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furgo
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Joined: September 06, 2016
Posts: 944
Location: Southern Germany
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 Posted: Tue Feb 06, 2018 8:51 am Post subject: Late Bay fuel sender calibration values |
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A while ago I calibrated my fuel sender with the fuel gauge, and while at it, I did some measurements and took some notes. Recently, a question about the Late Bay sender prompted me to take some additional readings and to put those notes into this post.
Note: as far as I know, neither VDO nor VW published the sender specs –other than perhaps specifying its min and max resistance. I'm sharing the values I measured as orientative measurements in the hope that they can help others to either calibrate or diagnose a bad sender. I do not claim them to be the nominal values from the manufacturer. Depending on the sender itself, the matching gauge and how well the gauge's voltage regulator steps down and regulates the battery voltage, the gauge reading might vary from bus to bus.
Here's a visual summary:
Here's the data:
| Code: |
Fuel tank level (l) Fuel tank level (gal) Fuel sender arm angle (°) Fuel height (mm) Fuel height (in) Fuel sender resistance (Ω) Fuel level status
------------------- --------------------- ------------------------- ---------------- ---------------- -------------------------- -----------------
56 14,8 60 200 7,9 10,1 Full
28 7,4 40 130 5,1 21,0 1/2
14 3,7 25 75 3,0 33,2 1/4
5 1,3 12 35 1,4 45,1 R
0 0,0 0 0 0,0 79,1 Empty
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Here are the diagrams:
In order to calibrate the sender, you'll need to perform two steps:
Electrical calibration
This is simply to ensure that the sender's max and min values of the resistance range correlate to the gauge's empty and full position.
Fig. 1: bending the stopper tabs
To see if you're in the ballpark:
1. Use a multimeter to measure the resistance at the lowest float position (empty tank): it should be around 77-80 Ω. The measurement can be easily done across the sender's terminals.
2. Then measure the resistance at the highest float position (full tank): it should be around 10 Ω.
3. If your measurements fall within that range, you're good to go. You can optionally double-check the 1/2, 1/4 and R points with the diagrams above, but that's usually not necessary. You can now jump to step 5.
4. If your measurements are off, by say +/-5 Ω, you'll probably need to readjust the sender. You do this simply by carefully bending the tabs that stop the travel of the float's arm (see Fig. 1). Bend the tabs as necessary and reiterate through steps 2 to 4 until the values are within spec.
5. For the best accuracy, you will want to test the sender with your gauge before it goes into the tank. This is optional, but it will ensure that the two are well matched.
6. Disconnect the old sender (which is hopefully still sealing the fuel tank) from behind the dash. You only need to disconnect the wire that goes to the gauge's leftmost terminal when looking at the instrument panel from above.
7. Connect the ground terminal of the sender under test to chassis ground.
8. Connect the remaining terminal of the sender under test to the now free terminal of the fuel gauge (leftmost terminal when looking at it from above).
9. Turn the ignition key on.
10. Actuate the sender's arm manually to double-check that the lowest and highest float points correspond to the empty and full tank indicators on the gauge. Bear in mind that the gauge does not respond instantaneously to resistance changes. For instance, it can take a minute or two for the needle to fully travel from the empty to the full position.
11. If the empty and full marks match the sender's output, you're all set. Otherwise carefully bend the stopper tabs as necessary.
Mechanical calibration
More than calibration, this is a fix or workaround for the fact that the late VDO and other aftermarket senders seem to provide an output that causes the maximum level displayed on the gauge to be only 3/4 of a full tank. Apparently this is due to the fact that the early VDO senders had a float with a flat top (like a cylinder sliced axially), which allowed the floating point to be as close as possible to the top of the tank. Later senders feature a round, fully cylindrical float: its extra width makes it hit the top of the tank, thus never reaching the resistance value that correlates to a full tank.
The workaround implies bending the sender's arm so that the wiper can reach further into the direction of decreasing resistance (towards full).
As this is something I've not done myself yet, I'll simply refer you to this thread with some more information.
I must admit I don't quite visualize how exactly the arm must be bent here and the implications for the other side of the range (towards empty). The float is going to hit the top of the tank before and after the mod, that can't be changed. So I would think either the angle of the float's arm elbow is increased by bending it, or a second elbow between the first one and the float is created, so that the arm will end up looking like an elongated Z. Looking at another post (see picture below), it seems another option is to bend the long segment of the arm into an arc. I guess I'll see it when I get there.
Picture by Samba member Dave Cook (vwwestyman)
In either case, it might be good to get some data on how folks ended up doing the bend, so that not everyone has to go through this fix by measuring the fuel level in the tank.
Perhaps the approximate angle to bend the elbow could be worked out or simply the required distance from top of the float to top of the lid. That at least would get other members in the ballpark. I know, easier said than done, especially with the sender already in the tank.
_________________
'79 Westy, P22 interior, FI 2.0 l Federal, GE engine (hydraulic lifters)
Decode your M-Plate
Last edited by furgo on Tue Feb 06, 2018 10:17 am; edited 6 times in total |
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furgo
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Joined: September 06, 2016
Posts: 944
Location: Southern Germany
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| Posted: Tue Feb 06, 2018 8:51 am Post subject: Re: Late Bay fuel sender calibration values |
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For those of us who use SI units, here's also the metric version of the diagrams 
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'79 Westy, P22 interior, FI 2.0 l Federal, GE engine (hydraulic lifters)
Decode your M-Plate |
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Blue69Baja
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Joined: September 04, 2008
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Location: Fair Oaks
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| Posted: Fri Dec 14, 2018 1:01 pm Post subject: Re: Late Bay fuel sender calibration values |
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Hay, nice write up....
Too bad the info is spread over several forums.
After some conclusions are made maybe we can have Everett consolidate the various threads....
I did not get a chance to measure a good sending unit so this is helpful.
As a note, I wish we could find the resistance charts for the Type 1's and Super Beetles.
Thanks for posting.
Jim
_________________
69 Baja with a 1914...
72 Super 100% orig...W/ factory air... 56,000mi
Even the spare is original..
Last edited by Blue69Baja on Sat Dec 15, 2018 11:17 am; edited 2 times in total |
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SGKent 
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Location: Citrus Heights CA (Near Sacramento)
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| Posted: Fri Dec 14, 2018 1:06 pm Post subject: Re: Late Bay fuel sender calibration values |
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I've published the data before on this. Original floats were flat topped and that allowed the floats to right higher. Now the metal arm wraps around a round float which means the float arm never allows it to reach full. The only thing people need to know is to bend the arm a little to allow for that. One can also use a wooden yardstick to lift the float to full and then compare that to the gauge.
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cmonSTART
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Posts: 1915
Location: NH
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| Posted: Fri Dec 14, 2018 2:30 pm Post subject: Re: Late Bay fuel sender calibration values |
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I have to say making a 4" access hole for mine made this really easy. I think I had 3 rounds of trial and error with it and nailed it exactly. I used a 4" chimney plug and some silicone to seal the hole up.
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de K1IGS |
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Zed999
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| Posted: Sat Dec 15, 2018 3:27 am Post subject: Re: Late Bay fuel sender calibration values |
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I hope it's not too off topic to mention voltage here?
The gauge if effectively an ammeter (i=v/r) and relies on stable constant voltage to indicate changing resistance. If your sender is set up as excellently described above but your gauge is still out of whack, check the voltage from your voltage stabiliser. IIRC it should be 5.2v. Someone had carefully tried to insulate mine from the dash, the output was 4.3v and the gauge under read by about 1/4 tank. |
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telford dorr
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Location: San Diego (Encinitas)
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| Posted: Mon Dec 17, 2018 11:01 am Post subject: Re: Late Bay fuel sender calibration values |
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Nominate for "sticky" designation...
_________________
'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST) |
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WhirledTraveller
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Location: Cambridge, MA
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| Posted: Mon Dec 17, 2018 11:19 am Post subject: Re: Late Bay fuel sender calibration values |
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| Zed999 wrote: |
I hope it's not too off topic to mention voltage here?
The gauge if effectively an ammeter (i=v/r) and relies on stable constant voltage to indicate changing resistance. If your sender is set up as excellently described above but your gauge is still out of whack, check the voltage from your voltage stabiliser. IIRC it should be 5.2v. Someone had carefully tried to insulate mine from the dash, the output was 4.3v and the gauge under read by about 1/4 tank. |
It's not so much an ammeter as a power meter. That's why the calibration curve of the float sender is not linear. Basically the gauge is a resistance heating element and a bimetallic strip, and heat corresponds to power (I^2 R)
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1977 Westy, Automatic. Big Valve heads, CS Cam. |
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Amskeptic
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| Posted: Tue Dec 18, 2018 10:04 am Post subject: Re: Late Bay fuel sender calibration values |
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Please note the wrench in the fan:
Many replacement senders are using linear potentiometers versus the original senders which used exponential ones:
The original potentiometers have the slow drop in resistance between full and 1/2, then the speeded-up reduction down to empty. This of course mirrors the gauge which has a short distance between full and 1/2, and that streeetch to reserve.
I have an annoying replacement sender in my 1977 now. Even after adjusting the gauge head, I have a stunning drop in the displayed gauge reading from full to 1/2 in only three actual gallons consumed, then it levels out to an actual 1 gallon remaining at the indicated reserve mark.
Please note, do not bend the float arm too much in your effort to get the gauge to read full. Better to try a slight adjustment on the full-stop tab, then go directly to the gauge for a tiny tiny little nudge of the range screw.
Colin
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Blue69Baja
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Location: Fair Oaks
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| Posted: Tue Dec 18, 2018 11:16 am Post subject: Re: Late Bay fuel sender calibration values |
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| Amskeptic wrote: |
Please note the wrench in the fan:
Many replacement senders are using linear potentiometers versus the original senders which used exponential ones:
The original potentiometers have the slow drop in resistance between full and 1/2, then the speeded-up reduction down to empty. This of course mirrors the gauge which has a short distance between full and 1/2, and that streeetch to reserve.
I have an annoying replacement sender in my 1977 now. Even after adjusting the gauge head, I have a stunning drop in the displayed gauge reading from full to 1/2 in only three actual gallons consumed, then it levels out to an actual 1 gallon remaining at the indicated reserve mark.
Please note, do not bend the float arm too much in your effort to get the gauge to read full. Better to try a slight adjustment on the full-stop tab, then go directly to the gauge for a tiny tiny little nudge of the range screw.
Colin
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Nice write up Amskeptic!
If things work the way I expect them to we might have a solution to combining the non OEM gauges and the needed voltages/current to make them work fairly accurately....With our sending units 69 and up.
Like you said, a Nudge of the Range adjustment is needed for getting a full swing of the gauge, It is a delicate adjustment though.
I currently have 3 non-oem gauges working on the bench, showing Full ,1/2, 1/4, R, and empty....All within 1mm of position readings.
I used the reference resistances made by forgo to verify the measuring, adjusting.
All was a balancing act. Raise the voltage, adjust the Range etc.
Seems that the final dc voltage is in the 7.2vdc rang with the non OEM gauge 33ohms and the Sending units resistances.
I have Zener Diodes coming for final testing. If they ball park the gauge then we can remove the 5.1 Zener and install the 7.2 in the aftermarket diode regulators. Then bench adjust the 33 Ohm Gauge range to obtain full needle swing and be pretty accurate. A 10 ohm resistor and another combination of resistors to get about 72. OR better yet a good sending unit. or so would be needed... along with a 9vdc battery, 12vdc car battery or bench regulator.
The little Buck step-down regulator really works great and much simpler to integrate, but not as reliable??...hahahaaa.
Forgo's diagram...
Jim
_________________
69 Baja with a 1914...
72 Super 100% orig...W/ factory air... 56,000mi
Even the spare is original.. |
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Blue69Baja
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Location: Fair Oaks
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| Posted: Tue Dec 18, 2018 11:21 am Post subject: Re: Late Bay fuel sender calibration values |
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Amskeptic, I notice that your gauge appears to be an OEM gauge or something else. Have not seen one of those. I have little experience with Bus types.
Can you check the resistance and see if it is 15, 33 or something else resistance.. would be nice to know...
Jim
_________________
69 Baja with a 1914...
72 Super 100% orig...W/ factory air... 56,000mi
Even the spare is original.. |
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Amskeptic
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Joined: October 18, 2002
Posts: 8568
Location: All Across The Country
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| Posted: Tue Dec 18, 2018 11:41 am Post subject: Re: Late Bay fuel sender calibration values |
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| Blue69Baja wrote: |
Amskeptic, I notice that your gauge appears to be an OEM gauge or something else. Have not seen one of those. I have little experience with Bus types.
Can you check the resistance and see if it is 15, 33 or something else resistance.. would be nice to know...
Jim |
It is an oem gauge. Unfortunately it resides back in the cluster over the past eight months and 21,000 miles of reliable annoying readings.
Colin
(but, it is seriously accurate at the reserve mark, and it is full when full)
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Wildthings
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| Posted: Tue Dec 18, 2018 11:56 am Post subject: Re: Late Bay fuel sender calibration values |
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| If any one has a busted OEM late Bay sender as Colin shows I would love to play with it and come up with a decent rebuilding how to. |
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Blue69Baja
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Location: Fair Oaks
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| Posted: Tue Dec 18, 2018 12:29 pm Post subject: Re: Late Bay fuel sender calibration values |
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| Wildthings wrote: |
| If any one has a busted OEM late Bay sender as Colin shows I would love to play with it and come up with a decent rebuilding how to. |
That would be cool. I attempted to do the 72 Super Beetle sender which resistive wise the same for 69 up. The Nichrome resistive wire was worn through in the mid area along with the tab on the slider.. Toast.
_________________
69 Baja with a 1914...
72 Super 100% orig...W/ factory air... 56,000mi
Even the spare is original.. |
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Amskeptic
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Joined: October 18, 2002
Posts: 8568
Location: All Across The Country
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| Posted: Tue Dec 18, 2018 10:08 pm Post subject: Re: Late Bay fuel sender calibration values |
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| Wildthings wrote: |
| If any one has a busted OEM late Bay sender as Colin shows I would love to play with it and come up with a decent rebuilding how to. |
Would you like a photograph of a disassembled burnt up one?
I threw it away. I hope I don't regret that.
Please note, the wire is coiled tightly. I do not understand the loosely wound units where the sweeper can snag each strand!
Colin
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Wildthings
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| Posted: Tue Dec 18, 2018 10:20 pm Post subject: Re: Late Bay fuel sender calibration values |
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| Thinking about it, it might be easier for someone with electronic knowledge to design a circuit to modify the output of a linear sender to mimic the output of the original exponential sender. |
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Blue69Baja
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Location: Fair Oaks
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| Posted: Tue Dec 18, 2018 11:11 pm Post subject: Re: Late Bay fuel sender calibration values |
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| Wildthings wrote: |
| Thinking about it, it might be easier for someone with electronic knowledge to design a circuit to modify the output of a linear sender to mimic the output of the original exponential sender. |
Haw, I have been shown how a Gia and bus sending units from the 60s- mid 70's bu never was curious about the resistance...hahahaaaa
Straight wire type.... 8-80 ohms. Never dissected it to see how the piece of wire can present that kind of resistance.
https://www.jbugs.com/product/271919051B.html
Jim
_________________
69 Baja with a 1914...
72 Super 100% orig...W/ factory air... 56,000mi
Even the spare is original.. |
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WhirledTraveller
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| Posted: Wed Dec 19, 2018 12:57 pm Post subject: Re: Late Bay fuel sender calibration values |
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Even better would be a circuit that could take the output of a reliable, cheap, no moving parts capacitive fuel sender and drive the original gauge.
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1977 Westy, Automatic. Big Valve heads, CS Cam. |
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telford dorr
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| Posted: Wed Dec 19, 2018 1:37 pm Post subject: Re: Late Bay fuel sender calibration values |
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Both of those are possible using a small 8 pin microcontroller (think: ATtiny85, programmable using an Arduino). Connect the sender to a processor Analog-to-Digital Converter input pin (using suitable biasing circuitry). Connect a Pulse-Width Modulated output pin to the gauge (using suitable driver circuitry). Generate a data conversion lookup table in the processor which correlates the sender readings to the values needed to make the gauge read correctly. Processor reads the sender input, looks up the correct output value in the table (piecewise linear approximation), and sends the corrected value to the gauge.
This can be done to use any sender with any gauge. Great for engine swaps into old cars still using their original 6 volt gauges. Yeah, it's going to take some time to design, build, and calibrate unit #1, but could be worth it...
_________________
'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST) |
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Blue69Baja
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| Posted: Wed Dec 19, 2018 4:03 pm Post subject: Re: Late Bay fuel sender calibration values |
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Well someday one or both of these solutions might be needed as parts become more rare....
Think many want to maintain orig type operation... Resistance and voltage.
The capacitive sender and micro controlled, programable controller to an OEM looking gauge would be great. That is a project I am not going to get into. Too many other projects and at 74, not enough time...hahahaaa
Jim
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69 Baja with a 1914...
72 Super 100% orig...W/ factory air... 56,000mi
Even the spare is original.. |
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