Fuel gauge acting up - PeachParts Mercedes-Benz Forum

t walgamuth · #1 11-26-2015, 08:42 AM

123 240D. The fuel gauge never goes much above about 2/3 no matter if full and doesn't go below 1/3. The warning light is not working either (don't ask how I know!).

Any ideas as to the root of the problem?

Mxfrank · #2 11-26-2015, 09:30 AM

The low fuel light and the level are separate sensors, although both are in the same tube. The only conclusion is that the float is actually getting stuck somehow. You're going to have to pull the sensor for a look see. It's easy enough to get to on a 123. You remove the first aid kit holder, and there it will be. I use a monkey wrench to loosen them. When you remove it, pull it out slowly...it's full of fuel and it's designed so that it takes a long time to drain out.

t walgamuth · #3 11-26-2015, 10:08 AM

Thanks Frank! I'll do that. I have a spare tank in good shape so I can swap it out if need be.

charmalu · #4 11-26-2015, 10:24 AM

Tom...did you skip class the day the teacher was explaining the sender units? LOL....

Here is a good thread with pictures....

W123 300D Fuel tank sender question

Swapping one from your spare tank might be as gunked up as the one in your 240.
They are really easy to clean out.

Pull the first aid kit.
Pull the wire harness.
Place some rags around the area and seat.
Unscrew the sender unit
Gently lift it out, and let the fuel drip off and or run out the tube if it is a full tank.
Have a pan to set the end in as you take it out of the car so it doesn`t drip on the seats etc...
Unscrew the bottom nut.
Remove the bottom plate.
Slide the tube off the sender innards.
Use some brake/carb spray cleaner to blow out the gunk.
Clean out the tube.
Put it all back together and you are good to go.

Here is a thread on the Socket size. 46mm or 1 13/16".

Power loss at speed 1983 300SD

Charlie

sent from my pos computer

t walgamuth · #5 11-26-2015, 10:41 AM

Oh well, if there was not a forum this is what I would have done, but since there is a gauge and wires it was pretty easy to ask first in case there was a simpler solution.

Thanks Charlie! (I was going to pm you about this but figured somebody else might be living with the same problem)....you know, public service and such.

fahrvergnugen · #6 11-26-2015, 01:03 PM

Probably already a warning in that thread, but Be Careful on cleaning that thing. I broke a second hand until because I was playing with it connected to the vehicle harness while apart... I forgot I had it connected, tried to move away, and snap; the wire that holds the float went kaput. $60 mistake right there... Here's my sign.

Redpeople · #7 11-26-2015, 01:04 PM

Interesting coincidence that you would post this this morning. I'm in the process of fixing all the little things that need fixing on my new to me 240D and I decided last night that I would tackle the fuel sender on thanksgiving morning. Same symptoms as you; gauge wouldn't go higher than half full and didn't really seem to do much at all.

I found the sending unit to be really easy to remove. I just used a big channel lock. Because it has a rubber gasket I can't imagine it being stuck to the point where a socket would be necessary, but I may be wrong. Definitely need to let the unit drain back into the tank. That probably took five minutes.

As mentioned take the unit apart very carefully so you don't break the sensing wires. As suspected things were completely gummed up. I cleaned it up with brake cleaner. Unfortunately, the top 2/3 of the rod on mine was also very rusty and corroded. (See photo) I am guessing it was probably stored for a long time with a mostly empty tank. I scraped the major rust off with a utility knife and then very carefully used steel wool. The float now goes up and down relatively freely, but not as good as new. I may ultimately have to replace the whole unit.

Put everything back together and it works! All told it took me less than two hours.

cooljjay · #8 11-26-2015, 01:11 PM

Just make sure the contacts don't slip off the wires.

Redpeople · #9 11-26-2015, 01:13 PM

Another photo. Still haven't figured out how to attach more than one at a time from my phone.

fahrvergnugen · #10 11-26-2015, 01:13 PM

OH, I should add;

On my '83 300TD, my float would never go to the top hash mark, only the one before it. Turns out this changed when I took the trouble to solder my instrumentation rheostat, instead of just keeping a wire inbetween the two prongs. Gauges were of course brighter, and the gauge now reads full when full. Also, the swing of the gauge is more consistent as the tank empties.

OtisMagotis · #11 11-26-2015, 01:21 PM

Would this be the same process for cleaning the sender unit in a 81-85 300sd?

fahrvergnugen · #12 11-26-2015, 04:46 PM

I believe so, but I have no direct experience on the SD fuel senders.

charmalu · #13 11-26-2015, 10:31 PM

Yeah, 123, 124, 126 all the same type (more or less) senders.

The 80 - 83 240D has a 17 gallon tank. All previous 240`s use the 21 gallon tank as do the 300D NA and Turbo.

So the sender for the 80 - 83 240D is a bit shorter as I remember.
Also the 82 and 83 240 use a plastic tank.

Here is a link showing a W124 sender cleaning.

DIY w/pix -- W124 fuel sender disassembly/cleaning/testing

Charlie

sent from my pos computer

t walgamuth · #14 11-27-2015, 03:13 AM

Thanks Charlie!