The fuel guage on my 1980 240D hasnt worked since I bought the car. When I turn the key on, the gauge always shows full. My youngest son ran out of fuel yesterday, so I guess it's time to troubleshoot the situation. With the information I have provided, where do I start? I have read where people have fuel gauges that jump around, but havent came across one where it reads full all the time. Would that perhaps be a grounding problem?
thanks,
Jim

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1979 Mercedes 240D (Sold to a neighbor last year,now I have room for a new one,looking for a 300D this time)
1980 Mercedes 240D(bought for a parts car, but I have fun driving it!)

cleaning el-trick connectors is a good starting place
along with grounds simple unplug replug a few times may do the trick
spray oil or cleaners help too
a intank cleaner may help to free a stuck sender

if that fails try a VOM to check sender wires and guage for voltage and connection or do that while trying the unplug trick

unsure if pulling the sender is eazy on your car
most can be cleaned or rewired as needed
new senders canbe over priced but worth a check

I'd start with pulling the fuel sending unit out and cleaning it. It's can be accessed if you remove the first aid kit and box it sits in. You will need a big pair of channel locks to get it out. Or the proper end wrench that is some ungodly size.

Once you have it out. There is a nut on the bottom that comes off, then it all comes apart. Be gentle with it there are three tiny wires inside that can broken fairly easily. If any of them are broken you'll need a new sending unit. If not clean it with some break cleaner, and make sure the float moves easily. You can also test it to make sure it works. Hook it up to an ohm meter and you should get a range of resistance when you move the float up and down. The FSM has the details.

Usually when the gauge is pegged the gauge itself it toast, but the float could be stuck in the full postion as well. So start with the sending unit, then go to the gauge.

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Yup, I agree with BioD 300TD. I just went through this with my 300SD. The float guide rod was rusted in the middle, so a half tank is what my gauge always read until it pulled the sender unit and cleaned the guide rod with fine steel wool.

Yea i know how that is running out of diesel, i had the same problem but the guage was stuck on half, well i my 15 year old son found the sending unit and he pulled it out got some newspaper and shook it, alot of crap came out. then i soaked it in diesel again and it came out clear and it was fine again. it works like a champ

Ok, I pulled the sending unit, and disassembled. There was quite a bit of "grudge" inside the tube, almost the consistency of grease. A couple of of globs. I cleaned it all out with brake cleaner, run an emery cloth up and down the shaft so the float moved freely. Used the emery cloth on the brass/copper washer type contact on bottom of float, and scuffed up the contact places on top of container head. Installed it back, and turned the key on. Still shows full, although I just filled it up this morning, so it should be full. Wish I had come back in to read the part about checking the ohms while I had the float out, but forgot about reading that part of the reply! :-( I may just tell my son to take off and drive it, and keep track of the mileage and we will see if the fuel gauge moves. What else can I do to check whether the gauge itself in-dash is working or not?
thanks,
Jim

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1979 Mercedes 240D (Sold to a neighbor last year,now I have room for a new one,looking for a 300D this time)
1980 Mercedes 240D(bought for a parts car, but I have fun driving it!)

Your wait and see approach is good at this point. If you are impatient reading the gauge resistance in place can be done. This system seems to be set up simular to general motors cars. High gauge reading is usually an open circuit on the sensor line.Or sensor in the tank. Or bad ground. Actual gauge failure of the dash unit can occur yet is not too common. The actual test is to ground out the sensor wire that attaches to the tank. Gauge and wiring good the gauge hand will go to empty every time.

If I remember right, there were 2 wires(perhaps (3)?, which one of the prongs do I ground out, or does it matter? I am going to pull the dash out this morning, as I need to replace the bulbs as we have no dash lights. Perhaps if I disconnect everything, and clean all the contact points I will have lights and fuel gauge?

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1979 Mercedes 240D (Sold to a neighbor last year,now I have room for a new one,looking for a 300D this time)
1980 Mercedes 240D(bought for a parts car, but I have fun driving it!)

I do not know the color code from the dash gauge to the tank. If you have the dash cluster out you may get the color from the wire that feeds the actual gauge. That may not be good enough so a wiring diagram may be required to get it.
The three wires at the plug should be the actual gauge, the low fuel light indicator and probably the third is the ground. Come to think of it just short each one to ground at the sender plug end to see if the gauge does respond. It will go to empty. There will be no damage as each function is set up to deal with that senario when in actual operation. Until you prove the actual gauge is out do not mess with it. It is still the least likely thing to be bad. I have dealt with more than a few gas gauge circuits in my time. Does not make me an expert though.
Your dash gauge sitting at the top means it is being fed its operating voltage. Also if you do not feel comforatable shorting the rear pins to ground. You should read some voltage on two of the three pins on the plug back there to ground. Yet the shorting out is the real test for the gauge function. It should go to empty.
I also think you can get a reading on the tank sendor gauge if you wish. One terminal may be a dead short, one will be open, the other one should read some resistance below 150 ohms but not a dead short or open. The above just for what it's worth. Again, statistically if something is wrong on these particular cars it will usually be the tank sendor in the vast majority of cases.

Barry,
This is what I just checked. Turned key on, and fuel gauge went to full. Turned key back off. Unplugged wire at sending unit. Turned key back on, and fuel gauge showed empty(which it should with no juice to it). Unplugged wire at sending unit, there are 3 wires in connector. Checked first wire with voltmeter, it read 11.8 volts, and made the low fuel light come on in the fuel gauge. Checked wire number 2 and it read 4.66 volts and made the fuel gauge register half full. Checked wire number 3 which was apparantly the gruond, as nothing happned to the gauge. I am still going to pull the dash today, and check my connections and replace the dash bulbs, but I thought what I have checked so far might give you some indication of what else I need to do?
thanks,
Jim

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1979 Mercedes 240D (Sold to a neighbor last year,now I have room for a new one,looking for a 300D this time)
1980 Mercedes 240D(bought for a parts car, but I have fun driving it!)

From what you describe I think the tank sender is bad. Not totally sure as I worked primarily on american gas gage set ups years ago but it sure smells like it. Mercedes might have thrown a curve into the mix. You have in a way identified two of the terminals though. The one with twelve volts and the one with 4,88 volts. On the sendor you could now read the resistance on those two terminals. One I believe should read open and the other some resistance below 150 ohms if the element is working. If both readings show no resistance then it is defective for sure. I think you are really close now.

With the meter switched to OHM, would I put the red lead on one of the terminals I had voltage on and the black lead on any ground or the ground peg on the sender? If this is the case, then I would check the other terminal that had voltage and ground the other lead the same?

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1979 Mercedes 240D (Sold to a neighbor last year,now I have room for a new one,looking for a 300D this time)
1980 Mercedes 240D(bought for a parts car, but I have fun driving it!)

With the meter on ohms sometimes indicated as a upside down U you could use the prong on the tank sender that had no voltage as a ground. It does not matter if you use the red or black probe to measure the simple resistance you are looking for. From that ground just measure to your two connections that correspond to the terminals with voltage. Or even simpler the tank sender face should provide a good ground to measure all three from if you wish. I am trying to make this less confusing with little success..

Ok, Barry, see if you can decipher any of the following. With the wire plugged into the sending unit, the fuel gauge displays full when the key is turned on, and empty when the key is turned off. If I unplug the wire from the sender, the fuel gauge reads 1/2 tank when the key is turned on, and empty when the key is turned off. No wire is hooked up to the sending unit! Now, on to my ohm readings. My voltmeter, when turned to ohms, reads "1", when I touch the leads together, it shows all zeros. On the sending unit, the connector that the 12 volt feed goes into, reads "1" when I have one lead on it, and the other grounded. On the sending unit, the connector that read 4.3 or 4.8, when I touch it and ground the other lead, it zeros out my display. On the sending unit, when I touch the connector that is the ground pin, and ground out the other lead, I get all zeros on my display. Now, does all this information tell you anything? Really has me baffled when the fuel gauge moves, when the wire isnt connected to the sending unit.
thanks for the help!
Jim

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1979 Mercedes 240D (Sold to a neighbor last year,now I have room for a new one,looking for a 300D this time)
1980 Mercedes 240D(bought for a parts car, but I have fun driving it!)

Later thia morning I will go look at a gauge set up and examine it in one of my 123 cars. Mercedes may have a balancing bridge type of circuit of some sort in there. Or maybe something else. I will see.
It is the first car that I have ever known to seek the halfway point on the gauge with the sender disconnected. It might be a weak ground in the cluster though or even the gauge.
From just reading about your rehostat I see someone has been in your cluster before. Why someone would just remove a bad rehostat is beyond me as well and do nothing else. Or perhaps they did do something else that is affecting the gauge circuit.. The speedometer looks like something just let go in the speedometer head during the extraction of the cluster unfortunatly. Or perhaps,when and if you painted the needle. Since the odometer is working you hooked the cable up right. Again I will try to have some usable input after checking one of my cars.
There is almost a comedic component here if you can get by the frustation of it all. Before you could not really see how fast you were going at night as you could not see where the speedometer needle was. Now the dash lights are working you can at last see the needle at night but it is now meaningless.
Nothing to really get concerned about as there are tons of surplus clusters out there if it comes to it. They can be had quite cheap if not free as well with a little effort. Hopefully what you have can still be made to function.