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  #1  
Old 12-12-2006, 10:57 AM
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Location: Gavle Sweden
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Red face W123 Fuel gauge problem

Between models on W123 Sedan , Coupe , StationWagon , Gas , Diesel ,200 , 250 , 300 etc, etc, it's a lot difference in fuel capacity, now to my question,,,
Is there a lot of different sending units or is the difference in the gauge itself ??????
my orginal gauge went south and i tried a couple of different clusters/gauges
but the reading on the fuel gauge is never the same as it was and YES i checked the sending unit and all the reading on that was by the book and also the wiring.
My Mercedes is a -82 300TDT.
Thnx i advance Kjell Jonsson.
P/s sorry for my english but im a Swede !!!!
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  #2  
Old 12-12-2006, 12:48 PM
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I don't know about the gas models, but the fuel tank in the wagon sits flat, as opposed to the vertical placement of the sedan. Because of this, I believe the sending units are different.
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  #3  
Old 12-12-2006, 02:04 PM
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Quote:
Originally Posted by MattBelliveau View Post
I don't know about the gas models, but the fuel tank in the wagon sits flat, as opposed to the vertical placement of the sedan. Because of this, I believe the sending units are different.
You nailed it. The wagon sending units are different, they are shorter.
But all the 123 diesel sedans have the same one.
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  #4  
Old 12-13-2006, 10:41 AM
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Smile Fuel Gauge ???

Ok thats was that regarding the sending unit, what about the gauge itself,
is there anything different in that one between models ????
Thanks for the answers thou
Regards Kjell - Sweden.
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  #5  
Old 12-13-2006, 11:12 AM
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I think I have the answer you need. They should be the same. Here is why I think they are. Recently, just the other day, I was testing a fuel sending unit out of a sedan in my wagon. The unit in my 240 is toast, but since I am moving it has already been moved so it it 500 miles away. I wanted to see if the sending unit was any good. I plugged it into the wagon and manually moved the level unit up and down to see if the gauge followed suit. It did.
It plugged into the wagons system so thats the same and the gauged worked with the sedan sending unit. So I belive that the guages for all the 123 diesesl is the same. I can't speak about the gas version, but I would guess they are the same too.
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  #6  
Old 12-13-2006, 12:34 PM
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Pictures

Long fuel sender => '82 300D
Short fuel sender => '83 300TD
Attached Thumbnails
W123 Fuel gauge problem-wagon-sedan-fuel-senders.jpg  
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  #7  
Old 12-13-2006, 12:43 PM
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Hi Captainchris,

Could you give me the lengths of both from the O ring seat face to the end of the senders? It looks like the sedan is a little over 12 1/2".

I am thinking of using stock sensors in aux fuel tanks.

Can you tell what the threads are? Are they the same as the fuel pickup screen, 36mm x 1.50 pitch?

I suspect that they are both 90 ohms on empty and 0 ohms on full. That is one of the VDO standard ohm ranges.

Thanks,

Ron
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'85 300 Turbo Diesel 2 tank WVO
'83 300 Turbo Diesel 2 tank WVO
Some former WVO vehicles since ~1980:
'83 Mercedes 240D
'80 Audi 4000D
'83 ISUZU Pup
'70 SAAB 99 with Kubota diesel
'76 Honda Civic with Kubota diesel
'86 Golf
Several diesel generators
All with 2 tank WVO conversion
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  #8  
Old 12-13-2006, 01:59 PM
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Hi WD8CDH,

The sedan fuel sender is 12 7/8" long (to the nearest 1/8"), including the small plastic nut at the bottom. (Not visible in the picture.)

The wagon fuel sender is currently in use, so I can't get an exact measurement. Here's another picture. It looks like a little over 7 1/2", plus some accounting for the nut.

As for the threads, I'm not familiar with the "36mm x 1.50 pitch" notation, so I measured the outside diameter of the threads of the sedan fuel sender and got 44mm. (+/- 1mm as I don't have a proper caliper.)

As for resistance measurements, these two fuel senders were part of an experiment. The wires in my wagon fuel sender were shot, so I used this sedan fuel sender as an organ donor. My assumption was that the wires had the same rating in terms of ohms/unit length, and the wagon compensated for the difference in length using the 325-ohm resistor in the head of the fuel sender. (Pics attached.)

The result is that my fuel guage now deflects between half and full. (Half equalling empty.) At least my reserve light now works. I think I will have to fool with the resistor to get an accurate deflection.
Attached Thumbnails
W123 Fuel gauge problem-wagon-sedan-fuel-senders-2.jpg   W123 Fuel gauge problem-wagon-fuel-sender-resistor.jpg   W123 Fuel gauge problem-wagon-fuel-sender-resistor-size.jpg  
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  #9  
Old 12-13-2006, 02:15 PM
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Darn, those pics by captainchris aren't giving me much hope that taking the sending unit out for a cleaning is actually going to do anything for me.

The gauge in my 80 does not register above a 1/2 tank (but is accurate when the level dips below 1/2) and the reserve light is always on.

My 87 on the other hand is accurate above a 1/2 tank, but once it dips below the needles tends to get a little crazy. I haven't seen the reserve light functioning in the 87, but I've never dared let it get much further than 1/4 tank b/c the needle is so crazy by that point.

I wish those sending units weren't so expensive
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  #10  
Old 12-13-2006, 02:24 PM
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the resistance values are just about the same across the full line of W123 chassis. about 4-5 ohms full & around 80-85 ohms empty.
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  #11  
Old 12-14-2006, 08:31 AM
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Quote:
Originally Posted by captainchris View Post
Hi WD8CDH,

The sedan fuel sender is 12 7/8" long (to the nearest 1/8"), including the small plastic nut at the bottom. (Not visible in the picture.)

The wagon fuel sender is currently in use, so I can't get an exact measurement. Here's another picture. It looks like a little over 7 1/2", plus some accounting for the nut.

As for the threads, I'm not familiar with the "36mm x 1.50 pitch" notation, so I measured the outside diameter of the threads of the sedan fuel sender and got 44mm. (+/- 1mm as I don't have a proper caliper.)

As for resistance measurements, these two fuel senders were part of an experiment. The wires in my wagon fuel sender were shot, so I used this sedan fuel sender as an organ donor. My assumption was that the wires had the same rating in terms of ohms/unit length, and the wagon compensated for the difference in length using the 325-ohm resistor in the head of the fuel sender. (Pics attached.)

The result is that my fuel guage now deflects between half and full. (Half equalling empty.) At least my reserve light now works. I think I will have to fool with the resistor to get an accurate deflection.
Hi Captainchris,

Thanks for the info. Good point about the invisible plastic nut.

It looks like the threads of the sender are larger than the threads of the fuel pickup screen on the bottom of the tank.

The wagon unit would have to have finer wire since it needs the same resistance for it's TOTAL length. If you know the length from float contacts up to the top of the sender, I can calculate the gauge of resistance wire that you would need. If the float size is the same on both the wagon and sedan senders, I could estamate the wagon dimensions from the sedan dimensions.

The resistor will not help you on the wagon. Removing it compleatly will maybe move the needle about 1/4 of a needle width closer toward empty.

The resistor is just for the manufacturer (VDO) to make fine calibration of the empty reading of the about 50 different length senders that they make without using 50 different diameters of resistance wire.

One of VDO's most common OEM fuel gauge meter movements requires about 90 ohms for an empty reading so all of their "Tube Type" senders have similar resistance at empty.

Do you have a picture of the reserve contact end of the sender?

Thanks,

Ron
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Ron Schroeder
'85 300 Turbo Diesel 2 tank WVO
'83 300 Turbo Diesel 2 tank WVO
Some former WVO vehicles since ~1980:
'83 Mercedes 240D
'80 Audi 4000D
'83 ISUZU Pup
'70 SAAB 99 with Kubota diesel
'76 Honda Civic with Kubota diesel
'86 Golf
Several diesel generators
All with 2 tank WVO conversion
LI NY
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  #12  
Old 12-23-2006, 06:48 AM
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Lightbulb Biodiesel and sticky fuel gauges

I am not a mechanic, but have a theory that seems to work for biodiedel and sticky fuel gauges. I have three early 80's MB and have run them on Biodeisel for the last 12 months. All developed a new symptom of sticky "wacky" fuel gauge past 1/4 full within 3 months of each other. On the first one I replaced the sending unit for lack of anything better to try. It worked. When the second started I became suspicious. So at low fuel tank levels I added a good diesel stabilzer (PRI-D) which has been a good one for me. Then added a tank full of B20 and no more probelems. My theory is that the B20 I switched to was dissolviong lots of old petrodiesel debris from the system and pressumably being lighter, it floating to the top of the fuel. As the tank fuel level dropped to use the last of the fuel, the stuff temporarily clogged something in the sending unit. The additive seemed to purged this stuff some way and now I am not having problems. A $15 bottle of PRI-D beats new sending units if the result is the same. I don't think it can hurt to try it. There is also a PRI-G for gasoline as well as PRI-D for diesel engines.

T-bone
84 300sd
84 300D
83 240D manual
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  #13  
Old 12-23-2006, 10:55 AM
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Quote:
Originally Posted by Tbone View Post
I am not a mechanic, but have a theory that seems to work for biodiedel and sticky fuel gauges. I have three early 80's MB and have run them on Biodeisel for the last 12 months. All developed a new symptom of sticky "wacky" fuel gauge past 1/4 full within 3 months of each other. On the first one I replaced the sending unit for lack of anything better to try. It worked. When the second started I became suspicious. So at low fuel tank levels I added a good diesel stabilzer (PRI-D) which has been a good one for me. Then added a tank full of B20 and no more probelems. My theory is that the B20 I switched to was dissolviong lots of old petrodiesel debris from the system and pressumably being lighter, it floating to the top of the fuel. As the tank fuel level dropped to use the last of the fuel, the stuff temporarily clogged something in the sending unit. The additive seemed to purged this stuff some way and now I am not having problems. A $15 bottle of PRI-D beats new sending units if the result is the same. I don't think it can hurt to try it. There is also a PRI-G for gasoline as well as PRI-D for diesel engines.

T-bone
84 300sd
84 300D
83 240D manual
Have you tried cleaning the sending units. There is old diesel buildup in it I'm sure, and the BD is probably breaking it up and moving it around which could plug the holes in the sending unit.
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'77 300D ~ Sold
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