240D - 460,000 miles

I took out my fuel tank because my tank screen kept getting clogged. I had some serious "build up" in the bottom of the tank which I had boiled out.

I removed the sender but my "buddy" had cut the lines because he couldn't figure out how to get the connector off the top of the tank (after he cut it I showed him the two screws in the first aid kit box!).

While the tank was being cleaned I replaced all the rubber hoses out back and proceeded to clean all the "varnish" off the sending unit.

I used "easy crimp" electrical connections to reattach the electrical leads to the connector. (I know the first question will be to make sure I correctly connected the leads back to the correct wire - YES, they are connected correctly)

Reinstalled the tank, screen, hoses, sender and connection. As soon as I turn the key to "Acc" the gauge jumps to FULL.

I thought maybe the sender needed to "recalibrate" so I filled her to the brim and drove her.

NOPE, stuck on Full.

I used Parts Cleaner and Brake Parts Cleaner to remove the varnish on the sending unit . . . could that have caused a problem.

Stumped

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1982 240
460,000 miles total
140,000 on Rebuild

You need an ohmeter to see if the tank element is open I think. At least if the problem only started immediatly after you had the gauge out. It pretty well is the gauge assembly or no working ground on it. This suggestion is based on them being set up simular to the gm fuel gauge circuit I think.

I could not tell by reading your post if you opened the sending unit or not. If you did not then it is possible that you would have to clean it up. I just worked on mine a few days ago and the inside was pretty dirty... Once cleaned up you can connect the sending unit, turn ignition on and turn the sending unit upside down, the float should move freely and you should see your fuel gauge move as well.
Watch out for the 3 very thin wires inside the unit...

Attached Thumbnails

 

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1980 300TD(euro non turbo): Eng: 617.912 /Tran:722.118
1985 300SD: Eng: 617.951/Tran: 722.416

I did NOT take the unit apart (did not know it could be torn apart or I would have done so)

I appreciate (very much!) the exploded view picture.

How does the unit come apart?

Thanks so much! I love how quickly I can get a response (usually with pics) on here.

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1982 240
460,000 miles total
140,000 on Rebuild

1)- Take the unit out

2)- At the bottom you'll see a small nut, remove the nut and pull the 2 bottom plates(pic2)

3)- Slide the metal tube slowly, inside are 3 hair thin wires you don't want to brake. (see pic3&4)

4)- Use some brake cleaner spray to clean the unit. At the bottom of the float is a metal ring that should get in contact with the 2 contacts located at the bottom of the unit. Make sure that the metal ring is clean as well as all the contacts. This is for the "low level" light in your cluster.

5)- Re-assemble the unit and slowly turn it upside down, you should hear the float going up and down. You can plug the unit and test it with the ignition "on". You then should see your gauge going from empty to full.


6)- Put the unit back in the tank.

My only advise is : take your time, be gentle, you really don't want to damage the hair thin wires...

Hope this will solve your problem.

Picture here:

http://web.me.com/flangenard/Site/Fuel_gauge.html

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1980 300TD(euro non turbo): Eng: 617.912 /Tran:722.118
1985 300SD: Eng: 617.951/Tran: 722.416

Thanks for the advice and pics.

I'll try this tonight and re-post my results.

Thanks again!

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1982 240
460,000 miles total
140,000 on Rebuild

Hello, my 1979 300D just began to have this problem.
When the ignition is off, the needle drops to empty as normal. When the ignition is turned on, the needle jumps to full, regardless of true fuel level.

How is the fuel sender removed?
Where is it located?
Does one have to remove the fuel tank first to get to the sender ?

Any other possible cause for this gauge malfunction?

Thanks,

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1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
********************
1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD

Follow the above instructions.

The sender is located on the drivers side underneath the first aid kit/box.

Remove the box and the sending unit is right below it.

Undo the electrical connection . . . it's pretty straight forward after that.

Make sure you have some rags handy! Allow ample time for the sending unit to fully drain before you attempt to remove it completely from the tank . . . also make sure the tank is close to empty . . . makes it easier to get the fuel out of the sending unit!

This repair/fix worked VERY well . . . guage functions normally now.

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1982 240
460,000 miles total
140,000 on Rebuild

Once y'all have cleaned and reassembled the Fuel Sender...
Regular use of Startron will prevent re-occurrence.

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'84 300SD sold
124.128

You crimped the wires? Thats probably it. They need to be soldered. Resistance based stuff is REALLY sensitive.

For example, on my Mark VIII I was having all sorts of throttle and idle issues. My engine was not stock and I had to extend the throttle position sensor. It works the same way, a resistance "dial" if you will. Well I crimped the wires. It was not good enough. Removed the crimps and soldered and all the issues went away. I then read into it more and found that solder is always the way to go, especially for circuits that depend on variable resistance.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Thank you all for the advice and directions.

I'll try to get to it in the near future and post my results.

Thanks again.

__________________
1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
********************
1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD

OK so, about 2 weeks ago I had fed 3/4 qt. of tranny fluid into my fuel tank AND a small bottle of STP diesel treatment (for cleaning injectors etc.).
The idea was to allow the tranny fluid lubricants and detergents to work as well as the other treatment.
Today I removed the first aid kit (it has a sticky gasket around the underside).

I unplugged the electrical harness and systematically tappped on the top surround of the sender retaining head. I used the platic butt of my medium sized screw driver. Very fast repetitive taps in order to induce vibrations of high frequency.

It worked to dislodge the gas gauge from being pegged on 'FULL'.

maybe the moral of this story in my case is to NOT top the fuel off when filling !
That way the sender is not forced up all the way. (I always use diesel fuel conditioner-- Redline and the sorts).

__________________
1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
********************
1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD