I have a well maintained 1991 300E with 150K miles. Recently it started to exhibit what I can describe as a "bouncing" fuel gauge indicator when around 1/4 tank of fuel is in the tank. It will bounce between E and 1/4 until I fill it. Any suggestion on the problem or possible solution(s)? Thanks.

Sounds like your fuel sending unit. Sometimes they get dirty or just need replacing.

I'm not sure how to access it in a 124. I'm sure though someone will chime in with the proper info. Until then, you might remove the tray that holds the first aid kit and look under there,(that's where it is on a 123). There is an electrical plug that fits into a big (44mm) hex cap. Unplug it and unscrew the cap. Carefully withdraw the cap, (the sending unit is attatched), wait for fuel to stop dripping, use a rag to catch any drips and withdraw the sending unit.

You can use parts cleaner to remove any crud. There are very thin and delicate wires that run the length of the sending unit. If they are broken, a new one is probably needed.

Good luck

Mr Bill, thanks for your reply. WHy would it only happen when the fuel level is low if the sending unit is the culprit?

I also have a similar problem... I have a bouncing speedometer on my 1988 300e, I wonder if both these problems are linked

Gas Gauge Issues
 

The speedometer is driven mechanically by a cable that runs to a gear in the transmission. Most of the time bouncing is caused by the cable lacking lubrication inside it's sheath, causing it to bind slightly at low speeds. Alternatly, it can also be caused by the tip of either end of the cable not being completely seated into its slot and slipping a little.

BOUNCING GAS GAUGE
The gas gauge bouncing is another issue entirely. Bouncing is often caused by a loose electrical lead from either the back of the gauge pod or the sending unit on the tank. Before going to the trouble of removing the sending unit try removing the plug from both ends and re-seating it.
**NOTE: Be sure to check inside the plug and recepticle for corrosion. If you find any, clean it out. Apply a small amount of di-electric grease to the connector and plug it back in.

If you do end up removing the sending unit to clean and inspect it it's always good to do some research and find out if there are any seals or filter that you may wish to replace. Starter fluid works great for cleaning out a gummed up sending unit, but be sure to wear gloves as it WILL give you chemical burns after prolonged contact.

This fluttering petrol gauge needle was an issue that afflicted our 1992 300E.
It was in existence when we bought it and we chose to tolerate it (it was a minor issue on the list with that car). The cause was the illumination rheostat - dimmer knob for the instrument cluster - as it also sounded crunchy and crackly when rotated. It is a common item in Europe. It only ever fluttered when under half a tank of fuel was left.

The fuel gauge works on the principle of a rheostat except the wires ( 2 wires ) are straight inside the unit. The float varies the resistance and it is read by the gauge. You have a weak/dirty spot on the wire whereas the contact with the float is not made, hence bounce or show 0. This is a very common mode of failure. I do not know how to remove the sender on a w124. Do a search here and there is a wealth of info on how to replace, refurbish, repair etc.

Just got through trouble shooting this issue on a 92 300D. The fluttering gauge, in this instance, was caused by a cold solder joint at the connector where it pushes onto the fuel level sending unit. With the key in the ign watch the fuel gauge while wiggling this connector. If there is a problem at this location then the connector needs a little attention. On my situation not only was this going on but the fuel gauge itself has an issue in the fact that it won't read below about 1/4 full. Sending unit in tank is good. Check reading at the level sending unit and pins 1 and 3 at the plug in for the instrument cluster. If they are close then the problem is in the fuel gauge itself.

Had the same problem. Added 1 quart of Marvel Mystery Oil to a full tank and the problem is gone. Your results may vary but it doesn't hurt to try.

[QUOTE=Had the same problem. Added 1 quart of Marvel Mystery Oil to a full tank and the problem is gone. Your results may vary but it doesn't hurt to try.[/QUOTE]

Seems Marvel.Mystery oil cleaned out the sender unit! .
Shortly after replacing my sender unit i got a flickering needle . I suspect the cleaning additive added to the fuel coated the sender unit , removed and cleaned the delicate resistance wires in the sender gauge and back to a steady needle again.
Ad