const D3DXQUATERNION& q = l_transform->getRotation();
D3DXVECTOR3 axis;
FLOAT angle;
D3DXQuaternionToAxisAngle(&q, &axis, &angle);

Every time this function is called, the angle is positive...

Update:
I am wrong, sorry, I got some problems of the truck always turning right (the cab of the truck)

Thanks
Jack

Make the angle negative? If you're trying to figure out the direction the truck is turning by using QuaternionToAxisAngle, that's not really a good idea. What you can do is limit the angle to 180 degrees or PI radians, and subtract 360 or 2*PI from it if it comes up higher.