You would think this is simple right? Well, I have heard several people in online forums messing up their Electronic Climate Control while doing this simple procedure. With all the hidden electronics logic which we can not visualize, it is a bit of mystery how this occurs to me. The best I can imagine is if ECC is "on" somehow while you dis/reconnect your battery, it might be the cause of this.
Before you say "But my key is out!" The ECC actually has a temperature fan that remains running for a few min after the key is out. So waiting for a period of time before disconnecting the battery is Volvo's procedure for doing this. Furthermore, Volvo's procedure for reconnecting the battery is to have the key turned to position II (accessory on, headlights on). So I always turn everything off on the ECC before shutting the car down (AC off, fan off, not in automatic temperature mode) when I plan to disconnect the battery. I figure when I reconnect the battery, it is best if the ECC doesn't surge to all of its functions on right away while the battery terminal connection pulses the system with rapid power on/offs.
Well, first, the battery is in the back and requires removing a couple of covers and brackets. The following sequence is what Volvo specifies whenever you dis/reconnect battery
When the battery is connected, the headlights and various electronics will power on instantly. This feels weird compared to most cars but it is the proper procedure on this car according to the factory service manual.
An example procedure is here
When I have read people frying their ECC after dis/reconnecting battery. I'm guessing they might not have followed this sequence to a T. ECC is actually still operating for a little while even after shutting down the car and keys out. Maybe they left the AC or fan on that caused this problem. There is probably no way to know unless you get to understand the ECC software + electronics inside.
Sometimes, after battery dis/reconnect, some of your upper cabin electronics (sunroof, homelink, dome light near the rear view mirror) may not work. I hear this is due to electronics subsystems that didn't reinitialize properly. Just lock and unlock your doors with driver door switch or remote. It will initialize properly.
After taking off the sunroof, I found the liding mechanism consist of a lot of fairly losely assembled plastic parts. Seems like many new Volvos have this problem. The easy fix is to tighten the 4 screws that holds the sunroof to the rails/guides + adjusting the front and rear roof edge height properly. If the sunroof rides too low, then wind pressure seems to press the sunroof on these loose guide/rail parts to make rattle noises. I've already done this but still have some rattles. Seems like my and others commonly have worned out rails/guides which can be expensive to fix. I still need to find a low cost option to repair mine fully.
The 01-04 V/XC 70s had glass integrated headlight assembly. If the front glass cracks, then you have 2 options.
1. Change the whole assembly needs to be changed. Parts $350 Labor 1+ hour. DIY directions can be found here.
2. Just get the glass and repair them. I read about it here
There are also plastic aftermarket covers that can be glued over them. But some people seems unhappy with them as they peel. I've not tried these yet.
As the ETM begins to fail. It seems to consistently put the engine control module (ECM) in a sluggish performance mode. When a new ETM is installed along with ECM reset, my car felt more powerful.
There seems to be a lot of mystery surrounding this area. I was able to figure out that when my car felt sluggish, A ECM reset (battery disconnect for a period of time. This erases the ECM's adaptation memory) would improve the performance. But the failing ETM seems to put the car back in the sluggish mode not too long after (just a few days)
When I had the ETM replaced. The car felt sluggish afterwards. So I did a ECM reset and got the performance back. With the new ETM, it no longer puts the car in sluggish mode.
Anyways, someone with in-depth understanding of the ECM program can probably explain this. These were the patterns I found dealing with this problem. Some people also suggest resetting the ECM might not be good for the engine depending on the reason it adapted toward more sluggish operation. I suppose the ECM maybe protecting the engine by running it less optimally.
In any case, this doesn't seem much of a problem now as the new ETM software prevents the car from frequently entering this conservative performance mode. See ETM section under the "Failed inside 45k mile" link.