The blower kept revving up and down regardless if it was on hot or cold and it wouldn't switch from hot to cold or cold to hot without shutting off the car. I got a used transistor from the junkyard. This stopped the revving up and down but still won't switch from cold to hot. I know it could be faulty since it was used but could the problem also be the controls?

 

There is an arm that snaps or gets out of sync that controls how wide open the vent is. It's on the passenger side footwell panel, around where the radio is. There is a procedure to sync it back up with the 0 position once fixed. I'll see if I can scrounge up a link. I know there is a pdf on one of the popular boards. I think it's MVS or UK boards.

 

This one is for the arm - it has several links to pdfs on the post - http://www.matthewsvolvosite.com/for...hp?f=8&t=34858

This one is for the random controls - which might be your problem if it does in fact switch - see the two links to 30forty50 and swedespeed on post 11 - http://www.volvoforums.org.uk/showpo...5&postcount=11

 

Thanks for the reply, come to think of it, I had pulled out the controls to change a bulb, I might have accidentally knocked off the arm, will take a look at it again.

 

Ok so I checked the arm and it's in place. This morning though it was switching from hot to cold and cold to hot but not all the time. I got a used control panel and gonna try that, I'm thinking though that the used transistor I put in could be bad. Ill keep you posted

 

I changed the control unit and it seems to have resolved all issues, the revving up and down of the blower and the switching from hot to cold and from cold to hot. Although it is only a small sample size, jist changed it this morning. Hope this helps anybody having the same issues.

 

So far the 2000-2004 S40 seem to have the same issues over and over. It's nice to have someone find something that works so that we may all learn.