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Air Sensor/Valve/? under the dash hissing


BlueFlame
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Help! Our 2004 Freightliner Columbia started hissing air from behind the dash when the husband stepped on the brakes. Yikes! The dash is now torn apart and the air seems to be coming from the back of this "sensor/??". (See the attached picture, yellow arrow points to the item in question.) What is this sensor/valve? Why so many of them? We are looking for any clues as to what is going on.

2020-02-01 12.26.45-1.jpg

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Lots of sensors. It could be low air warning, stop light activation. If it is leaking between the "collar" and the plastic, replace. If it is leaking where threaded into the block, maybe it has loosened. You may get lucky and a deep well socket will fit. 

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Turn the key to "On", whichever air gauge doesn't work is connected to the leaking sensor. It's probably either 1 of the low pressure alarm switches, or a gauge sending unit. There's a collection of them, due to monitoring 2 separate air systems, with backups for safety. Unthread or disconnect the Push to Connect fitting and replace it. This might be a Freightliner specific part, but other brand dealers might have it, if one's closer. 

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I have a 99 century,  changed several of those switches before i figured out it was actually the manifold that was leaking so changed the whole thing out.   Anyway,  the attached diagram helped me figure things out.   Not sure if it will cover the 04' but worth a look.  I think i just used some small channel locks and i removed the one below it first.   Good luck!

Edited by Fire guy
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1 hour ago, BlueFlame said:

I'm assuming it won't hurt to drive the RV this way until we get to a bigger town.


It depends on how fast the air is leaking.  Actually iirc you aren’t supposed to drive with audible air leaks.  The acceptable air loss rate for your unit would be 3 psi in one minute.  Checked by bringing pressure to 100 engine off, key on, parking brake released and full application of brake pedal, watch gauge for one minute.  Personally as long as the compressor will bring the pressure up full at idle I would be tempted to drive it to the first shop that could fix it.

Nigel

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Thinking it's your cruise control cut out sensor.   It will only leak air when you push on the pedal.   It kills the cc when you touch the brake pedal, pretty cheap part but if your cruise still works and shuts off when you touch the brake then it's probably just leaking at the threads which is either just loose or the manifold is cracked.  Just make sure you get the right switch to replace.   They all look the same but some are normally open and some are normally closed.   Good luck

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I have an FLD120 that developed the same issue, it was the sensor that disengages the cruise control when you step on the brakes, they are the most common one to fail. They also built the truck around it. It took me 4-5 hours to replace it. And thankfully the White dealer near Iron Mountain, MI actually had one in stock. I now carry a spare but have zero desire to replace it a second time.

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