I found this problem on a transport system that was icing the evap coil.
Here's the system: Carlyle 06D 208vac, 6 cylinders, with pressure (non-electric) suction cutoff unloaders on 2 of the cylinder heads. R22 with a dual coil evap - 2 solinoid valves, 2 TXVs. A fairly simple system.
In full cooling mode, both TXVs working, get normal operation, no problems.
But in reduced cooling mode, one TXV working, get icing on the evaporator. The suction pressure goes down to 32 -35 psi. At this low pressure, the two cylinder heads should have unloaded. But they don't unload, because I check the current draw, and it's 27A, which is a fully loaded value.
Next step: I change both unloaders (but not the cylinder heads) and adjust, but I get the same result. When I changed the unloader, I checked the unloader valve body (located in the cylinder head) and it seems to move normally under spring pressure.
I can't figure out why neither cylinder head will unload. This compressor is a recent rebuild, so something may have gone wrong during the rebuild.