Hi, I have a 450 SEL 6.9, the AC blower will start and work only when DEF is pressed, if any other speed button is pressed , no air is blowing, I replaced the board , still same, please can anyone advice how I can find a fix for this.
Thanks,
Sebastian.
It's almost certainly a vacuum leak. I would check the condition of the lines going into the servo (on the passenger side) and ensure they're connected. You can also get a cheap vacuum tester and test the lines to see which ones don't hold steady pressure.
When you say you replaced the board, which component are you referring to?
Hi, I replaced the Push button where blower speed are selected.