1984 300D fuel gauge - ideas anyone?

My fuel gauge always reads empty, and the reserve light is on. Here is what I have tried: 1. Filled tank 2. Removed and disassembled sending uint. All looks good: wires intact,
contacts good, float moves freely, everything is clean. 3. Swapped another fuel gauge into cluster. Same zero reading.
Is there any kind of control unit between the sender and the gauge? Am I down to the only possibility being a wire issue? Any other ways to check?
Thanks again for all the free advice
Michael
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wrote:

I don't understand the logic myself but contributors to the forum on the UK Mercedes Owners' Club suggests that using low quality petrol can play havoc with the accuracy of the fuel gauge. It suggests either putting a fuel cleaner in the tank or filling up with top-of-the-range petrol and this should remove any unpleasant deposits from the system.
The relevant forum is at:
http://forums.mercedesclub.org.uk/showthread.php?t 921&highlight=fuel+gauge
--
Alasdair.

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The needle and the light are on two seperate circuits. If the light is on, the float is sitting on the bottom. The float has two contacts on the bottom of it. It has to sit on the bottom to turn the light on.
Unscrew the float tube out of the tank. Plug it back in. Turn the key on. Turn the tube bottom up so the float moves. Light should go out and the needle should move.

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Karl wrote:

I did this when I had the sending unit out. When the float is on the bottom the gauge shows full. When the float is in any other position the gauge shows empty
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That is obvious enough. The sender is bad.
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szcz wrote:

If my reasoning is correct, the float guage should operate like a variable resistor. you should be able to hook up a multimeter and check the guage's proper operation from that..K
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