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.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member

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

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J.B. Hebert

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Current Vehicles:
'76 Ford Bronco
'78 Volvo 262C Bertone V8
'80 Alpina B7 Turbo Coupe (For Sale)
'94 Ford Explorer
'95 Mercedes E300D Sportline+

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.

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1986 300E 5-Speed 240k mi.

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

__________________
1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member

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

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car