Full tank, reserve light on?
 

Lately the reserve light has been on constantly, even though the the tank is full. Is this an instrument panel issue or sending unit issue? This is on our '87 TD.

I had 2 123 chassis cars with the same problem, and both times it turned out to be the sender.

2nd vote for the sender.
The reserve light is controlled by a relatively simple on/off switch in the sender unit. Less complex than the gauge part. Perhaps you could get it to work just by cleaning it up or something.

You could even try whacking the top of the sender with something (carefully) first to see if you can shake it loose.

How do you get to the sending unit in a W124 TD? Keep in mind I have the third row seat in the back. Thanks

In the 123 ( and I'm sure Mercedes would follow the same method of design) the sender has a float that when at the bottom, IE empty, makes contact with 2 probes on the bottom. This sends the signal to the guage light. However the reserve light test curcuit goes thru the seat belt buzzer module. So determine if the illumination is due to the seat belt module or the sender.
To test the seat belt module disconnect the buzzer module, just after turning the key on. This is when the test of the light is done. If it is still on then disconnect the sending unit. Once the car is started, which turn off the test curcuit
Where is the seat belt buzzer module in your car? I do not know.
Also the test curcuit is dimmer than the actual low fuel mode.

Dave