I've started getting erratic readings from my fuel gauge. It's not really consistent i.e. one minute I'm at 3/4 the next I'm at 1/4. I ran out of fuel the other day when it was 1/4 full.
Is it the gauge or the sender in the tank? Or ???

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Jeff
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1982 300D Turbo 150K
PlantDrive Single tank WVO conversion
6000 veggie miles and counting!
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1985 Ford F250 - 132K
PlantDrive Two-tank WVO conversion
15,000 veggie miles and counting...

Driving for FREE!

The easiest thing to check first is the electrical connection to the tank fuel sender. If that is starting to get corroded it will result in flakey readings. If that is not it then I would target the sender itself. It can get gunked up over time. Take it apart and clean it, just be careful with the thin (0.003") nichrome wire.

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Bill Reimels
Now down to one:
1972 300SE 3.5 W109 (Euro delivery)

how do i get to the sender & is the electrical connector right there?

__________________
Jeff
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1982 300D Turbo 150K
PlantDrive Single tank WVO conversion
6000 veggie miles and counting!
-----------
1985 Ford F250 - 132K
PlantDrive Two-tank WVO conversion
15,000 veggie miles and counting...

Driving for FREE!

Pull out the first aid box. It opens up to the tank sender and the connecter is right on top.

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Adam Lumsden
(83) 300D
Vice-President of the MBCA International Stars Section

The first aid kit box is secured by two phillips head screws. They must be removed first prior to lifting the box out. Good to hear the sender can be cleaned as have one to do and thought it would just be worn out. Anything special about getting into the tube? My gauge is so erratic I was getting concerned the pivot points on the actual gauge may break or wear out. Tips like the tank unit may be cleanable are priceless. Remember to pull tube with fuel tank low if possible and let unit drain down for awhile before completely lifting out. Does not hurt to position rags to prevent spillage on your seats etc either. This of course is after cleaning the connection on top does not solve your problem.

I had to do this same thing almost a year ago. Getting into the tube was a bit tricky for me. I don't have any pictures or I'd post them, and as I said it's been a year so I don't remember exactly, but at the bottom of the tube is a retaining nut, but I don't believe it's designed to be used w/ a socket. It does have a slot like for a screwdriver, but the bolt coming through it is in the way. What I wound up doing was taking a removable flathead screwdriver bit and filing away at the middle of it until it would clear the bolt. Once I'd done that, removing the nut was simple. There's probably a much easier way to do it, but that worked for me.

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Doug

1984 300D 130,000 miles "Silver Blue Metallic"

Not to ruin anyone's high hope, but cleaning the fuel sender unit is not necessarily going to cure all ills. I've an erratic fuel gage myself, it works OK up to about 1/2 of the tank, and then starts jumping around. I pulled out the sender unit, opened it up and found an amazing amount of crud inside. Not just "some residue", but big (2-3mm) pieces of hard crud, plus lots of smaller sand-line pieces. It's amazing that the sender was working at all. Anyway, I cleaned all of this out, also tried to remove some residue from the thin wires, the best I could, and put it back in. Very little difference observed. My guess is, since the fuel level is apparently determined by measuring the resistance in the circuit where the lenght of the thin wire between the float and the base is the main metric, the key part of cleaning is getting those thin wires as clean as possible, which is hard to do without breaking them. I've read someplace else on the forum of other people attempting to clean their fuel sender units to no avail, ending up shelling out $80 for a new unit.

With an ohmeter and unit apart you may be able to determine if unit really functional again before reinstalling. Just move wiper up and down full length of element.Also for comparison check before cleaning to establish benchmark. Not a particularily good part to buy used as a lot of senders have problems. But if wrecker is willing to guarantee good maybe worth a chance.

grabbed one at the wreckers off an 85 for $15 - easy to swap out; worth a shot...

__________________
Jeff
___
1982 300D Turbo 150K
PlantDrive Single tank WVO conversion
6000 veggie miles and counting!
-----------
1985 Ford F250 - 132K
PlantDrive Two-tank WVO conversion
15,000 veggie miles and counting...

Driving for FREE!

With an ohmeter and unit apart you may be able to determine if unit really functional again before reinstalling. Just move wiper up and down full length of element.Also for comparison check do it before cleaning to establish benchmark. Not a particularily good part to buy used as a lot of senders have problems. But if wrecker is willing to guarantee good maybe worth a chance. Sometimes you can lightly retension wiper on some designs. I have not had a mercedes sender apart yet. Also good time to inspect little piece of 1/2 round metal that will be the actual wiper contact. If its worn badly you are just wasting your time.