Chrysler Repair: 2001 T&C LXI 3.3 FF: cabin blower speed?, drb iii, computer logic

Question
QUESTION: Hello Roland,

When starting the vehicle and first turning on the a/c after it has been sitting in the sun and the interior is hot, the air coming from the vents constantly goes from blowing on high to low like it is surging. The air is cold like it should be. This "surging" keeps happening until the inside temperature cools down some and then it will blow normal like it should. I have the 3-zone automatic HVAC system in my vehicle. Thanks for your time and assistance.

ANSWER: Hi Rennie,
The fan blower speed is controlled by computer logic in the automatic temp control unit, as is the fraction of the air recirculated vs drawn in fresh air, and this changes as the cabin conditions change. I believe what you are observing may be a normal operation under the extreme temp/humidity situation that you posited. There is said to be a rotary knob that will allow you to override the automatic controls if you don't like the way the fan is operating (check the owner's manual). But I suspect that you may have to accept this 'glitch' as the normal way it works in the automatic temp control mode. The system can be quizzed by means of a Diagnostic Readout Box III (DRB III) at a dealership and if the controller has noticed a flaw it will have coded the fact and the shop manual may explain a repair. Let me know what you learn if you pursue that alternative. I don't believe there is an obvious, simple flaw that can be affected without intervention such as I described.
Roland
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---------- FOLLOW-UP ----------

QUESTION: Thanks Roland. I should have mentioned that the problem has only started happening about a month ago - sorry about that. It has never happened before then. Dumb question but is the automatic temp control unit located in the dash or somewhere else? Could the problem be in the control unit itself? In your opinion is there anything else besides the control unit that could have something to do with this? Thanks for your continued help.

Answer
Hi Rennie,
The control unit in the dash center console and the body computer located under the dash are the two units that are involved in the opeation of the ATC system. There is unfortunately no obvious reason why the blower speed will vary during the start-up of the van with high internal temperatures. There is an in-cabin temperature sensor that might by relevant but the manual I have doesn't address this sort of malfunction in specific. You could ask at a dealer what would be the cost of simply quizzing the system using the DRB III to see what fault code it has stored, and what remedy it suggests. Then once you know that I can give you the info on what would be the part/repair that would respond to that code. The control unit of course chooses the blower speed and also manipulates electro-mechanical motors that determine how the air is handled by the unit and those parts are on the main air unit that is under the dash.
Roland