The reason why it doesn't work is that you don't have an equation that directly solves for y.
Equation (1) solves for w, equation (2) solves for x, equation (4) solves for s, equation (5) solves for u, equation (3) solves for v, equation (6) solves for z. So equation (7) must solve for y, but it's too difficult for the solver to get diff(u(t)) from the other equations.
From equation (5), it follows that
diff(u(t)) = e*diff(x(t))
Inserting diff(x(t)) from equation (2) gives
i + a*g + b*h - y(t) = e*(- y(t) + a + b)
thus
y(t) = (i + a*g + b*h - e*(a+b))/(1-e)
So if you remove equation (7) and insert this constant for y(t), it should work.
