How much does your fuel gauge fluctuate? - Page 2 - PeachParts Mercedes-Benz Forum

#16 08-22-2002, 11:21 PM

Usually when the gauge is getting near empty the needle will jump some, take a dive to R, then recover...just gets a little squirelly I guess.
Wierd, though, Supposedly the tank is 18.5 gallons + 3.0 gallon reserve. I take that to mean that when the idiot light comes on you have 3 gallons left. I've run it down to that point but can only top off the tank at around 15 gallons. Does the 18.5 include the 3 gallon reserve?

lrg · #17 08-23-2002, 02:01 PM

I've taken apart and cleaned the fuel sending units in both my W123 and W124 which solved most of the problems you have described. I believe yours are probably the same or very similar. The sending unit has a float that rides up and down on a set of wires. As the float moves on the wires it changes the resistance and varies the current to the fuel gauge. If your gauge is moving smoothly then the problem is almost certainly in the sending unit. The works of the sending unit is contained in a metal tube that has several small holes in it that allows fuel to flow in and out slowly. In the case of my two units, both had some algae built up inside the tube that was affecting the operation. Interestingly I've never seen any sign of algae in either tank, just in the sending units. It is possible to remove the unit and clean it up so it will work better. There is a small nut on the bottom of the tube that you can remove and then carefully remove the "works" from the metal tube. If you have algae it will be a black gunk that is likely on the float and the contacts at the bottom that make the reserve light go on. I carefully cleaned the float and all the contacts, including those on the sides of the float with toothpicks and mineral spirits. Make sure to wipe the contact wires throughly with mineral spirits, mine were very dirty and the uneven resistance gave me bad readings. Also note that there is a small drain in the plastic piece at the bottom that should be cleaned too. It has a small hole that allows the fuel to slowly drain out of the tube as the tank runs down. If this clogs, your gauge will not read below 1/4 and/or will be eratic.
All in all this is any easy project that will most likely solve your flucuating readings. The sending unit is not a bad design but it is very susceptable to clogging from algae. If you have algae in your tank you will need to get rid of it or even a new sending unit will rapidly get clogged up. Good luck.

Dcraig · #18 08-25-2002, 02:44 PM

I did take out the sending unit and cleaned with carb cleaner. I hope that wasn't a bad thing, but I tried not to touch the contacts and the wires. I made sure it was really dry before I put it back. That stuff evaporates.

Maybe I didn't put it back together the right way. Originally I took the sender out because the gauge didn't seem to be giving me the right readings. I can't remember how much it fluctuated.

It would seem that the fuel in my sender is leaking out to fast. If I'm on a curvy road, the gauge can move from 'R' to '1/2' and then settle back at 1/4 on a straightaway. If the fuel leaks to fast, it seems the gauge would do that.

Maybe I should take it out and check to make sure the bottom of the tube is on correctly. I don't remember any holes in the tube itself.

Craig

#19 08-25-2002, 06:06 PM

The fuel gauge on my 300SD works great (1981), level readings are always accurate, little fluctuation. However, the low fuel light is nearly always on, often flickers off then back on. Anyone know any simple fix for this short of cutting the wires? Is there an obvious reason this would happen?

lrg · #20 08-25-2002, 09:55 PM

Markulta,
Pull the connector at the top of the sending unit and drive for a while to see if the reserve light goes on. If it does you have a short in the wire. If it doesn't then the problem is almost definitely in the sending unit. The contacts are at the bottom (you'll have to remove the tube cover) and the circuit is normally closed when the float sinks all the way down. Something is likely shorting it.

Dcraig,
I agree that the sudden fluctuations indicate the fuel is leaking out of the tube way too fast. I'd take it apart and try to see where it might be leaking. The drain at the bottom is actually a kind of serpetine maze that the fuel needs to run through. Also, there should be one or two small (pencil lead sized) holes in the side of the tube. If these have been somehow enlarged that could be the problem. Also, you may want to clean the wire contacts that the float runs up and down on. I found that mine were really quite dirty and the contact was being compromised so I was getting an eratic reading. I used mineral spirits and a Qtip, being really careful not to let any of the cotton threads from the Qtip to get stuck in the mechanism. The wires look pretty fragile but I found if I was gentle it was no problem cleaning them.

Kyle Blackmore · #21 08-26-2002, 12:14 AM

My 123 only acts strange when it's almost empty,drops to R and then bounces back after a while.My 124 has the wild needle movements below 1/2 tank , I'll have to remove the sending unit and clean it up,thanks lrg.
Shawnster-the 18 gallons includes the reserve.

Dcraig · #22 08-26-2002, 12:15 AM

I saw that little maze. I think I put it on the outside of the tube, not the inside.

I can't think what might have enlarged the holes in the aluminum tube. Well, looks like I need to pull it again and check it out.

Craig

The Warden · #23 08-26-2002, 03:25 PM

Where do you pull the sending unit from?

I've got a bit of fluctuation between 1/2 and 1/4 tank, and I think my reserve light comes on a bit to early.

BTW, the owner's manual says that my 300D's tank is 21.1 gallons, including a 2.8 gallon reserve. The most fuel I've put in my tank was 16.4 gallons, and that was after going about 430 miles. The reserve light was on at that point, but theoretically it should have gone through another gallon or so before the light came on...unless the venting system on this tank's as bad as that on my Ford (I can get another 4 gallons or so in my pickup's tanks by topping off; it also typically leave me at a fueling station for over an hour though)...

However, if memory serves, wagon tanks are a few gallons smaller, so the 18 gallon number on the TD is likely right.