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1971 220D
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86 Posts
Discussion Starter #1
Does anyone know the proper resistance for a W115 fuel level sending unit?
My fuel instrument gauge is stuck on 'F', so I pulled the unit out of the fuel tank. It wasn't horrible dirty and the float is working fine. I have a resistance of 9 ohms when it is up and 70 ohms when it is down. If I remember my physics, V=IR, so I should be getting current differences of 1A down to 0.12A.
So I'm thinking it's the gauge...
And the copper wire in the sending unit to operate the low fuel indicator has corroded off :rolleyes:, but I can fix that...
Thanks, Bob
 

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1971 220D
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86 Posts
Discussion Starter #2
I've answered my own question by hooking up various resistors across the leads of the sending unit. The gauge registered half-full at 1000 ohms, so I have a short in the sending unit, and the gauge is fine.
 

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I've answered my own question by hooking up various resistors across the leads of the sending unit. The gauge registered half-full at 1000 ohms, so I have a short in the sending unit, and the gauge is fine.
If you come up with a fix for this please share it with me. I have having the same issue and was wondering if I can repair the sendor/sensor or if I need to find another.
 

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'72 250C, '74 280C, '85 500SEL, '81 300CD.
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1,041 Posts
Senders can be cleaned but you've got to be gentle. The thin wires snap easily. The senders aren't that expensive and are available. Replace the gasket at the same time no matter how good you think the old one looks.
 

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Registered
1971 220D
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86 Posts
Discussion Starter #6
The copper wire for the "empty" warning light had corroded off, and I discovered that the wires for the gauge are coated with something to increase the electrical resistance. So, apparently, that corroded off the other wires as well.
I ordered a new one...
Thanks, guys.
 

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1972 MB 280SE 4.5
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19 Posts
My fuel gauge shows full though the tank is not full. When the sending unit is unplugged the gauge is at empty. i had to tap the instrument panel before this problem to get a reading. The needle would move upon tapping. It is a new sending unit and I tested for the resistance before installation, the are correct. Any ideas on what else I need to check? Thanks 1972 280SE 4.5
 
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