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  #1  
Old 02-13-2014, 03:33 PM
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Fuel gauge vs fuel sending unit

Good Afternoon
I would appreciate some help with my 1971 280SE convertible.
How do I determine which model, W111, W108, W113? I have the vin. Does that tell me.
I have a fuel gauge that does not work. I have power to the sending unit in the trunk. I removed the fuel sending unit but it is a solid tube and did not know how to take that apart or if I should mess with it. The car sat in the barn for 10+ years so lots of things gummed up!
I was going to remove the dash cluster to check the fuel gauge but got stuck. How do I remove the cluster? I read on the forums there are knobs on the back of the cluster that if removed would allow me to remove cluster but I cannot find any knobs. Not only that I have big hands for a woman but have no idea of how a mans hands would fit behind the dash. It would be alot more fun to drive if I knew how much gas I had!
Any help appreciated. I can order a new sending unit but hate to start throwing parts until I diagnose the problem
Thanks
Pam

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  #2  
Old 02-13-2014, 05:03 PM
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111. The sending unit should make a noise when you tip it up and down. If the tank is gummed up it should be cleaned. Drain it and look at it from the sending unit hole. Most likely not the gauge.
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Old 02-13-2014, 09:41 PM
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Thanks

Thanks for the help. It does make noise and gas comes out so I guess OK. Already drained and cleaned tank. It sat in the barn for many years so changed all the fluids. It is running fine just no gauge.
Guess my next step is taking out the cluster to check the wiring up there
Thanks again for the help
Pam
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  #4  
Old 02-13-2014, 09:45 PM
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If I recall there is a small nut on the bottom. If you remove that the case will slide off. There are very thin wires inside. Three maybe? I forget. But its possible they are damaged, corroded or dirty. If it sat for this long I would recommend taking a close of look possible at all parts or you will just be chasing a vicious circle. Somewhere on here is the proper ohm readings for the sending unit. So with those you can read it as you manipulate it out of the car. If its fine you can probably put it back and never have to question it again.
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Old 02-13-2014, 11:43 PM
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Quote:
Originally Posted by LandYaghtLover View Post
If I recall there is a small nut on the bottom. If you remove that the case will slide off. There are very thin wires inside. Three maybe? I forget. But its possible they are damaged, corroded or dirty. If it sat for this long I would recommend taking a close of look possible at all parts or you will just be chasing a vicious circle. Somewhere on here is the proper ohm readings for the sending unit. So with those you can read it as you manipulate it out of the car. If its fine you can probably put it back and never have to question it again.
Let me add a little information to this. Pam, do not remove the cluster yet. It is MUCH more likely to be the fuel sender.

The fuel sender is held in by 5 or 6 nuts which I assume you have removed since you indicate the sender makes a sound when you tip it upside down, etc. If you wish to dismantle it, on the bottom there is a plastic disk surrounding a nut. Remove the plastic disk/nut, use pliers if necessary. It will just unscrew. Then carefully remove the metal disk from the bottom of the sender, and then carefully remove the plastic disk under that. At this point you can carefully slide the aluminum tube down from the top of the sender, wiggling it left and right a little. What you are left with is the sender completely exposed.

It has two guide wires that have contacts touching them on the float that goes up and down. When the float is at the very bottom, it also makes contact such that the empty fuel light comes on.

My recommendation is that the first thing you do is test the sender (assembled). Attached is a pin out diagram with the resistance between pins for different positions of the sender. You will need an ohm meter to do this. If you don't have one, find someone who does and borrow it. It's an easy test and will let you know if you need a new sender.

If the sender doesn't test as per the attached image, clean or replace the sender. If you wish to test the gauge, you can do it by shorting out the pins on the plug that goes to the sender corresponding to my diagram. For example, if you short the socket corresponding to pins 2 and 3 on the diagram, the low fuel light should illuminate on your dash. With the plug disconnected and nothing shorted, the gauge should read full (or past full). With pins 1 and 2 shorted the gauge should read empty.

The gauge will only read things if the ignition is in the on position.

Does this help?

cheers
Scott
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Fuel gauge vs fuel sending unit-mb-tests.jpg  
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  #6  
Old 02-14-2014, 10:47 AM
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yes agree with Scott, test if it fails, I have repaired a few, but it is not easy nor is the wire easy to source

http://www.peachparts.com/shopforum/vintage-mercedes/136823-anyone-ever-rebuild-gas-gauge-sending-unit-w114-w115.html
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  #7  
Old 02-14-2014, 10:55 AM
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Thank You

Thank you to everyone for the great help. I really appreciate the wiring diagram I will check it out today. I am hoping that is the problem sure seems easier than rewiring or pulling out the dash cluster.
Thanks again
Pam
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  #8  
Old 02-14-2014, 01:16 PM
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Hi Pam, I think it's likely the sending unit too. Yes, there is not much room behind the dash, I have small hands and am only ever thankful when I need to fix something under there. The sending unit "guts" may still slide inside but if one of the thin wires are broken where it connects the unit won't function so check those carefully.

It's encouraging to see an owner doing their own work, most of us on this site do to some degree. You may also wish to visit us at the links in my signature, I started a club specifically for our cars, both coupe and cabriolet like yours. Lots of members on Facebook who love to share info and stories and we are hoping to plan a get together later this year. Where are you located by the way?
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http://img.photobucket.com/albums/v3.../SideSmall.jpg

The Coupe Group (W111/112 coupes and cabs) official website
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Old 02-14-2014, 01:21 PM
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I've never experienced a faulty fuel gauge. Either the entire cluster is disconnected or it's the sending unit, but easy to verify with a short piece of copper wire or a paper clip to short the plug and see what the gas gauge does...
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  #10  
Old 02-14-2014, 10:14 PM
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Guess not sending unit

Nothing happens when I short the wires or disconnect the sending unit electrical plug. Any ideas of an easy way to get the dash clusters out to check the gauge itself? I hope to get the cluster out and not the whole dash. I do not see a nut to disconnect the cluster but will look again
Thanks again for the help and schematic
Pam
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  #11  
Old 02-14-2014, 10:45 PM
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Quote:
Originally Posted by spatengal View Post
Nothing happens when I short the wires or disconnect the sending unit electrical plug. Any ideas of an easy way to get the dash clusters out to check the gauge itself? I hope to get the cluster out and not the whole dash. I do not see a nut to disconnect the cluster but will look again
Hi Pam -

I don't know how to remove the cluster but I've attached a PDF of the instruments from MB Electronic Parts Catalog. You don't have to remove the dash but you do have to be very careful with the water temperature capillary tube that goes from the engine block to the water temp gauge. If you kink it or break it, it's pretty much destroyed and needs to be rebuilt. Also the oil pressure probably has a rigid line going to the cluster. So my best guess is that to remove the cluster you need to:

1. remove whatever retaining nut(s) are in back
2. push the cluster out a little from the back (the dash doesn't need to come out)
3. You need to disconnect the rigid oil pressure line
4. you need to remove the water temp sender from the engine block
5. You need to unplug the big plug shown in the diagram
6. Then try to slowly pull the cluster out while someone else feeds the water temp capillary tube through the firewall. You may want to attach a string or something so that when you re-install you can follow the same path.

Just to confirm:
a) you tested shorting things with the car in the ON position
b) your other instruments work and light up - amp, high beam, etc
c) the instrument lights in that cluster work

If someone has a pin out for the big plug, you might be able to unplug the plug and test using the plug without removing the cluster. I don't have that info but I'm sure someone does.

You might also ask on the sl113.org forum - the cluster on your car is basically the same as the cluster on a R113 Pagoda.

Scott
Attached Files
File Type: pdf W111CabrioInstruments.pdf (36.2 KB, 313 views)
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  #12  
Old 02-15-2014, 09:49 AM
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Thanks again

Thanks again sure do appreciate all the help. I will tackle the cluster today and hope I can do it without creating more problems. Might be easier to just keep the fuel topped off as I do not drive it far!!!
Thank you for all the wonderful ideas and followup
Pam
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  #13  
Old 02-15-2014, 06:18 PM
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Quote:
Originally Posted by ScooterABC View Post
3. You need to disconnect the rigid oil pressure line
Be careful here or you will make more work, this is a solid line that physically pushes oil into the gauge to measure oil pressure. Also, do not disconnect while the car is running and be sure it's totally connected properly when refitted to avoid an interior full of splashed oil. These are some of the reasons you may wish to fully confirm the sending unit isn't faulty before you dig into the cluster.
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1962 220SE W111 Coupe, 2nd owner

http://img.photobucket.com/albums/v3.../SideSmall.jpg

The Coupe Group (W111/112 coupes and cabs) official website
The Coupe Group on Facebook
MotoArigato: Roadworthy News & Humor
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  #14  
Old 02-15-2014, 07:24 PM
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I agree completely. I also would definitely experiment with the plug to the cluster maybe with a nine volt battery before removing the cluster...
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  #15  
Old 02-16-2014, 09:02 AM
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Exhaust all diagnostic options before disturbing the instrument cluster. Probability is high that the problem is with the sending unit or wiring rather than the gauge itself.

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