My 1995 S500 Coupe Gas Gauge is mostly showing no Fuel & reserve light is on... It occasionally works while driving though....

I bought the car this way a year ago & have found a great source to test the IC in the Cluster to confirm its not the cluster itself... It will not however show me if I have a ground problem... http://k6jrf.com/MB_IC.html

I have to correct this as well as a few other issues to sell the car to a friend & a Benz W140 guru I know has said there could be a corroded ground...

My American car experience would agree with this since that is the way a fuel gauge works, using a variable resistor that is on a "float" and feeds the gauge a voltage referenced to the float position and because it is mostly on empty & occasionally rises to the level a bit closer to full after filling the car up with fuel and driving... vibration from driving might be shaking that ground point & making marginal connections. When stationary, its always empty and yellow light on.... And yes, I know, it could be the sending unit, I will do a resistance test on that with an ohm meter... If it's a "Open Circuit" that's the problem... I get it...

So... if anyone out there could let me know where to locate the gas tank ground ( if there is such a under body or in the trunk area connection ) and where I might find it... THAT would be awesome...

FYI, the underside of the car, below the tank was rusty... I've scraped it clean & painted it and its still structurally ok, but I think it could very well be that rust and a corroded wire connection is the culprit....

 

I do have access to the WIS info system and have looked into the R & R procedure for the Tank Sending unit.... I have a couple other W140's as well so parts will be available after I drain off some fuel as in the WIS data...

VIN is: WDBGA70E7SA215127

 

I wanted to Thank everyone who responded to my request because THIS is what I was looking for.... A 2 wire, analog answer... I don't need to look at the underside of the body... I DO need to verify the sender is ok.. ( I have a couple other W140's to use for parts to validate sender & verify the IC on the Cluster up front which I posted a previous link to so NOW I have the directions to head in.... I have 12 cars.. 11 are Mercedes... I have a busy life.... too busy & I just wanted to share what I've learned.. Hope this detailed "R & R" & inside explanation of the sending units "guts" will make others with the same problem I have go that much smoother...THANKS!

LWB250 has just replied to a thread you have subscribed to entitled - 1995 S500 Coupe Fuel Gauge problem - in the W140 S-Class forum of Mercedes-Benz Forum at BenzWorld.org.

This thread is located at:
http://www.benzworld.org/forums/w140-s-class/2661106-1995-s500-coupe-fuel-gauge-problem-new-post.html

Here is the message that has just been posted:
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The gauge sender is mounted through an opening in the top of the tank that can be accessed through the trunk, I believe. There is a large "nut" arrangement with a connector on it for the harness to attach to it. No separate ground.

The sender is an aluminum tube with a float that slides up and down a rod flanked by two nichrome wires inside the tube. The float has fingers on each side that slide/contact the wires, providing the variable resistance necessary to drive the gauge.

It's purely analog.

At the bottom of the tube/rod is a disc that two contacts on the bottom of the float will touch, closing the circuit to turn on the reserve light. The bottom of the tube is held in place with a small 8mm nut with a nylon insert. The bottom is a nylon disc sandwiched between two large metal discs that has grooves molded into it that allow the fuel to leak out the bottom of the tube slowly, so changes in the tank level from cornering or going up/down hills won't affect the sender.

You can carefully remove the sender from the car and disassemble it for cleaning and testing. Be careful when doing so, as fuel will take some time to leak out of the bottom of the sender, so if you remove it immediately without allowing it to drain you'll have gasoline all over the place. Once you get it on the bench, remove the bottom of the tube by unscrewing the 8mm nut, remove the nylon disc, slide the tube off the assembly and the float and nichrome wires will be exposed.

Just remember that you're dealing with a tank full of gasoline, so heed all necessary safety precautions before doing so.

Later (post facelift) gassers used a more traditional float with an arm and float at the end. They also incorporated a pressure transducer in the top of the float assembly as well.