Never underestimate the impact of poor grounding either. Crappy grounds are very common on these cars and any resistance anywhere in the circuit will skew the sender/gauge accuracy.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

I bought several fiberglass pens to clean IC board for soldering. They remove the surface corrosion on board. They may not be abrasive enough to clean the wires but they clean contact well.

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84 300SD
85 380SE
83 528e
95 318ic

I ran a new ground line from the cluster to the bolts behind it. Didn't do much, unfortunately. Soldering a small wire from the post of the fuel gauge to the round body stopped it from bouncing. Temp gauge still flickers from time to time.

Seems unlikely the float would fail, so I must be missing something in the cluster.

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1981 240D 4sp manual. Ivory White.

I have a hunch if you fix the flickering temp gauge, it will also fix your fuel sender issues.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I still favor a high-resistance connection somewhere in the circuit, not necessarily a ground.

Although the factory diagrams may lead you to believe otherwise, the gauges don't work on 12V. Most gauge systems work on either a regulated 10V or 5V. There is always a miniature voltage regulator built somewhere into the dash pod or the gauge itself. If the regulator fails, the gauges can read incorrectly or erratically. There are no replacement parts or DIY instructions, but if you can figure out where the regulator is, you may be able to either recap it or cobble together a replacement with a 7805 or 7810, depending on the voltage requirement. The attached photo shows a 124 guage regulator. If it went bad, I'd take a stab at replacing the big capacitor. This particular pod also has a mystery rheostat which possibly is there to allow fine adjustment of the power supply.

Attached Thumbnails

   

This sounds interesting... I'm fairly certain there's something up with the gauges themselves. This hasn't been a favorable hypothesis around here. The number of ancillary wires and solder I've had to run back there is beyond absurd.

I'd happily replace that part of the cluster if I was able to find another for a reasonable price...

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1981 240D 4sp manual. Ivory White.

@Shern ;

You say no fungus yet your very first photo clearly shows fungus.....

You also didn't test the gauge properly like I told you how to so you have no idea what's what vis - a vis the sender / gauge head .

Stop jumping around and follow the proper steps, you'll get where you want to goo much sooner .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

You’re right. Seems like the most obvious diagnostic.
Re: fungus, I suppose there was a minor amount of sludge but I blew that away with brake cleaner, ran startron, pulled tank strainer and ran my fingers along the bottom of the tank (pretty clean).

Will perform your diagnostic.

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1981 240D 4sp manual. Ivory White.

I know I've been pushing Startron hard for a long time but I recently read about BiBor JF right here, trotted out and bought a jug, WOW ! it did the job in two tank fulls, I've been struggling with Startron for over a year now and always had this tiny little bit of fungus I couldn't *quite* kill / get rid of .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

I used a combination of startron and bio-kleen (power service product). Guess one suspends the water and the other kills the bugs.

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1981 240D 4sp manual. Ivory White.

Okay Nate -pulled the sender last night and did the test. Gauges are perfectly in sync with the float level. Full range of movement, etc. Cleaned and reinstalled. So either the gauge is faulty, or the float isn't floating as it should be.

Am I missing anything else?

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1981 240D 4sp manual. Ivory White.

Nope ~ if you cleaned it and it works properly when tilted by hand, you should be good to go .

I've never heard of a Mercedes fuel sender float 'sinking' although it happens on other brands .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

Low fuel light came on about 20 miles ago so I refueled this eve: 13.8 gallons. My understanding was that the low fuel light came on around the 2 gallons remaining mark. Out of a 17 gallon tank, it came on a bit early no? My MPG is a bit down as well... calculated about 23.8 mpg which is way low for me.

Between the mild air incursion and the operating temp of around 79/80 something is eating my fuel. Possible it's time for a new thermostat. Possible I need to figure out where I'm getting air into the fuel lines. (Likely a combination of the two). Very frustrating...

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1981 240D 4sp manual. Ivory White.

I hear the frustrating part .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

Two possibilities I can think of.
1. Collapsed tank
2. Wrong sender. I.e. too short

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked