If you own a 1998-2000 S/V70 or a 1998-2005 C70 and ever swap out your ECC with another module pulled from a junk car then you need to be aware that the year of the car and year of the new module matter.
I'm opening a new thread for a thread I opened back in 2013 (now archived) because I finally found the resolution and it's one of those WTF moments of looking deep into the heart of how complicated the Electronic Climate Control is and where it gets its inputs from to spin up the fan and maintain temperature. See the link below and I've recopied the description of the problem so someone else finds it if they search.
Here's the solution copied from @sleddriver over at MVS and linked back to his thread.
"The key was programming the ECC using my indy's Autodiagnos Volvo scanner. As previously stated, there are several features that can be adjusted. The one that solved this problem was found here:
Note: In order for the new ECC CM to function correctly, the speed signal must be programmed. From and including production week 832, the ECC CM has been updated with a new function for the programming of the speed signal from COMBI. This is because the functionality of the COMBI has been changed. Previously COMBI had two output signals; a 12 pulsed and a 48 pulse signal. The new COMBI will only have a 12 pulsed speed signal. Therefore the ECC CM is connected to the 12 pulsed signal. The new ECC CM replace the older versions and can be installed both new & older cars. To obtain the correct functionality, the new ECC CM must be programmed for the correct type of speed signal.
The new COMBI has the text "MAX" instead of a digit at the max. fuel level in the fuel gauge.
On older COMBI the ECC CM must be programmed for 48 pulses. On new, they must use 12 pulses.
When I entered programming mode, the ECC reported it was set to a speed signal of 12 pulses. I changed it to 48 pulses. After driving off, I noticed that I could now manually set both the air temp and cabin blower speed and the later would remain CONSTANT regardless of vehicle speed."
So essentially here's the way it works:
ECC: 1998 (no change necessary)
ECC: 1999+ need to change the ECC CM to 48 pulses
ECC: 1998 need to change the ECC CM to 12 pulses
ECC: 1999+ (no change necessary)
Symptoms if the ECC don't match as described above and the settings are not changed:
Blower motor slows down or even cuts off once the car accelerates to 35-40 MPH / ~2000 RPM.
Once the car slows to below 1000 RPM the fan starts to spin back up and comes to full speed once the car comes to a complete stop. The fan still blows some air so it's not completely off - usually.
I'm fully aware that the AC spins down / cuts off when the car goes into boost / accelerates beyond ~3000 RPM - the AC is not on in this scenario.
Settings when this happens:
1. Set the ECC fan switch to full (all the way to the right) or to Auto (all the way to left) - really any setting besides 0 but it's most noticeable when the blower is running full.
2. Set the temp switch for both driver and passenger side to anything above cold (fully turned down to the blue) or full hot (turned to red)
3. Set air circulation to outside air
Settings when this doesn't happen:
1. When temp switch is to cold (fully turned down to the blue) or to hot (fully turned up to the red)
2. When air circulation is set to recirculate internal air
3. The defrost mode is turned on for the front blower