parameter estimation with recursive least square estimator in simulink

I have this equation of a quarter car model mx''+dx'+cx=0.I am comparing this Quarter car model with real car data. As a comparision between model and real car i took the x'' (acceleration) of my car body.I used RLS estimator in simulink to estimate the parameter d (damping coefficient) so that the acceleration of my car model is equal to the real car. I need the parameter to be in positve axis only but i get parameter changes in positve and negative axis. Im not sure i used the correct equation for the Regressor.