The blower regulator is anything but a resister. It is actually more properly described as an end stage amplifier.
It is just a big transister. Basically with the three wires all transistors use: control wire (1-5v from PBC), incoming circuit (from blower), and outgoing circuit (to ground in the 140's case). The 140 also has a term 15 power lead (ignition hot).
As the control signal voltage goes up the connection to ground is made "like" an infinitely variable resistor, but it is done with transistor control.
As to evaporators, I think their problems are much greater in certain areas and certain climates. We don't seem to do as many as shops in large metropolitan areas. I have two theories to this: first is the possibility of air polution/acids facilitating the external corrosion process. Second is the feeling that many large city shops are swayed by the CSI concept I formulated in flourescent dye discussion.
Breifly respoken, many feel that in order to have a good CSI (consumer satisfaction index - no comebacks)it justifies replacing every posible problem. (this is the way I see the first post in this series; was it posssible that you let three major problems accumulate with your A/C ? - it sure is unlikely that they all happened at the same time).
I am seeing a severe decline in many repair skills in my trade due to this concept. How do you all feel about this senario:
Car is brought in with defective A/C manifold assy on a 300e with fuel cooler (a $600 hose). Shop A does the whole job and repairs the hose by cutting the crimped connections and welding new barb fittings and then recrimping modern barrier hose in its place. The job will be the same as Shop B who replaces the hose except the bill will be $500 greater and with a old style rubber hoses. Easy question but now comes the kicker 1 out of 20 welds will have a microscopic leak with a resulting comeback.
To many the possibility of such a comeback is the deciding factor on whether to use such skills and techniques.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician