I have a 1980 300SD. The fuel gauge was stuck in the empty position. I added some Biobor to the fuel when I filled the tank. After a short while, the fuel gauge went to the full indication. I removed the sending unit from the fuel tank and cleaned it with carborator cleaner. It was not clogged with slime, but it had some slime on it. Now it seems to be stuck on the full indication.

Do I need to replace the sending unit.

Testing fuel gauge for Sedans/Coupes
 

Is your float able to slide up and down freely? Is it possible you injured the sender when you cleaned it? Or it is not grounded properly?

Here is how to test the sending unit. PLEASE BE CAREFUL OF SPARKS WHEN WORKING AROUND THE FUEL TANK. I just had to say that.......

Remove the sending unit from the fuel tank.
Connect an ohmmeter to terminal G and terminal 31.
Hold the sending unit in the upright position, like it would be if it were installed. The float should have slid all the way to the bottom, thus engaging the reserve warning contact. If it didn't slide down, you have some problem with the pole and float. With the float at the bottom the ohm reading should be: 56.9 plus or minus 1.9.

Flip the sending unit upside down. The float should slide to the full position. The ohms now should read: 1.6 plus or minus 0.7.

Now test the reserve warning contact. hold the sending unit in the upright installed position. Connect an ohmmeter to terminal W and terminal 31. The value should be 0 ohm, and flipped upside down it should be infinity ohm.

Test the wiring harness: Do not make any sparks!!!! I just had to say that again.....

Measure resistance from jack 3 on wiring coupler and vehicle ground. Nominal value 0 ohm. If you get an infinity ohm reading then the grounding line is interrupted.

Reinstall the sender, and measure the resistance on terminal G and terminal 31 on the sending unit. The reading depends on the amount of fuel in the tank. Between 1.6 (full) to 56.9 (empty). You should know now if you need a new sender. Plug wiring coupler back on installed sending unit.

Here is how to test the gauge in the dash:

Pull coupler from back of fuel gauge and measure the resistance between jack 3 and jack 15 on the coupler. The value should be under 2. A slight deviation due to length of wires is permissable. If the value is attained the harness is in order. If the reading is higher or at infinity, then there is a poor connection or interruption in the wiring harness somewhere.

If all these tests are okay, then you have a faulty fuel gauge on the instrument cluster.

This is from my W123 chassis manual, but I believe it is the same for the W116.

The reserve light has been changed starting September 1982. With the ignition switched on, the reserve warning lamp will light up(checkup function). As soon as the engine is running, the lamp will go out, provided the tank is not empty. I recall this question not being answered in a previous post as to when this feature began...

Hey Patsy,

Thanks for the information, that is absolutely amazing. That is exactly the information that I needed. But I just finished taking the sending unit apart to inspect it again and one of the wires was broken. I am sure that it was my inspection process, but the float was not operating smoothly. So, it looks like I need to replace the sending unit.

But you might be able to help me out with another challenge. I have a 1983 240D that the temp gauge always reads max. I have replaced the sending unit. If I measure the current to the sending unit, it is over 12 V. I have replaced the circuit board for the gauges and swaped out the temp gauge from the new board (I tried the temp gauges from both circuit boards). I am ready to buy a new set of gauges, but I am not sure that would solve the problem, if I replaced the board and had the same problem with two different gauges.

What is the diagnostic process to determine what is going on?

Tref, I am glad to help, even though it was not necessary. I am not sure about the test for the temp. gauge, but I will keep looking. It sounds like the gauge is not grounded??? Maybe someone else will know.

Tref, I believe your temp. sensor goes to the glow plug relay, and then to the gauge. See if all your connections are in order. The resistance of the temp. sensor is:

At coolant temperature:

0 Deg. Cel. is 8200 Ohm
+25 Deg. Cel. is 2440 Ohm
+80 Deg. Cel. is 290 Ohm

If two gauges did not work, it is most likely a loose connection somewhere. Good Luck