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Old 03-16-2008, 09:30 AM   #1 (permalink)
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Date registered: Aug 2006
Vehicle: 96' S500
Location: Florida
Posts: 134
Fuel level sending unit changeout

On my 96' S500 the fuel tank gauge sometimes drops to empty and then slowly will come back to correct level.

I searched the forum postings and it sounds like replacing the fuel level sending unit would be the likely fix. There is a diagnostic for the cluster available on Alldata so maybe I will do that first.

Looking at the Alldata procedure it appears that replacing the sender unit is an easy DIY change out and since I did a similar job on my Q45 (fuel tank pump), I figure I can do this.

I have two questions though. I see that there was an issue with certain intrument clusters with this indicating problem when the tank is less than 1/4 full. In my case it happens at various levels so I'm hoping it is not the cluster. Have any of you had to change a cluster because of this?

I also see that the replacement part is a lever sender type unit and replaces an immersed pipe sender. Seems lower tech to me but does my 96' have the immersed pipe style as factory installed or am I likely going to swap a lever type for another lever type unit?

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Old 03-17-2008, 07:58 AM   #2 (permalink)
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Date registered: Aug 2006
Vehicle: 96' S500
Location: Florida
Posts: 134
Part 2...

I think I will hold off on purchasing the sending unit for now. I found this TSB on this issue and I guess it's possible there might be a problem with my gauge cluster circuit board.

On my car when the gauge drops to zero it also turns on the reserve light. In Alldata there is this comment regarding the level sending unit.

"NOTE: If an open circuit in the wiring exists, the fuel gage in the instrument cluster indicates empty; the fuel reserve warning lamp (A1e4) does not come on."

Since mine comes on then there isn't an open circuit...right? But I guess it could still be the level sender if there is a problem with the potentiometer.
Fuel gage sensor design

The fuel gage sensor (B4) is a lever sensor with sliding contacts potentiometer.

Fuel gage sensor function

The resistance of the fuel gage sensor (B4) is analyzed in the electronics of the fuel gage.
low resistance = fuel tank empty
high resistance = fuel tank full

I did find that MB went with a lever level unit starting in Jan 95'.
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Old 04-26-2008, 07:52 PM   #3 (permalink)
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Date registered: Aug 2006
Vehicle: 96' S500
Location: Florida
Posts: 134
Well this was a easy fix.

I followed the procedure on Alldata to replace the fuel level sensor. As DIY jobs go this was simple stuff anyone can do. All that is involved is pulling down the back panel of the trunk to expose the sensor at the top of the fuel tank, pull two connectors, rotate out and remove the old sensor and then reverse for the new one. I mistakenly ordered some other fuel tank gaskets but be assured, the new sensor has a gasket attached and no other parts or gaskets are needed or required.

One caution...I did not have the MB specific tool to loosen the sensor and cracked the old sensor when I removed it. I was very careful not to damage the new one. It only takes a small degree of rotation to lock the sensor on the tank so I used a bit of gasket lube to lock the sensor back into place using basic tools and hand pressure.

Contrary to the MB service bulletin, it appears my gauge circuitry was fine and the problem was the fuel level sensor. As a reference, my car is an early 96 and it had the float type sensor and not the immersed pipe style.

Now I can once again accurately track that precious dinosaur juice on it's way out the tailpipe.

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Old 04-28-2008, 10:14 AM   #4 (permalink)
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Good job! Thanks for sharing the info.
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