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Old 08-11-2006, 04:19 PM   #1
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I'm wondering if the AC has a bypass valve on it that is not operating correctly on my coach. During acelleration, there is a sound like high pressure air that comes on, and it goes off if I turn off the AC, or when I reach crusing speed. I had it in to a Workhorse service center in Utah, and they were able to recreate the problem, but couldn't identify the source. Do I have some AC valve that's not working right?
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Old 08-11-2006, 04:19 PM   #2
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I'm wondering if the AC has a bypass valve on it that is not operating correctly on my coach. During acelleration, there is a sound like high pressure air that comes on, and it goes off if I turn off the AC, or when I reach crusing speed. I had it in to a Workhorse service center in Utah, and they were able to recreate the problem, but couldn't identify the source. Do I have some AC valve that's not working right?
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Old 08-12-2006, 03:19 AM   #3
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Hi wilanddij,

Does it sound like your A/C blower fan just gets louder or the airflow within the dash ducts is changing?

There's no A/C "bypass valve" to my knowledge, but if you accelerate to full throttle with the A/C on the engine computer will disengage the compressor to allow maximum engine power. This typically does not result in any audible changes, but sometimes a driver will notice the A/C get warm briefly during the hard acceleration.

I'm wondering if your vacuum supply reserve canister is disconnected or not functioning for some reason.

The dash A/C duct doors are vacuum operated and they receive this vacuum from the engine. During heavy acceleration the engine vacuum can be low, so a reserve canister and a check valve are used to keep the vacuum level high in the lines to the A/C control head which distributes vacuum to the various mode doors. If the reserve system isn't intact, everything may work OK at idle and at cruising speeds when engine vacuum is high, but the system will switch to its default or "no vacuum" mode when engine vacuum is low (heavy acceleration, climbing a grade, etc.). This may cause mode doors to change position in the ducts and make the blower fan and air pressure sounds within the dash change.

Take a look under the "hood" for a spherical black plastic part about 4"-6" in diameter that should have 2 vacuum lines going to it. If it's there, trace the lines, one should go to the engine manifold and the other to the dash A/C control head. Hopefully there's just a line disconnected or something simple like that. Also, the lines could be installed backwards or the check valve missing or improperly installed or the vacuum reserve canister is cracked or missing.

I found a few vacuum lines that were disconnected on my rig (or were never properly connected in the first place) and re-connected them before I noticed any strange happenings.

Hope this helps.
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Old 08-12-2006, 06:54 AM   #4
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HPFan has the right starting point for sure. If you secure a vacuum gage and hook it to the supply line under your dash at the controls and start the engine and rev. You should see no difference in the amount of vacuum on the gage if everything is OK. If there is a drop in vacuum, then start the search for wrong lines or missing connections.

Another common sound found in some evaporators is the expansion valve changes and cycling of the compressor causes a "rush" of R134 which makes a higher pitch resonating sound in the HVAC system. If this is the sound, then there is not much that can be done about the noise. I have seen this in the Denso evaporator a lot since the expansion valve is internal.

Do you notice any temp change or flow change at the vent when this sound is present?

Hope you find the source!

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Old 08-12-2006, 10:50 AM   #5
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Hi Willan,
Was jusr reading your post and I have the same problem on my 04 Winnebago. It doesn't make any noise in 5th. I only get the noise when in 3rd or 4th.
I don't think it is a vacuum line because if so it would show up all the time and then other things would not work.
A relief valve sounds more like it but where? I think I will contact Denro and see what they say.Please let me kmow if you find the problem.
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Old 08-14-2006, 11:55 AM   #6
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i have the same problem does not seem to hurt anything but i do have that sound.you can turn fan off and it quits.does yours?
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Old 08-14-2006, 05:26 PM   #7
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The sound definitly goes away if I turn off the AC, as well. Another reply to the "whining while accellerating" thread states that the problem has been diagnosed as a problem with the AC compressor.
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