As usual, I am coming into this thread a tad late. I have read quickly all the previous posts. If the '93 850 ECC is similar to the '98 V70 ECC, then there may be another culprit: a dirty or failed Cabin Temperature Sensor. If, like mine, it has a tiny built-in fan, and it has never been cleaned, then you have about 16 years accumulation of "stuff" in there.

In "AUTO" mode, this sensor tells the ECC the current temp in the cabin. The ECC then configures the system to get you to the desired temp.

Step One should be to remove and clean the sensor. If that does not solve the problem, you should consider replacement.

I just checked FCP Groton, and they have a Cabin Temp Sensor for a '93 850 ECC.

http://www.fcpgroton.com/product-exec/product_id/13874/nm/1993_1997_Volvo_850_ACC_Cabin_Temperature_Sensor/category_id/77

I am not saying this is the problem, but you have already tackled the major stuff.