Using joint->SetParam for velocities has no effect in Gazebo7

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I migrated my old differential kinematic plugin from Gazebo2 to Gazebo7. While it didn't worked out of the box, I had a look at on the following posts which recommend to use SetParam for joints to set the proper ODE parameters:

• gazebo_ros_diff_drive.cpp
• SetMaxForce deprecated in gazebo 5.0
• ODEJoint.cc

So I changed to call in my OnUpdate function from:

this->leftWheel->SetMaxForce(2, 2.4); // Nm
this->rightWheel->SetMaxForce(2, 2.4); // Nm
this->leftWheel->SetVelocity(2, someLeftVel ); // rad/s
this->rightWheel->SetVelocity(2, someRightVel ); // rad/s

to

this->rightWheel->SetParam("fmax",2 , 2.4);
this->leftWheel->SetParam("fmax",2 , 2.4);
this->leftWheel->SetParam("vel", 2, w_l );
this->rightWheel->SetParam("vel", 2, w_l );

But the vehicle does almost nothing. Very very slowly it moves to the desired direction if steered. By the away, what is the difference between fmax and max_force when using SetParam?

Testing the SetParam function:

Additionally, the output of the following lines:

std::cout << " +++++----- " << this->leftWheel->GetParam("fmax",2) << "\n";
std::cout << " +++++----- " << this->leftWheel->GetParam("max_force",2) << "\n";
this->leftWheel->SetParam("max_force",2 , 100);
this->leftWheel->SetParam("fmax",2 , 100);
std::cout << " --------- " << this->leftWheel->GetParam("fmax",2) << "\n";
std::cout << " --------- " << this->leftWheel->GetParam("max_force",2) << "\n";

produce the output

...
+++++----- 0
+++++----- 0
 --------- 0
 --------- 0
...

So actually, the SetParam function does nothing. Am I missing something here?