My "testing" of them is limited to plugging one side, then hooking a garden hose up to the other side and putting some pressure thru 'em and making sure they don't leak.. then opening the other side to make sure they have flow.. then I hooked up a battery to see if the fan would blow.
They just have the lever for "defrost on/off" which is just a metal flap in the inside that directs the air to either the top round air outlets (presumably for the two window defrost vents) or the main outlet for cab heat.
The '46 1/2 ton looks to have what might have been the original heater.. looks like some kind of old time toaster with a little door looking thing on the front. It's currently not hooked up.. don't know if it was just bypassed the last summer the truck was driven regularly, or if it leaks.. but it's funky and rusty.. and the AD heaters just look neater, and seem like they'd provide a little more heat.
If it isn't raining tomorrow i'll stick one in and report back.
