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Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST)

[mikedjames]

Another way to measure is to put a low pressure gauge below the tank and measure the height of fuel by pressure.

Alternatively you can buy an ultrasonic pinger that measures the distance to the top of the fuel.

Both of these work by using a user-provided depth versus surface area model to get the volume of fuel above the pressure sender, or below the ultrasonic sender.

I have used both (made by Maretron) on boats, we rejected the ultrasonic sender because we had an single cylinder diesel engine running at 3000 rpm (generator) for 50Hz AC .. and the ultrasonic sender went wild unless we isolated it from the vibration.
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Ancient vehicles and vessels

1974 VW T2 : Devon Eurovette camper with 1641 DP T1 engine, Progressive carb, full flow oil cooler, EDIS crank timed ignition.
Engine 1: 40k miles (rocker shaft clip fell off), Engine 2: 30k miles (rebuild, dropped valve). Engine 3: a JK Preservation Parts "new" engine, aluminium case: 26k miles: new top end.
Gearbox rebuild 2021 by Bears.

1979 Westerly GK24 24 foot racer/cruiser yacht Forethought of Gosport.
1973 wooden Pacer sailing dinghy

[bsairhead]

I see the brain trust have spoken Fill till pump shuts off, drive till 1/2 tank, fill till pump shuts off. That and 5th grade math should be enough.

[SGKent]

[bsairhead]

As I am not sure about my gauge.This spring when I do my 5 year fuel line change I will note where 4 liters reads. I'am less than a 1/2 mile to my filling station and will take notes.

[metahacker]

here are a couple microcontroller "adapter boxes" to sit in between a sending unit and a gauge - particularly useful just for eliminating slosh on early bay, i'd imagine.. but basically let you read the sender actual reported range and then decide what values are actually sent to the gauge
https://www.tanksinc.com/index.cfm/page/ptype=product/product_id=748/mode=prod/prd748.htm
https://www.opgi.com/air-fuel-delivery/fuel-tanks/...23120.html
https://classicdash.com/product/universal-fuel-signal-adapter/
they're all pretty neat and the option for the low fuel light on some of them is rather cool IMO

here are some VW specific ones that include a new gauge movement/needle to be mounted on the original gauge face ... it uses a stepper motor for the movement (so it's all software positioned) and appears to have RGB LED to go amber and red as fuel is getting low
https://www.c-e-g.eu/product-categorie/gauges/fuel-gauges/
he also has an updated microcontroller driven OE style sending unit
(it's not listed on the site - you have to ask)
to order that Classic EVO stuff in the states u will need to message him
https://www.facebook.com/harold.fransen.3

telford's approach is the coolest, though :)

also, i believe the curves of the output created by original senders and _all_ replacement senders are also different?

[mikedjames]

The original late bay senders have a curved former under the resistance wire in the sender. So that each turn is a different length.. that produces best accuracy.

12 years ago I bought an aftermarket sender and it had three blocks of windings with a different pitch so it was close but not so perfect.

Last year I bought an aftermarket sender and it was wound in a linear fashion.. so all of the range of the gauge vanishes in about 100 miles then you are deep in the reserve for the next 100 miles or so on the gauge.
_________________
Ancient vehicles and vessels

1974 VW T2 : Devon Eurovette camper with 1641 DP T1 engine, Progressive carb, full flow oil cooler, EDIS crank timed ignition.
Engine 1: 40k miles (rocker shaft clip fell off), Engine 2: 30k miles (rebuild, dropped valve). Engine 3: a JK Preservation Parts "new" engine, aluminium case: 26k miles: new top end.
Gearbox rebuild 2021 by Bears.

1979 Westerly GK24 24 foot racer/cruiser yacht Forethought of Gosport.
1973 wooden Pacer sailing dinghy