My ACC seems to operate fine at low or moderate speeds. But when I get to highway speed, or if the RPM's are around 2000 or higher, I lose heat. Only cold air blows out. any suggestions?

sounds like your mono valve( heater valve) they sell rebuild kits for it. its located on the firewall, mine stuck on my way back from salt lake on a very cold winter day, good luck

How about the auxiliary water pump? When is it supposed to run?

when the heater is turned on or defrost

ok. thanks alot for your help. I've ordered a monovalve kit. After installing it, what is the best way to tell if the aux pump is running because when I checked in defrost and max heat modes it seems not to be running. can i simply disconnect it and apply 12vdc?

that will work, you don't here about them going out much, mine was a 83 and when I sold it last year it had never been changed with 260000 miles, good luck