I took the 240D fuel sender out a few weeks ago and did the "shake shake shake" fix and it cured the jumpy fuel gauge needle, however, the reserve light was still constantly dimly lit when it shouldn't. When I unplugged the connector to the sender, the light goes out so I was pretty certain the problem was in the sender.






Fuel sender pin T is gnd or common, G is sender, W is reserve switch

float down: W to T should be zero ohms (switch closed)
float up: W to T should be infinite ohms (switch open)

I was getting a resistance reading of anywhere from 280 ohms to 400k ohms between pins W and T. I tried cleaning the switch mechanism with solvent quite a few times but the resistance got higher but would always come back lower after I installed it and the bulb remains dimly lit. Turns out I had to partially disassemble the reserve float switch mechanism (remove the 7/32 hex nut) to literally scrape away whatever gunk that was shorting the switch out to gnd. The resistance changed to 2 to 3 meg ohms which is not perfect (infinity) but good enough to not keep the light constantly on ( which was bugging me )

Anyway, don't know if this is a common problem or not with the fuel sender but this may be useful info for someone in the future.

btw I am very impressed with how well built the sender is: Good quality materials (alum, stainless, phenolic, plastic and rubber). Gold plated contacts. If you've opened one up, you can tell it was designed to be disassembled and repaired.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Thank you for this info.

One thing does not make sense. You must mean the reverse:

float down: W to T should be ZERO ohms (switch CLOSED)
float up: W to T should be INFINITE ohms (switch OPEN).
The light circuit would be sent to ground to cause bulb to illuminate on a low/empty tank.

__________________
Dionysius

Your right. I musta been on crack when I wrote that. I'll see if I can edit it.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I am seeing the Reserve light always on but dimly on my 1979 300SD.

I like it this way since I have a current check that the lite bulb and circuit is operational. When a real alarm occurs the lamp glows to full brightness.

This is one of those rare occasions where a malfunction results in a useful feature.

From your findings the cause would appear to be a buildup of algae across the contacts acting as a resistor and conducting some current to ground.

__________________
Dionysius