Intermittent Warning Sound, Stops w Brake

[CVD]

Just picked up RV after week in storage, chassis is 1998 Freightliner XC (air brakes). I now have a high pitched warning sound coming from the dash (like low air pressure warning), that is intermittent (sometimes on, sometimes off). There is no warning light accompanying the sound. I can be parked or moving and it may come on. I have narrowed the conditions a little:

1) The sound always stops when I apply either brake (parking brake or peddle brake).

2) When sitting at idle, with parking brake active – no sound (No sound once air pressure is up, otherwise I get the expected low pressure sound and light). If I release parking brake (level ground of course) it might make the sound. If I press brake peddle – no sound. When I let up brake peddle – always makes sound (even if it wasn’t making the sound beforehand).

3) When sitting at idle with no brake and the warning sound “on”, I can eliminate the sound by reving the engine up to 1,000 – 1,200 rpm. When the revs drop back to idle the sound comes back. Again, if I put the peddle brake or parking brake on – sound goes away.

Suggestions?

[Passin_Thru]

Very probably what it is.

[fcooper]

This sounds very much like the problem I had on a 2000 HR Endeavor on a Freightliner chassis. Warning sound would come on at random times. Stopping the coach followed by cycling the parking brake switch would silence the alarm for a while, then it would come back.

The cure was an air pressure sensor located on an air line, driver side, along frame rail just in front of rear axle. If I remember correctly, it threads into a manifold, and has two wires.

For diagnosis help, call the Freightliner help desk (1 800 ftl-help). It's free and they are good. Be sure to have the last 6 digits of your vin number available as they will ask for that to find the info on your chassis.

I did a little research that may help. Look at this thread linked below from the Freightliner chassis forum on IRV2. Post # 15 will have a picture of the air pressure sensor as it is mounted on the frame rail.

thread on irv2 on air pressure switch

Fred

[ScottG]

I get a noise like that too but it stops after she falls asleep in the passenger seat again.

[MrWizard]

I have never worked on air brake vehicle
But have worked on lots of "air controlled' machinery

What you describe, sounds to me like a sensor, valve, etc. With a response problem
It's sticky, it does closer or open like it should, allowing for drop in air pressure or a false alert signal which is then disappears

[CVD]

Not idle specific. The warning sound comes on any time I stop braking.

Just drove it again. Put it in 2’d gear, got up to 2,000 rpm (sound had stopped), tapped brake (while keeping rpm up), sound came on immediately after letting up on brake. Sound then continued for 5 – 6 seconds and stopped on it’s own.

[rgatijnet1]

Is the current idle below spec?