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450 SEL 6.9 AC blower speed control issue

Started by bnunes39, 15 November 2022, 01:56 AM


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.


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?
1979 300 SD "Liesel", 269k Miles


Hi, I replaced the Push button where blower speed are selected.