I thought I had fixed my fuel gauge problem where the needle is steady for a long time like it should be (lets say at 1/2 tank) then you glance down and it's suddenly showing a full tank.

I know the fuel sender in the tank is not the problem because it's been cleaned, checked, and with a replacement unit in the tank it does the same thing. So the fuel sender has been ruled out.

We've noticed too that if we use the turn signal, sometimes that causes the fuel gauge to suddenly move 1/4 tank or more. So does that mean there's a short somewhere or something and if so how do we fix it?

jeff 1991 300d

Did you check the chokes on the gauge itself?

Jeff ;

The fact that the gauge goes wonky when you use some other electrical consumer tends to point to either a bad ground near the dash binnacle or a power supply issue, 99 + % of the time this will turn out to be a ground issue.

Check the MPC connections into the back of the binnacle ~ I've found many that were ever so slightly loose, not quite plugged all the way in and some that the contacts were brownish in color, they worked fine after ever so gently cleaning with brass polish on the end of a Q-Tip .

Sometimes it's as simple as an aftermarket item, like a radio.......

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

I usually take a small copper wire and some small nuts (forget the size) and add an extra ground to the studs for the fuel gauge and the light. I take them with a long lead to the main ground point. Helps sometimes. Hurts to climb under there always.

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

Make sure all the screws that hold the three components of the instrument cluster are tight. thats how everything is grounded.

__________________


1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

This is what I would do. The gauge shows the volume of fuel based on the resistance of the wire. The fuel gauge has a 3 prongs connector at the top. Try to find out which 2 prongs gives resistance to the dash. 0 ohm is full, I think 200 or 300 ohm is empty. Disconnect the plug and clamp a 150 ohm resistor in there such that the gauge would show constant fuel level. The fuel gauge shouldn't fluctuate at all. Drive around for a few days. If it does bounce around then you have a wiring problem from the plug to the dash

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

Good call. That’s sound advice if we know the appropriate resistance levels..

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

Do you mean at the dash? If I can avoid taking the instrument cluster out, I will.

The fuel sender is a replacement. Both the original and the replacement give the same problem so I've ruled the sender out. When you say the gauge does this mean the instrument cluster has to come out?

Are you just talking about the wires plugged into the back of the instrument cluster? If I can avoid taking that out again, that would be good.

No. You unplug the connector to the fuel gauge in the trunk and put a resistor across the 2 pins. You need to determine which 2 pins is the float wiring first.

Depends on what you found. It is possible that you may need to trace a lot of wirings.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

Yes. The likely problem is that the choke coil on the side of the gauge is damaged. Any time the windings touch the case, the reading will go wild. Pull the cluster already and fix the problem. It's not the fuel sender, and it's sure not the turn signals. Your problem isn't a bad ground.

So once again. This is a photo of the gauge, not the sender. Each gauge in the pod has a choke coil, which damps the movement of the needle. This one has a broken spool, which allows the coil to short against the side of the gauge. The only way you will see this is by removing the dash pod, and removing the gauge cluster. To fix it properly would require a new gauge, but a reasonable repair can be made by taping the side of the case and slipping a length of shrink wrap over the coil. Otherwise, you will be chasing this forever.

Thanks Mxfrank, I'll try it. I have the cluster removal tools on the way.

BTW , yes, binnacle is the dashboard gauge cluster and MPC is the Multiple Pin Connector, the big round plug on the back of the dashboard .

Remember, when removing the binnacle : only slide it out maybe 20MM or so, then reach up and unscrew the speedo cable and then use an open end 10MM wrench to loosen the oil gauge connector .

It's a fiddly but not terribly difficult job, the first time is the worst....

Because it's your car and YOU CARE you'll do a better job than almost any garage / dealer....

Take you time, it's easy to get frustrated, sit back, relax and go back later if so .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better