What purges air when you turn key to on?

Discussion in 'Volvo Forum' started by wr2001, Sep 3, 2021.

  1. wr2001

    wr2001 Light Load Member

    Apr 27, 2012
    Las Vegas
    When you turn the key to the on position I always hear clicking and at the end a nice purge/blast of air. This is before you turn the key to completely turn on the truck. Where is that purge of air coming from? I had my steering shaft replaced at the dealer and they disconnected the air lines. Now I don't hear the air purge anymore. I took it back they inspected it and they said it's fine, but I don't like that suddenly the truck stop purging after this repair. I don't know if it's necessary to purge like this and wondering if I should keep worrying. From what I remember the purge came from a line in the pass through or from the foot valve. I would appreciate your diagnosis. Sorry, forgot to say it's VNL670 2007 D12
    Last edited: Sep 3, 2021
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  3. God prefers Diesels

    God prefers Diesels Road Train Member

    Jun 26, 2020
    South Texas
    Mine does it too. When it does, my brake application gauge jumps. My guess is some kind of ABS check.

    Also, not sure if you're supposed to, but I don't wait for all those clicks and stuff. I crank the truck right away. Because after it starts, it just does all that stuff again anyway.
    Rideandrepair and wr2001 Thank this.
  4. Lyle H

    Lyle H Heavy Load Member

    Apr 16, 2014
    On newer trucks you want to wait after you turn the switch on to let the computers do their "check downs", before you crank the motor over.
    Failure to do so "may" cause communication errors and cause possible issues.
    An easy way I did this was to turn the key to the on position before I got into the truck. By the time I got in the seat, enough time had passed for the start up tests to run.
  5. Arctic_fox

    Arctic_fox Heavy Load Member

    Sep 16, 2016
    Yup, i do this with my bullfrog every time. And in the rare case it doesnt finish by the time i buckle in and fiddle with my ELD it is done.
    Rideandrepair and wr2001 Thank this.
  6. Ross_Tech

    Ross_Tech Light Load Member

    Aug 31, 2021
    It is the ABS controller cycling the modulator valves. Some call it the chuff test. The purge of air is coming from the modulator valves. If your truck isn't doing it anymore, it usually means your ABS module isn't working, or there's a fault. If you get clicking, but no purge, you may have low air pressure. If you're holding your foot on the brake when they cycle, you will hear the purge from the treadle (foot) valve.
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