Fuel level sending unit question
 

My low fuel level light only comes on when the tank goes completely empty. Furthermore, once I get down to about 8 gallons in the tank, the fuel level gauge goes down to R and stays there forever. From what I found out from searches and from my own suspicion, this means my fuel level sending unit is messed up right? Just wanted to get the opinions of techs or people who have gone through this before I purchase a new sending unit and start the replacement task. Thanks. Oh, car is the '84 300SD.

Also I forgot to mention that my temperature gauge isn't working properly. I already changed the sending unit but it still flickers sometimes. Now that I think of it, the temperature and fuel level gauges are on the same part of the instrument panel. Maybe that is really messed up.

Some considerations

This post is based on my W123 work, I dont' have a 126 manual anymore since the 300SDL is gone. So minor variations may exist. Someone with a 126 will correct me if I'm wrong, I'm sure.

I played with the fuel sending unit on my daughters 240D.
The lack of a correct indication could be due to algae/crud on the sending unit wires. Remove the unit and open it up. Clean it with carb cleaner. It will take 2-3 cans of carb cleaner.

Removing the sending unit.

http://www.peachparts.com/shopforum/diesel-discussion/102003-w126-removal-1st-aid-kit-box.html?highlight=fuel+sending+unit

Then open the sending unit by unscrewing the nut at the bottom. Slide the tube off the assembly. Be careful not to damage the 2 wires. Clean the unit completely with carb cleaner. Check the resistance at the plug with an ohm meter while sliding the float up and down, this will make sure your unit is making contact. Also check the reserve light contact. Reassemble and install with a new Oring.

Here is the SM for the W123

http://skinnerbox.steaky.org/Service/W123/Index/617T/47FuelSystem.htm

Also the reserve light goes thru the seatbelt warning relay. My daughters light did not work. I replaced the seatbelt relay and it now works. Test the light first. Test the reserve contact in the sending unit. If they both check OK then it could be the seatbelt relay.

These proceedures shold be covered in the Service Manual.

Dave

My 240D didn't have a nut at the bottom of the sender so I could take it apart, I may pull it in a day or two and look at it closer. All I did when I had it out in the spring was spray carb cleaner into the holes and let it drain.

My 240 didn't appear to have a nut. It had a small round thing which screwed onto the center rod. I used channel locks to remove it CCW. It was full of junk which kept it from reading accurately. It had three wires inside. Delicate.

Dan

I had a similar problem with the gauges on my 300sd. Started of the fuel gauge didn't meaures the fuel in the tank properly. Learned a whole lot from dissassembling and reassembling various clusters. Now, five clusters later, I have a fully functional unit. I have 2 or 3 300sd gauge clusters laying around at the house, but can't remeber which one's have good gauges, and which ones have bad gauges. If it comes down to it you should be able to buy a whole entire cluster for less than 100 dollars. Check ebay, user id usedpartshunter, that is who I got mine from. If you have access to a gasoline speedometer though, you might be able to make it work to an acceptable degree. Of course the diesel units don't have the economy gauge, but other than that, the oil, temp, fuel gauges are virtually the same. You would though have to get a wiring diargram of the cluster and rearrange the pins to match the gasoline's board arrangement. Probably eaisier just to get another one though.

Thanks Dan, I'll give that a try tomorrow. :D

The carb clean works magic..
I ran a bunch through mine and it seems to be ok now
a lot smoother
Why does my 300SD w126 only seem to have a 18 gallon tank?
I looked it up and it is supposed to be more?

related (W123) fuel level sending unit question
 

Hello,

My low fuel warning light started to come on dimly this week, with more than half a tank of fuel (240D). When I topped off the tank, the light went out for a few miles. Then it came back on. Today, I replaced the voltage regulator to no avail, and checked the fuel level sending unit, but I did not get the fastener off the bottom, so I did not inspect the wires. It seemed like some black crud was coming out the top holes when I held it upside down. Is this probably a sending unit issue? I only replaced it seven years ago when I got the car because it was not working at all at that time. Price of a new unit must be over $100 by now. Don't want to spend it if I don't have to. BTW, this 240D now has 353,576 miles. :D

Get some carb cleaner and use the little tube to stick in the holes and clean that crap out of the inside, it may work by then. Also you will most likely need to clean the fuel tank and lines of this black gunk at some point.

That little nut on the bottom of the sender has something like a tight fitting little "round" of plastic tubing surrounding it. I just preyed if off, undid the nut, fixed the sender and used thread lock on the nut when reassembling.

Come to think of it I think I did finally get the nut off, because I did eventually get it apart, just been so long I forgot. My memory is not what it use to be to say the least.

LOL, Yah me too...what were we talking about?:D:D

I figured if I started prying that little plastic tubing off, that would be the end of the sending unit but I could bring the unit into the shop tomorrow and attempt it.

I got into one where the little wire was broken at the bottom. A friend held it and I soldered it back together. I tucked it away for a spare and didn't try it, but I think its fixed:P