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Old 11-10-2007, 10:09 PM   #1
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I have an intermittent recycling low air warning only on the rear air circuit of a 2002 Itasca Horizon 34HD (XC chassis). There are no visible or notable leaks. Today it never went off after an hour of driving. The air pressure hold steady, except after using the breaks, then air pressure will go down and the compressor will bring it back up. The other day, after 2 hours of driving, the rear air started to cycle - the alarm would sound, the system dumps rear air - goes to zero on the gauge, then right back up to 130 psi, then settles back down to around 110. Then it will do this again after about 1-2 min. It recycled about 10 times. After pulling over and shutting down the coach, then checking for leaks, the system ran fine for another 2 hours, but still had another 2 occasions of recycling the rear air. Is this indicative of a true leak, a sensor issue, regulator, air dryer issue, compressor, water in the lines, or? Is there a diagnostic that can be run. Are there locations where a pressure reading can be taken to test? If you have any ideas, let me know.
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Old 11-10-2007, 10:09 PM   #2
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I have an intermittent recycling low air warning only on the rear air circuit of a 2002 Itasca Horizon 34HD (XC chassis). There are no visible or notable leaks. Today it never went off after an hour of driving. The air pressure hold steady, except after using the breaks, then air pressure will go down and the compressor will bring it back up. The other day, after 2 hours of driving, the rear air started to cycle - the alarm would sound, the system dumps rear air - goes to zero on the gauge, then right back up to 130 psi, then settles back down to around 110. Then it will do this again after about 1-2 min. It recycled about 10 times. After pulling over and shutting down the coach, then checking for leaks, the system ran fine for another 2 hours, but still had another 2 occasions of recycling the rear air. Is this indicative of a true leak, a sensor issue, regulator, air dryer issue, compressor, water in the lines, or? Is there a diagnostic that can be run. Are there locations where a pressure reading can be taken to test? If you have any ideas, let me know.
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Old 11-11-2007, 04:03 AM   #3
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What you describe sure sounds more like sensor or gauge failure than an actual leak. If the pressure really went to zero it would take at least a minute for the compressor to recover to 130 psi, so I'm guessing there is no actual pressure loss.
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Old 11-11-2007, 01:57 PM   #4
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Call Freightliner and talk to a tech.
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Old 11-12-2007, 04:47 PM   #5
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I'm guessing that you have a Freightliner chassis. On many of the XC types, there are air hoses run to the Vehicle Data Controller (VDC) from the front and rear systems which then translates the air pressure information into data that the chassis can use and can be displayed in the gauges. In my case, the VDC is in my right rear passenger side compartment. It would be easy enough to test the air at that point to see if it really a pressure problem or whether there is a problem with the input house.

We were driving along in a rain storm one day and I noticed that the both air gauges were periodically cycling. Further examination determined that there was a crack in one of the sense lines between the chassis rails. I'm not sure what the duty cycle for the air compressor is but I didn't want to risk burning it up. I agree with Gary that yours sounds more like a sense problem than a real pressure problem but you just never know for sure what's going on until you actually locate the problem
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Old 11-12-2007, 05:43 PM   #6
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<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by chasfm11:
I'm guessing that you have a Freightliner chassis. </div></BLOCKQUOTE>

Yes, it is the XC (2002). Yes, the air is not actually dumping from the air tank system, but the gauge is resetting - going to zero, back to 130 and then settles, and then repeats again, each time sounding the alarm for awhile. I bled the tanks - no water. I will check on the lines that go to the VDC next. Thanks, Jeff.
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Old 11-19-2007, 05:00 PM   #7
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Took me about a year to solve the very same problemn you are having. Mine was also on a 2002 Horizon 36'.
I found that my gage would work okay when the ambient temperature was above 84 deg F and the engine was at operating temperature. Other than that and it bounced around like you described.
To make a long story short, it was a defective rear air pressure sensor on the VDC board. My board was obsolete as it had been superceded by a newer version. The newer one was about $325 at my Freightliner Dealer. No repairs or parts available from the board mfg.
With the help of a friend who has experience in electronics we removed the sensor from the board, found a part no and mfg name and located a new replacement sensor. Ordered one (less than $20) and installed it. Works fine now. Only downside is that it is not calibrated the same as the front sensor, but it is close enough for me.
If you want more details about what I went through I will be glad to supply them.
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Old 01-06-2008, 06:38 PM   #8
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<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Travlin Ken:
The newer one was about $325 at my Freightliner Dealer. No repairs or parts available from the board mfg.
With the help of a friend who has experience in electronics we removed the sensor from the board, found a part no and mfg name and located a new replacement sensor. Ordered one (less than $20) and installed it. Works fine now. Only downside is that it is not calibrated the same as the front sensor, but it is close enough for me.
If you want more details about what I went through I will be glad to supply them. </div></BLOCKQUOTE>

Thanks for this reply. My low air problem on the rear circuit continues and is annoying. Will have to fix it. Can the VDC board be owner installed?
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Old 01-07-2008, 08:37 AM   #9
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Yes you can change it yourself.
There are 2 wiring connectors to unplug and 2 airlines (green and red)to unhook. Note which airline goes to which fitting. Push the airline into the fitting, then press the retaining ring toward the fitting. While holding the retaining ring against the fitting, pull firmly on the air line and it will pull out. Un-do 4 screws that mount the VDC box to the panel above the engine.
Install the new box in reverse order. Attach the box with the 4 screws. Push the air lines into the fittings (I would guess the box comes with the fittings already installed but have not seen a new one) until they are seated. Plug in the wires and you should be good to go. Check for air leak after restarting MH.
Ken
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Old 01-09-2008, 06:07 PM   #10
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<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Travlin Ken:
Un-do 4 screws that mount the VDC box to the panel above the engine. </div></BLOCKQUOTE>

You have to get to the box from under the bed - there is panel to remove for access?
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Old 01-10-2008, 03:17 AM   #11
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Yes, the VDC is under the bed. Need to remove the upper cover (most rearward cover). The VDC is mounted on the vertical surface at the front of this opening toward the driver side on my 2002 Itasca Horizon. It has red and green plastic air lines going to it. Be sure and bleed off the air pressure before unhooking the air lines or they will start whipping around if you don't hold onto them when you unhook them. The air can be led off at the front of the coach by pulling on the manual drain valve cables from the passenger side.
Also, I said to remove the screws holding the VDC to the MH. Actually I should have said to remove the 4 nuts and washers then slide the VDC box off the studs to remove.
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Old 01-10-2008, 02:27 PM   #12
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check the air governer usually on or near the compressor- this controls when the compressor recharges the air tanks
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Old 01-12-2008, 06:26 AM   #13
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Another indicator my VDC is going out. Again on this last trip to Big Bend, driving along on US 90 to Amistad I all-of-a-sudden noticed that my oil pressor gage was ZERO. Wow. I pulled over on the HW. Checked the oil even know the motor is hot and reads plenty of oil. Started it back up and the oil gage is reads nominal again. Note that no buzzer warning ever alarmed and the dash low oil light never came on. Is the dash warning light pannel conneted to the VDC or direct to the Cat ECM?
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Old 01-14-2008, 04:34 PM   #14
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Suggest you call FL Customer Service with that question as I am not sure. Last night my voltmeter had no reading and there was no buzzer going off. Buzzer may only be with Low Air Preesure as it always buzzes after starting MH until air pressure builds up.
FL sent me a set of the electrical dwgs, but they are in my MH and it is at the Dealer shop having the leaky slide out hydraulic hoses replaced today and Tues.
Ken
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