  We study a directed coupled map lattice model in two dimensions, with two
degrees of freedom associated with each lattice site. The two freedoms are
coupled at a fraction $c$ of lattice bonds acting as quenched random defects.
In the case of conservative dynamics, at any concentration of defects the
system reaches a self-organized critical state with universal critical
exponents close to the mean-field values. The probability distributions follow
the general scaling form $P(X,L)= L^{-\alpha}{\cal{P}}(XL^{-D_X})$, where
$\alpha \approx 1$ is the scaling exponent for the distribution of avalanche
lengths, $X$ stands for duration, size or released energy, and $D_X$ is the
fractal dimension with respect to $X$. The distribution of current is
nonuniversal, and does not show any apparent scaling form. In the case of
nonconservative dynamics---obtained by incomplete energy transfer at the defect
bonds--- the system is driven out of the critical state. In the scaling region
close to $c=0$ the probability distributions exhibit the general scaling form
$P(X,c,L)=X^{-\tau _X }{\cal{P}}(X/\xi _X (c), XL^{-D_X})$, where $\tau _X
=\alpha /D_X$ and the coherence length $\xi_X (c)$ depends on the concentration
of defect bonds $c$ as $\xi _X (c)\sim c^{-D_X}$.
