  The Sunyaev-Zel'dovich (SZ) effect introduces a specific distortion of the
blackbody spectrum of the cosmic microwave background (CMB) radiation when it
scatters off hot gas in clusters of galaxies. The frequency dependence of the
distortion is only independent of the cluster redshift when the evolution of
the CMB radiation is adiabatic. Using 370 clusters within the redshift range
$0.07\lesssim z\lesssim1.4$ from the largest SZ-selected cluster sample to date
from the Atacama Cosmology Telescope, we provide new constraints on the
deviation of CMB temperature evolution from the standard model
$\alpha=0.017^{+0.029}_{-0.032}$, where $T(z)=T_0(1+z)^{1-\alpha}$. This result
is consistent with no deviation from the standard adiabatic model. Combining it
with previous, independent datasets we obtain a joint constraint of
$\alpha=-0.001\pm0.012$. Attributing deviation from adiabaticity to the decay
of dark energy, this result constrains its effective equation of state
$w_\mathrm{eff}=-0.998^{+0.008}_{-0.010}$.
