Sending unit's out; how do I take it apart? - PeachParts Mercedes-Benz Forum

The Warden · #1 01-13-2003, 09:17 PM

Okay, I've got the fuel sending unit out of the car...pretty easy.

Now, for the million-dollar question

How do you get the big metal tube out? I'm referring to the one that houses the float itself. I want to clean up the contacts while I have the car down.

I've done a search, and other posts refer to a small nut on "the bottom" of the tube, but I can't find it. I've got a plastic sheath around a metal part which I would assume is where the fuel drains out, at the very bottom. On the top, I've got the 3 electrical contacts, and a plastic dowel and nothing else.

How does this work? Thanks in advance!

jerryb · #2 01-13-2003, 09:35 PM

Been a while but I think there is a plastic nut on the bottom, unscrew and pull off the tube, inside is the float and the sender for the warning lite. There are 2 thin wires that go from top to bottom and they are fragile. That's for your warning lite.
the wires are stretched tite and soldered at the ends, if a wire is loose you can bend the tabs enough to resolder it then bend it back to stretch the wire, it will be obvious when you get it open.

#3 01-13-2003, 09:53 PM

Warden:

This is from a previous post of mine,

The sending unit is somewhat delicate, so treat it with care. The first thing I would suggest is disconnecting the battery. This is added insurance to make sure we don’t short something out. On my 300 SD, the sending unit was located under the hat shelf (as it is called in my book) under the first aid cavity compartment. I had some trouble getting that compartment out. My rear deck has seen some better days, and it seems the compartment was somewhat brittle. I think there were four screws that held it in. Under that compartment is the sending unit (another reason why MB are so great). Maybe before you go any farther get the pan and rags mentioned below,and a permanent marker, and some tape wide masking or duct tape would be ideal.
The sending unit has a connector on the top that pulls off, straight up. Now place a mark on the sending unit, and the tank. You will use these as your line up marks when reinstalling and tightening the sending unit, unless you have a torque spec and torque wrench and the correct socket in the next paragraph. The top of the sending unit is a huge nut (1.856" or 47 and some MM). Unfortunately I did not have a socket or wrench to fit it, so I used a large pair of channel locks. It is a right hand thread(righty tighty,Lefty loosy). I backed off the nut (which is the sending unit) and withdrew the unit. Have a pan or something because the sending unit will be filled with fuel, depending on how much fuel is in the tank. Like mine the holes were partially blocked, and I had to let it stand over the tank for a while to drain. (I had not planned for this, as I had nothing close by,or rags). The unit is about 3" in diameter and 8" to 12" long. At this point take that tape, and put an FME cover over the hole. (Foreign Material Exclusion) Nuclear plant terminology and a good practice for anything left open.
On the bottom of the unit I believe there was a nut, that holds the assembly together. I loosened the nut and gently tapped on the side to release the can around the sending unit (a coating from the fuel forms locking this assembly together). Once inside look at how it operates. The fuel fills up the can slowly through the holes in the side and moves the float up and down. Now observe the bottom plate. It has almost a circular mouse maze the fuel follows into the sending unit (I think this is used for dampening when the fuel gets down very low). Mine was very dirty. I carefully separated the sections (Make note how they come apart so you don’t mix them up) and cleaned them in mineral spirits. Going back to the sending unit, the float (if I remember correctly) rides up and down on a resistive rail. I cleaned this (the rail) with Q-tips and mineral spirits. At the bottom of the travel (float all the way down) there are two contacts. These activate your reserve light (Contacts on float touch contacts on housing assembly). I used Q-tips and mineral spirits, but I think I also took an eraser from a pencil and rubbed them a little. I had the liberty of checking this out with a ohm meter to check continuity.
I then took the unit back to the car, Plugged it in,(make sure it is not positioned in such a way that the internals can short out on something metal) hooked up the battery, turned the key on and exercised it and observed the gas gauge. Note:I don’t remember if I had to take the unit and ground it by touching it to the tank(it will respond instantly because it is now not dampened. I made sure if I moved it slowly the gauge would follow with no dead spots (Gauge bouncing or going in the opposite direction of the float movement). I then took the float all the way to the bottom, or if you have the sending unit vertical, let gravity take place. The light should come on. If everything works, just turn your key off, disconnect the battery, and assemble the unit the way you took it apart. When installing watch that the o-ring is centered in the bottom of the sending unit. You don't want to pinch it, or cut it. I would tighten to the same mark you place on in the beginning. Connect connector reinstall first aid compartment, and enjoy now knowing how many miles to the gallon these soot blowers get. If you have any questions feel free send me a thread. I will answer any questions you have before you attempting it

PeterG

79 300SD

The Warden · #4 01-13-2003, 10:05 PM

Sounds good...except I don't see this nut.

There's a plastic piece at the bottom, as mentioned before, but it a cylinder shape and doesn't appear to be able to unscrew.

Could they have changed the design prior to '85? Inside the plastic cylinder (it's maybe 1/4" long BTW) is a metal piece that has something sticking out the center and a slot that looks like a slot for a slotted screwdriver, except that it is interrupted by the part that's sticking out. The part that's sticking out doesn't appear to be a check valve or anything; it won't move at all.

lrg · #5 01-14-2003, 12:38 AM

Warden,
We have been calling it a "nut" but it's really a small round metal piece that is threaded in the center and has a couple of small notches on top. It's in the very center of the bottom plastic piece and looks like part of the bottom piece but it is not. I used a small pair of needlenose pliers and stuck the "nose" end in the notches and turned. It turns a bit hard at first but once started it should spin off easily. The plastic bottom comes off and then you can remove the whole aluminum tube exposing all the works. Be sure to clean the bottom plastic "mase" really well because this is what lets the tube drain when the tank gets low. The guts of this is fragile but not THAT fragile. It should easily take some gentle poking with a toothpick and Qtip. Note too that there is a notch in the aluminum tube at the top that you will need to line up with the top piece on reassembly. Have fun.
LRG

#6 01-14-2003, 12:23 PM

Senior moment, Irg your correct, it was not a nut, but some strange castle thingamabob. Sorry for the mislead Warden. The old brain is just a little Diesel soaked if you know what I mean.

PeterG

79 300SD

The Warden · #7 01-14-2003, 04:15 PM

Okay, I got it apart. Took a pair of needle-nose pliers that I didn't care about (I've got a bunch of them, but they're all too big!) and attacked it with a Dremel tool until it fit. Worked like a charm.

Talk about a precision instrument! At least it's straightforward...not to mention quite dirty. I don't have any mineral spirits handy; anything else you can think of to clean everything up? I'm thinking along the lines of rubbing alcohol, or even carb or brake cleaner

Mine is also quite dirty...

Thanks!

{added in edit} Also, is the float supposed to be snug against the center shaft, or is there supposed to be some play? The float feels rather loose...also, after doing a dry Q-tip wipedown, I plugged the unit in and had my girlfriend turn on the key...there's still some swinging around, but the needle will only go down to slightly below the halfway mark...I may not have had a good ground, though; that may be why...or, again, I haven't finished cleaning it yet...

lrg · #8 01-14-2003, 05:58 PM

I'd think alcohol or carb cleaner is OK. You just want to avoid a strong solvent that might damage the float or lower cap plastic. A small bit of gasoline might work too as long as you don't blow yourself up. The float should move easily up and down the full travel with no binding and it may feel a bit loose. Also, the contacts on the bottom (for the warning light), on the float itself (for the gauge), and the wires that the float contacts slide on should all be completely clean. I believe the unit works by the varying resistance as the float moves up and down on the wires so all the contacts and the wires themselves have to be clean. If the wires and float contacts are not clean you'll get bad readings. Also if the serpentine maze is not clean, the bottom of the tube won't drain properly and will never give an accurate read at the very bottom.

BOGDAN · #9 01-14-2003, 09:09 PM

HELLO EVERYBODY
I FALLOW THIS FORUM FOR 2 MONTS, FROM THE MOMENT I GOT MY FIRST MB 300 CD COUPE 85 AND I DO NOT NOW HOW TO ASK QUESTIONS FOR MY CAR MAYBE YOU GUYS WILL TEACH ME ( I DO NOT NOW HOW TO SEND THE NEW TREAD AT THE END OF MY MESAGE)
NOW MY SENDING UNIT INDIFERENT THE DIESEL QUANTITY , VERRY OFTEN IT SHOW THE REZERVE LIGHT ON BUT THE FUEL AT THE RIGHT LEVEL HALF OR FULL TANK
PLEASE , I APOLOGISE FOR MY ENGLISH , IT IS NOT MY FIRST LANGUAGE , I WAS BORN AND RAISE IN EUROPE.
ABOUT THE SENDING UNIT I REALLY HAVE TO TAKE OUT EVERYTHING OR I CAN FIX FROM OUTSIDE( ANY WIRE OR CONECTION PROBLEM)
THANK YOU FOR EVERYTHING

Piotr · #10 01-14-2003, 11:21 PM

Bogdan, send me a private message (in Polish if you like), we'll figure it out.

Now my gauge simply and suddenly began showing empty tank with at least 1/2 fuel still in. Originally, the needle would not move until the tank was about 3/4 full. However, the warning light works as it should. I pulled out and cleaned the sending unit, but that accomplished nothing. Is it possible, that a resistor at the gauge went bad? My simples solution would be to "recalibrate" the gauge by simply moving the needle, but I rather try other stuff first.

lrg · #11 01-14-2003, 11:31 PM

Piotr,
I wouldn't try to fool with the gauge unless you are absolutely certain that the gauge is the problem and even then only as a last resort. I think the sending unit is still the culprit. Sometimes they are just beyond repair. It sounds like you cleaned it pretty throughly but did you get the contacts on the float itself? Also be sure you don't have a hole in the float. If it sinks it will read empty. Try removing the sending unit and attach an ohm meter to the gauge contacts and see if the resistance changes evenly as you move the float up and down. If you have even a little bit of black algae crud in there it can screw it up.

BOGDAN · #12 01-14-2003, 11:52 PM

THANK YOU GUYS
I THINK I DID NOT MAKE MYSELF CLEAR : THE SENDING UNIT IT SHOW THE RIGHT QUANTITY OF DIESEL FUEL BUT BECAUSE VIBRATION IN THE ROUTE THE RESERVE LIGHT TURN ON AND OF VERY OFTEN
I DID CLEAN THE FILTER AT THE BOTOM OF THE TANK BEFORE - NO ALGAE SO I THINK IT IS SOME ELECTRIC CONECTION BECAUSE OF VIBRATION
THANKS AGAIN

Piotr · #13 01-16-2003, 12:11 AM

Bogdan, your English is fine. However, please turn off the Caps Lock on your computer. On the internet it looks like you are shouting.

As to the reserve light, I would look at the point where the wires plug into the sending unit. The light comes on when the float is at the bottom at closes the electric circuit. if the gauge shows the right amount, it cannot be the float, unless there is a loose piece of wire shorting the circuit.

As my problem-thanx, guys. the sending unit is the original. the gauge is not jumpy, but I notice that if I drive for a long didtance, the gauge is "less wrong" until a sharp turn or breaking. Then after the needle "dips", it does not come back. That seems to indicate a problem with the float. If it's an empty sealed plastic box, a pinhole could be a problem (simply siiting too low). However, the reserve light comes on when it should and NOT before, as it would with the low float. as you see, mixed symptoms- very frustrating!
BTW, I DID clean the contacts on the float.

nickofoxford · #14 03-23-2009, 08:53 PM

Quote:

Originally Posted by Piotr

Bogdan, your English is fine. However, please turn off the Caps Lock on your computer. On the internet it looks like you are shouting.

As to the reserve light, I would look at the point where the wires plug into the sending unit. The light comes on when the float is at the bottom at closes the electric circuit. if the gauge shows the right amount, it cannot be the float, unless there is a loose piece of wire shorting the circuit.

As my problem-thanx, guys. the sending unit is the original. the gauge is not jumpy, but I notice that if I drive for a long didtance, the gauge is "less wrong" until a sharp turn or breaking. Then after the needle "dips", it does not come back. That seems to indicate a problem with the float. If it's an empty sealed plastic box, a pinhole could be a problem (simply siiting too low). However, the reserve light comes on when it should and NOT before, as it would with the low float. as you see, mixed symptoms- very frustrating!
BTW, I DID clean the contacts on the float.

Hmm, I just had my sending unit apart and took a minute to learn how everything works in there. Sounds to me like the float is binding a little bit on the way back up. You may have bent the thicker wire that goes to the reserve contacts while putting it back together. I busted the solder joint (for the 3rd time

) unknowingly, and the gauge left me stranded again today so I took it apart and fixed it again. Thank god this is an easy job on a w123.

Did you use any fine abrasives on the resistive wires the float rides on? Another thing to look at is the way the wire rides through the float contacts, on mine I found that it readily slipped out of the contacts in the float and still made contact, but not much. Finally I got it down to a very linear and smooth resistance change from end to end on the ohmeter.

Sometimes it's just the little (tiny) things

Best of luck

Nick

Jeremy5848 · #15 03-23-2009, 09:34 PM

See this link for a DIY with pictures. DIY. For the purposes of the DIY, the 123 and 124 fuel senders are the same.