Author Topic: Fuel sending unit accuracy  (Read 2149 times)

johnk

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Fuel sending unit accuracy
« on: October 14, 2019, 17:27:31 »
On my pre-restoration check list was to improve the accuracy of my fuel gauge. As I remember it went from full to empty without much time spent in between, leaving me with little confidence as to how much gas I actually had left. With a new sending unit costing close to $300 is there a way I can test my unit to see if that is the problem?

I appreciate the advice gents!

John
John Krystowski
Avon Ohio
1968 Euro 280sl under restoration
2016 Jag F-Type R sold june 2021
1950 Alfa Romeo 6C 2500 SS For sale
2008 E350
2007 GL 450
2019 BMW 540

WRe

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Re: Fuel sending unit accuracy
« Reply #1 on: October 15, 2019, 07:49:43 »
Hi,
just measure the resistance of the sender.
There ist a lot of inforrmation in Technical Manual:https://www.sl113.org/wiki/Fuel/SenderUnit
and here: https://www.sl113.org/forums/index.php?topic=27267.msg195991#msg195991.
Hint: Fule sender must fit with fuel gauge (VDO 21/169).
...WRe

Pawel66

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Re: Fuel sending unit accuracy
« Reply #2 on: October 15, 2019, 11:39:26 »
The reading on my fuel guage is pretty proportional as the hand goes down, maybe speeding up a bit in the bottom half.

Maybe in the old tube the float got stuck somewhere high, then was falling down to fuel level closer to the bottom...

The resistance I measured for the G-class sender (similar type) was pretty linear from top to bottom.
Pawel

280SL 1970 automatic 180G Silver
W128 220SE
W121 190SL
G-class

mrfatboy

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Re: Fuel sending unit accuracy
« Reply #3 on: October 15, 2019, 11:56:13 »
I have had two pagodas in my life. The fuel sender in both seems to move faster to empty at the half way mark.
1969 280sl (Aug 1968 build)
Signal Red
4 Speed

Pawel66

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Re: Fuel sending unit accuracy
« Reply #4 on: October 15, 2019, 12:25:16 »
Yes - so my impression was correct, it moves faster from 1/2 down.
Pawel

280SL 1970 automatic 180G Silver
W128 220SE
W121 190SL
G-class

Mike Hughes

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Re: Fuel sending unit accuracy
« Reply #5 on: October 15, 2019, 14:28:56 »
Two reasons why: 

1.)  The bottom of the tank is shaped such that there is a shallow end and a deep end (like a swimming pool), so the lower half of the tank simply contains less volume of fuel than the upper half.

2.)  The "flower pot" displaces some volume and returned fuel is mostly retained within the "flower pot" during vehicle operation to provide a steady supply of fuel to the pump, rather than being returned directly to the volume of fuel outside the flower pot.  During vehicle operation at extremely low fuel levels the level of fuel within the "flower pot" may be quite a bit higher than the level of fuel remaining outside the "flower pot."  Thus the lower register of the fuel gauge may read just a little lower still after a few minutes of fuel circulation than when the ignition is first switched on.  This also explains the anomaly of the red low fuel level light being dark when first turning on the ignition, and then illuminating a few moments later even though the engine was not started!
- Mike Hughes  -ô¿ô-
  1966 230SL Auto P/S
  Havana Brown (408)
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doitwright

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Re: Fuel sending unit accuracy
« Reply #6 on: October 16, 2019, 00:49:00 »
Our cars use the same style sending unit that were in early Porsche’s. There are YouTube videos on how to open the sending unit. There is a small nut at the bottom end of the center rod that gets removed and allows the outer tube to be be removed from the top section. You need to be very careful if opening one of these up due to the very fine wires that stretch just inside the wall of the outer tube. What I found in the Porsche is that old gas can cause the float in the sending unit cylinder to stick. Cleaning the center rod and the wire allowed the float to move more freely.
Frank Koronkiewicz
Willowbrook, Illinois

1970 280SL Originally Light Ivory - Now Anthracite Gray Metallic

johnk

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Re: Fuel sending unit accuracy
« Reply #7 on: October 16, 2019, 01:06:18 »
thanks Frank and everyone. I will try testing the resistance this weekend and will open it up carefully to clean it.

John
John Krystowski
Avon Ohio
1968 Euro 280sl under restoration
2016 Jag F-Type R sold june 2021
1950 Alfa Romeo 6C 2500 SS For sale
2008 E350
2007 GL 450
2019 BMW 540

WRe

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Re: Fuel sending unit accuracy
« Reply #8 on: October 16, 2019, 06:08:00 »
Hi,
here you'll find a lot of pictures and information: http://www.hehlhans.de/tipp19.htm.
...WRe

johnk

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Re: Fuel sending unit accuracy
« Reply #9 on: October 16, 2019, 21:28:06 »
Excellent thanks Wre.
John Krystowski
Avon Ohio
1968 Euro 280sl under restoration
2016 Jag F-Type R sold june 2021
1950 Alfa Romeo 6C 2500 SS For sale
2008 E350
2007 GL 450
2019 BMW 540