380SL-gas gauge shows ~1/2 full, reserve light on

[Chris8]

Need to drain the tank. Gauge showed about 1/2 full. Siphoned about 7.5 gallons out - gauge reads about the same, but the reserve light is on now.

I read the sending unit float has a metal plate on bottom, which contacts a switch (2 contacts) when it gets down low. Assuming the float is stuck ~1/2 way down, what could make the reserve light come on?

It was hard to get the hose in, so wonder if I didn't hit the sending unit when jamming it in the filler tube, and do something (like the bottom plate fell down and landed on the contacts - I doubt this...).

The siphon hits air now, so I'd like to pull the plug from the tank line and let remaining gas drain into one container (an oil pan - easiest; or 2.5 or 5 gal can). I read one guy ran out of gas about 10 miles after hitting reserve (about 1/2 gallon), and another measured gas into a dry tank and said light went out at 5.8 gal. Seems 3 gal was consensus.

[meltedpanda]

the missing information is if your fuel guage was accurate before this draining endevour? You are correct on the function of the sender. So if your gauge did not work prior I'd say you have a bad sender If it was working , your guage could be the issue , or both
I doubt you hit it with the drain hose if you went down filler pipe to bottom of tank

[Chris8]

The gauge worked before - I don't know how accurate it was. I only got about 1/2 gallon when I drained the rest of the gas (basin wrench worked well to pull the hose/strainer came with it), so reserve light should have been on - and the float was at bottom of tank. With only 1/2 gallon less, the needle moved from just left of 1/2 to just right of 1/4. Put in 5 gal, the needle looks to be in the same place (~1/4), and the reserve light is still on.

I'll look at this more when I start driving the car, but you made me think to check gauge connections. Someday I'll pull the sending unit. Here's a good writeup on how to service them.
And so it begins.... - Page 13 - Mercedes-Benz Forum

Ah - maybe the resistive wires are dirty - so the float moves but reads wrong (would be higher resistance).

[meltedpanda]

I have repaired a few. Sometimes cleaning the wires works. The wires are nichrome and hard to source. I have replaced the wires but never with much success. Search my name and fuel sender repair.