If the light comes on during the initial test, when the key turns on then the problem is probably in the fuel quantity sending tube. the tube has 2 wires on either side that a float goes up and down on changing the impedances and the gauge position. When the float gets to the bottom, 2 contacts are made at the bottom of the tube and the reserve light comes on. I would pull the quantity tube in the fuel tank and clean it. Make sure the contacts are good. You may have to clean them up with some sandpaper.

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