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Doug, ASE Certified Technician
Category: Volvo
Satisfied Customers: 8618
Experience:  Volvo Enthusiast and extended work experience from a Volvo specialized shop
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2001 volvo S40: dash lights..the whole climate control module was

Resolved Question:

We just bought a 2001 volvo S40 for our daughter. The a/c comes on and off intermittently but we have no control as the buttons don't work, although if I press the DSA button that light shows up on the dash lights. We thought the whole climate control module was bad so we bought a used, switched it out but having same problem. We may drive for a half hour then all of a sudden the a/c will start blasting and the little ac light will go on. Any advice is appreciated. Thanks!

Sweating in NC
Submitted: 6 years ago.
Category: Volvo
Expert:  Doug replied 6 years ago.

For clarification, when the problem is occurring, do you have no cold air, or no air flow at all from the vents?

I want to be clear on what you mean when you say the light comes on and off but you have no control as the buttons don't work.

Customer: replied 6 years ago.
If i press the a/c button to turn it on, nothing happens, if I press the defrost button nothing happens, if I press the heated seat button nothing happens. If I press the DSA button the icon lights on the dashboard showing its on. So I've pressed all these button and nothing has happened then maybe 20 mins later...or 5 minutes later...or maybe 60 mins later the a/c light will come on and cold air will begin blowing from the vents but if we try to adjust the air flow we can't. Sometimes hot air will blow out instead of control. When the a/c goes on and the a/c light comes on, all the other buttons will then work but the a/c button will not, meaning we can not turn it off and we can't adjust flow or temp.
Expert:  Doug replied 6 years ago.

When the panel is not working, is there air flow coming from the vents?
Customer: replied 6 years ago.
Expert:  Doug replied 6 years ago.

When the panel is working and you have air flow, have you ever checked the heated seats to see if they work then?
Customer: replied 6 years ago.
yes, they work then
Expert:  Doug replied 6 years ago.

Give me just a few moments to double check a few things. While I feel confident about what your primary issue is, I'm a bit confused about the heated seat portion of the equation and want to look into something right quick.
Customer: replied 6 years ago.
ok...i'm not going anywhere.
Expert:  Doug replied 6 years ago.
Thanks for your patience.

The big picture issue that you want to be concerned with is if the control head is losing communication with the central electronics module. Unfortunately this is nothing that can be checked on your own, as it requires a Volvo factory scan tool to access the CEM... but realistically this is your primary suspect

As far as going separate routes, I am having a difficult time attributing the heated seat operation failure to the rest of the issues you are having.
The problem here is that the heated seats consist of a fuse, the climate control head, and the heated seat modules/elements in each seat. The only shared component in the entire heated seat system is the control head itself. Having swapped another control head and had the same reaction with the seats as well it is very unlikely that you would come across two units failed in exactly the same way. While these control heads are famous for failure, this particular failure you describe, not so much.

Regarding your AC controls going out; focusing on the air flow issue lets look at it the same way as the seats. You have fuses, the control head, a power relay, a power transistor, and the fan motor.
Fuses are solid state an are either good or bad. The control head has been replaced with a unit that is presumed to not be failed in the same way. We now have the relay, transistor and fan motor as potential failure points. If the motor was bad, it is not typical for it to come back to full operation, and even if it did, you would be able to control the speed, not be stuck at one speed. If the relay were to fail you would *lose high speed, not be stuck on one speed. Which brings us to the power transistor. When this fails you will lose operation entirely, and if you have a brittle connection inside one of the transistors in the pack, you will lock at a random speed and have no control over fan speed.
And guess which problem occurs very frequently...

Now back to the seats issue... there is really no connection that can be made here with one exception; I have seen power transistors fail in such a way that they back feed and cause issues with the temperature adjustment in addition to fan speed, etc. Theoretically it is possible that this could be occurring here with the seats as well since these are all incorporated into the control head.... it is a bit of a longshot though.

Honestly with the situation you describe, and -this is important- assuming the control head is OK since it was replaced, I really feel you are going to have a communication issue dropping out between the CEM and the climate control head. It is the only standalone issue that could cause all of your issues. I would feel most comfortable inspecting the communication between the CEM and the climate head before investing in any other parts.
If you were set on trying anything else before bring the car in to a shop, I would say try the power transistor under the passenger side dash as your AC-only related issues fit that particular failure perfectly... .the heated seats however are the ones that really kill that as a slam dunk in this situation. Otherwise I would be confident on a transistor failure.
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