W140 What is a Heater Supply Unit Circulation pup
 

I just checked the Air Conditioner Diagnostic codes on the LCD display.

I'm getting a

E0 76 (Heater Supply Unit Circulation pup (A31m1) over load
E0 77 (Heater Supply Unit Circulation pup (A31m1) over load

Can anyone be so kind as to give me a description of a Heater Supply Unit Circulation pup?

A31 is the Aux Coolant Pump. Its not much more than a small electrically driven pump assembly. I suspect the motor that drives the pump is seized.
Anyway, located in the front of the engine compartment on the driver's (left) side. Not hard to find; its got coolant tubing running to and from it.
Hope this helps.

This is what it would look like for my car:
http://img.eautopartscatalog.com/live/G305013421BOS.JPG