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1979 300SD Reserve Fuel light stays on

Started by Ray R, 22 April 2015, 06:55 PM

Ray R

As the title says, my reserve light stays on no matter what fuel level I have. I removed the wire plug at the sending unit as a test, and the light went off. I also removed the sending unit, took it apart and cleaned it, and the light still stays on. The sending unit did have some goop, but no where near as bad as some of the photos I've seen posted. I was also very careful not to damage any of those tiny wires.

The fuel gauge itself works fine.

Any ideas?

Ray R

I found a good used sending unit at a reasonable price which should get delivered in the next few days from SoCal Mercedes Parts (6 month warranty). Hopefully that will cure the issue.

TJ 450

Yep, it sounds like the float in the sender might be stuck.

1976 450SEL 6.9 1432
1969 300SEL 6.3 1394
2003 ML500

Ray R

Quote from: TJ 450 on 24 April 2015, 05:13 AM
Yep, it sounds like the float in the sender might be stuck.


Nah....the float moved freely when I took it out for cleaning.  And the gauge registers the fuel level accurately. If the one I'm getting fixes the problem, there must be something wrong with the contacts in the old one. But nothing I could tell visibly. The wires and float all looked good. And I'm not smart enough to test resistance and such in the circuitry. If someone has detailed step by step instructions, I could probably do it. I'm good at following directions. Even my wife thinks so.  ;)


Ray R

The sending unit arrived late this afternoon. I installed it right away, and my problem is fixed!


I hope this helps someone with a similar issue in the future.



My issue is that when I make sharp turns or fast take offs uphill, my 300sd wants to turn off.  My diesel slows down and just when you think its going to shut off the car gets a jolt and starts to move again. 

Please advise

Ray R

I'd guess it's a fuelling issue. One of the filters may be restricting fuel flow. You have three. One screws into the fuel tank (1 13/16" nut), a semi-clear one in line mid-engine on the drivers side, and one that looks like a spin-on filter in the upper front engine bay, driver's side.

Have you ever changed the filter in the fuel tank? Mine was so gummed up I would only get a trickle of fuel out the line when disconnected back there. Folks often forget there's a filter in there.

If you don't know the last time all three were changed, it's probably time to change them.

Ray R

#7 used replacement fuel sending unit has started acting up. My reserve light is constantly on again no matter what the fuel level is. The gauge reads accurately though. I bought another used one yesterday, and have the same result.

I know it's a problem with the sending unit. I know because I bought a "new" unit off ebay that arrived with the tiny fuel level wire broken. But the reserve wire was intact and when I tried that unit, the reserve light worked properly. (Testing them all with the unit out of the tank manually moving the float up and down).

A couple of things I learned so far along the way.....
Although the tiny wire looks like two, it is actually one continuous wire soldered on each end to the tabs at the top of the unit.
That wire is 40 gauge resistance wire (about .06mm diameter) with @ 67ohms per foot of resistance. I measured it using an ohm meter with the contacts placed a foot apart. I found some nichrome wire off ebay (vaping wire) that fits the bill. I'll try soldering the new wire to the "new" unit next once it arrives. Tedious soldering work for sure. But better than the $370 the dealer wants for a new one. And I'm enjoying the learning experience.


I know this is an old thread, but I found the info posted by Ray R particularly helpful when fixing my sending unit this weekend and just wanted to a) validate the resistance wire info above, and b) share some of my observations and pics after completing the same process.

My fuel gauge didn't work, and after removing the first-aid kit tray, I found the sending unit was disconnected. I know better than to think anything is ever that easy, and sure enough, upon reconnection it provided no reading--only a low fuel light (even though the tank was half full), which is I'm sure why it was disconnected in the first place.

I got lucky and happened to have a 46mm socket (for removing harmonic balancer bolts from my BMWs), and upon removal found the sending unit to be in bad shape. The thin aluminum tube was dinted and filled with sediment, the inner slide shaft was badly corroded, and the resistance wire was broken off of both tabs.

*** PSA *** When removing the unit, pull it out slowly and let it drain. There's a weep hole on the side near the bottom to allow fuel to drain from the tube. It will gleefully shoot a stream of fuel all over you and your seat if you just yank it out. Ask me how I know...

Took some time to straighten out the tube, remove as much corrosion as possible from the float shaft, and scuff the float brushes & low fuel contacts with some emery cloth, then set about re-assembling. It's a rather tedious and fiddly process, but beats spending $300 on a new one! Here are a few things to consider:

 - A project vise and/or helping hands are your friend. Lots of little parts to align all while keeping the wire taught and manipulating a soldering iron.

 - Speaking of solder, it doesn't really want to stick to nichrome wire or the tarnished contacts in your sender. Remove the old solder with some wick, polish the ends as much as possible, wrap the wire around each tab a few times to ensure it's secure, and use some flux.

 - The hair-thin nichrome wire has to wrap over and under two brushes on either side of the float. The brushes are both "V", shaped, and the bottom of the V is what should be in contact with the wire (sorry, no pic of this).

 - The nichrome wire needs to wrap around the outside of the tab or it will rub on the body of the float (see 2nd to last pic)

 - This is wire I used:


I forgot the most important photo!  :)  :)  :)