Reserve Fuel Light

[Mack Roark]

The reserve light on the fuel guage on my 1983 300D comes on as soon as, or even before, the guage reaches a quarter tank. I've removed and cleaned the sending unit, perhaps not adequately. Before the cleaning the guage was very erratic at about one-third tank. On previous 240Ds the light rarely came on - had to be almost empty. Question: Is this common? Is it guage or sending unit? I can live with it as I never wait past quarter to fill up.

Thanks for input.

Mack

1983 300D
1972 280SE

[Maxbumpo]

Usually the light is much more accurate than the gauge. Verify by filling tank as soon as the light comes on. If the tank can be filled with its capacity minus reserve, then the light is what you should pay attention to.

The inaccuracy of the gauge can be attributed to many things, including the sending unit, resistance in the circuit somewhere, poor ground, old gauge in the instrument panel.

[Alex]

Mack,

My experience is that both the gauge and the light have been accurate over the years, except when the sender failed.

While never waiting past the quarter mark to fill up may work for you most of the time, there will come a time when an accurate fuel gauge will prevent the gnashing of teeth.

I urge you to get to the bottom of the malfunction so that you end up with an accurately functioning fuel gauge you can depend on.

[Mack Roark]

First, thanks to both MaxBumpo and Alex for correcting my guage to gauge. Not sure how that happened. Wish I had someone to blame.
I filled up today as you suggested. The gauge was at quarter tank, and the light had been on for maybe 30 miles. It took 16.8 gallons. If the tank is 21.1 gallons minus 2.8 reserve, that leaves 18.3 gallons. This less the 16.8 I put in gives 1.5 plus the 2.8 in reserve yielding 4.3 at least in the tank when the light came on and it was at 1/4 on the gauge. Seems neither light nor gauge are correct.
I'll start by cleaning the sending unit again. As soon as I get a warm day.

Again, thanks for the help.

Mack

[Maxbumpo]

I don't think that 21.1 gallons is correct - are you sure about that? I'll bet you've got a 70 liter tank = 18.5 gallons.

18.5 - 2.8 = 15.7 gallons.
(drive 30 miles)
2.8 reserve - ~1 gallon of driving
=> should take about 16.7 gallons to fill.

If you get about 30 mpg, then the math works.

[Mack Roark]

Maxbumpo,

You may be right, but I'm going by the MB Service Manual Series 123 Volume 1 page 040/4 (hard copy) which gives capacity for 123.133 as 80.0/10.5 = 21.1/2.3. The 123.193 (wagon) is given as 70.0/2.9. There may be a misprint, or I may not be reading this rightly. If your numbers are correct my issues are answered. Please double-check this for me, at your convenience. I will be away from the computer until after Easter, but will catch up then.

I am consistently right at the 29-30 mpg, and on the highway sometimes over 30. Thanks again.

Mack