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Thread: Ultrasonic cleaning to restore the YH-409 HVAC sensor and park brake vacuum switch

  1. #61


    When I had tried if sealing up the brass housing works (before I had found my plunger) I had tucked a small pice of neoprene inside the hollow stem. Sealed up perfectly but did not work, of course.
    I do however think the plunger needs to be free for motion. If it only should stick out why in the world did they make it a costlier, moveable two-piece design instead of just a needle end on the brass housing like a carb needle?

    My sensorīs completey reassembled now so I canīt try that for I fear the bent end of the spring wonīt stand another straightening. It will sure break off.
    Maybe you can try that Lutrova? Could use a piece of wax instead of the neoprene, too. This is just for testing so the wax wonīt hurt and could be melted out with a hot needle.

    And yes, the "drifting" sounds like the plunger not seating correctly. I do think that the ultrasonic cleaning will help here since mine was prefectly clean inside. No debris or dirt tracks at all. Have not seen a non-cleaned disassembled though, so I canīt thell if theyīre clean or dirty inside normally.
    BUT from how dust loaded the bimetal was (and is on my donor car one) I really think that the exact dust adheres and builds up inside the brass housing. And that can prevent the plunger from fully seating into the valve body - making it stick/bind/ not move correctly.

  2. #62
    Carthago delenda est Lutrova's Avatar
    Join Date
    Dec 2019
    Fort Irwin, CA


    I don't know why Ford would've designed the plunger as a separate piece from the rest of the brass, but I also can't think of a reason it can't be fixed in the extended position. I filled the stem behind the plunger with a piece of basswood, which is just what was at hand. It should keep the plunger from falling into the stem without restricting its ability to rotate, whenever that may be needed. In bench testing without the spring attached I was seeing a drop of one inHg per nine seconds. And that's drawing vacuum from the supply port, relying entirely on the plunger to hold vacuum.

    With everything reassembled I'm currently seeing a loss of about 1 inHg/sec at room temperature with the temp bias set to full heat. Again, same test method. Any other temp position will drop vacuum rapidly, but at least build a little with a finger held over the servo vac line. Just what this will mean for servo operation in practice, I don't know. I probably could've stood to bend the bimetal some more. The spring adjuster is turned all the way in, so there's no room for adjustment if it generally biases too much toward cold. But now I'll have to install it in the car and see what happens.

  3. #63
    I'm an air-conditioned gypsy gadget73's Avatar
    Join Date
    Jan 2005


    unless maybe the seperate piece was a way to make up for sloppy manufacturing, basically letting the floppy bit self-center in the seat ? That seems unlikely though.

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  4. #64
    Beater gonna beat sly's Avatar
    Join Date
    Mar 2010
    Lewisville, TX


    Also, the failures I mentioned before are actually the tension spring, not the bi-metal.

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