rear sam
MBWorld Fanatic!
Joined: Jan 2005
Posts: 2,848
Likes: 15
From: Orange County
2005 C Wagon (No snickering please!)
The rear SAM does not measure the fuel in the tank. It collects data from the potentiometer in the fuel tank, digitizes the analog reading and sends it on the CanBUS to the instrument cluster. The problem can be any of the devices or the wiring in between.
If the rear lamps on the car work we can assume the CanBUS is working, and the processor and com circuits in the rear SAM work. The gauge in the instrument cluster can fail and not deflect the needle, but this is a very rare condition. This leaves us with the wiring to the tank and the potentiometer inside.
The wiring between the rear SAM and the gas tank is "indoors." The chance of corrosion on the contacts is greatly diminished, reducing our culprit to one. The unit in the trunk has a float that moves the wiper of the potentiometer. As the fuel sloshes in the tank the wiper is moving and causes wear, or one of the moving parts breaks. This results in a fixed reading or no reading at all.
You can certainly test the tank unit by removing the rear seat. To replace the unit you need a special tool to open the tank. It is also very dangerous to open the tank as it exposes gasoline fumes to the interior of the car. The smallest spark (even static from the carpet) can cause a deadly fire.
Considering the cost of the tool and the danger involved, I'd let a qualified shop do the repair.
If the rear lamps on the car work we can assume the CanBUS is working, and the processor and com circuits in the rear SAM work. The gauge in the instrument cluster can fail and not deflect the needle, but this is a very rare condition. This leaves us with the wiring to the tank and the potentiometer inside.
The wiring between the rear SAM and the gas tank is "indoors." The chance of corrosion on the contacts is greatly diminished, reducing our culprit to one. The unit in the trunk has a float that moves the wiper of the potentiometer. As the fuel sloshes in the tank the wiper is moving and causes wear, or one of the moving parts breaks. This results in a fixed reading or no reading at all.
You can certainly test the tank unit by removing the rear seat. To replace the unit you need a special tool to open the tank. It is also very dangerous to open the tank as it exposes gasoline fumes to the interior of the car. The smallest spark (even static from the carpet) can cause a deadly fire.
Considering the cost of the tool and the danger involved, I'd let a qualified shop do the repair.
