1984 300 D fuel reserve light.

Hi all. My fuel guage works with the exception of the "low fuel light". When I go to start the car the light shows so I know the bulb is good. It just does'nt light when the fuel gets low to the point where it should come on. Can I take the sending unit out of the tank and see if the contacts that would trigger the light are some how being impeded? Can I perhaps clean the contacts rather than spring for a new sending unit. I can live without the light but will try an easy cheap fix.
Thanks Mike G.

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Body and Interior Specific Aid

Chassis Model Number

W123

Gave it a try

I took the sending unit out the other day. It is kind of a sealed tubular unit with a couple of small holes in it to allow fuel to come in to reflect the fuel level. I squirted cleaning solvent in the holes and shook it a little and then drained it. A lot of sediment and black "stuff" came out of the small holes. I repeated the process a few times till what drained was relatively clear. The inner mechanism seemed to be moving a little easier and I thought I was onto something. However after re-installing the light still did not show low fuel warning. When I filled up, the car took close to 20 gallons, so I know the fuel level prior to filling was low enough to have the light actuate if all was working properly. Is there anyway to test the light at the connector that plugs into the unit so I can rule out broken wires and a lack of continuity?

Same fuel light on my '84 300D

I too have the same problem with mine. I haven't seen where the sending unit is, how did you access yours?
I plan on testing mine when I replace the rubber fuel hose that leaves the tank. As the tank drains, I am going to watch and see if the light comes on at some point.
The specs in my manuals state that the tank is 21 gallons with three gallons set as reserve. I would think that the light would come on when it gets to within 3 gallons of empty. I have run it as low as I dare to get to empty a couple of times and have yet to see the light come on.

Does anyone have a light that does come on? If so, how much fuel did it take to fill up?

black crud

One more side comment. The black crud you saw was probably algae. Have you tried a fungicide in your fuel tank?
I used Startron from West Marine in mine to clean out any algae that might have built up. I don't know if Kent sells a fungicide on here or not. That was before I knew about this wonderful resource site.

Location of sending unt

It is under the first aid kit below the rear speaker deck. You take out the first aid kit, (2 phillip head screws) and you will see the wiring connector. I used channel lock pliers to loosen it. Mersedessource sells a nice socket specially made for this and it also fits the fuel tank screen.

my light works

When the light comes on (I have an 86 300 SDL) and i fill it it takes about 20-21 gal. to fill so the 21 and 3 on reserve is about right. But one time i was running the fuel down as much as possible when the light came on I figured "I have about 3 gal. left" I put 4 gal. in a diesel can in the trunk and went driving 130 some miles later I figured I don't want to run completely out and filled up with about 23 gal. so light came on a little early or I get like 50-60 MPG on those last few gallons and 26-32 on the rest. I didn't know those little D's carried that much fuel I learn something every time i read a post!!