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Old 03-28-2005, 07:06 AM   #1
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When it gets hot, the air flow from the dash in our MH re-directs itself to the defroster vents, regardless of the switch setting (including "vent"). There are vacum driven controls in the dash and I can exactly reproduce the failing condition by removing the white vacum line from one of them. I believe that I can feel that the vacum in that white line is less than in the others but don't have a gauge to confirm that.

My dash system is this: Eveans Controls
THe vacum at the pump has been independently checked and is solid. I'm having problems figuring out where/how I'm loosing it. I'm following the "vacum troubleshooting guide" and believe my problem is either a kink in the white line somewhere or an internal problem in the "vacum mode switch" which I cannot find locate.

Thoughts?
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Old 03-28-2005, 07:06 AM   #2
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When it gets hot, the air flow from the dash in our MH re-directs itself to the defroster vents, regardless of the switch setting (including "vent"). There are vacum driven controls in the dash and I can exactly reproduce the failing condition by removing the white vacum line from one of them. I believe that I can feel that the vacum in that white line is less than in the others but don't have a gauge to confirm that.

My dash system is this: Eveans Controls
THe vacum at the pump has been independently checked and is solid. I'm having problems figuring out where/how I'm loosing it. I'm following the "vacum troubleshooting guide" and believe my problem is either a kink in the white line somewhere or an internal problem in the "vacum mode switch" which I cannot find locate.

Thoughts?
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Old 03-29-2005, 10:50 AM   #3
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chasfm,
Have you checked to see if there is a crack in the Vacuum Reservoir. When it gets hot under the hood the crack may expand and cause a loss of vacuum control.
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Old 03-29-2005, 05:59 PM   #4
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The failure mode in many HVAC control systems is to direct air flow to the defrosters. That's a safety feature designed to assure your windshield will stay clear if the failure occurs in weather where the windshield might mist or ice up. I wonder if yours is detecting some problem and intentionally shunting the air there? It is probably designed so that the "no vacuum" default position is defrost, so that you have defrosters if there is a vacuum line failure.

You said "When it gets hot...". Is that hot inside or hot outside and how hot are we talking?
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Old 03-30-2005, 03:57 AM   #5
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The additional question are related to what is going on when the problem occurs? Are you accelerating? Going up a hill? Does the problem occur and then reset by itself? some of these answers might help in troubleshooting the cause.

I suspect the problem is related to needing a larger or additional vaccum resiovar.
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Old 03-30-2005, 04:22 AM   #6
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Thanks for your replies. Here are the answers to your questions.

Yes, I'm aware of the "failure mode" design consideration. I didn't see that mentioned in any of the considerations in link that I provided, however. "Hot" in my original post refers to the dash area as in when the sun beats in through the glass in the afternoon. Just using the heater on a cool day does not produce the failure. I've had the failure when the outside temps are in the 60s but it most commonly happens when driving into the afternoon sun regardless of the outside temp.

The most common condition for the failure is while running along on the cruise control. In answer to your implied question about the vacum being reduced because of engine activity, the answer, I believe, is that the engine doesn't affect it. Mine is a diesel engine and the vacum is produced by a seperate pump, specifically for the dash air. In discussing my problem with a tech, one of his suggestions was moving the pump from the rear engine compartment (since I only need ignition switch driven 12volts to power it) to under my front hood area where my genset and house batteries are. He was the one who tested the vacum at the pump output and found it to be good.

My next troubleshooting act was going to be tracing the white vacum line to find out where it connects to the vacum control. I suspect that it goes through my "firewall" and under the coach. I have traced it from the dash valve, under the dash and back to the driver's console area but lost it in the wiring maze there. I cannot see it come through the firewall from the outside compartment door but it might run inside one of the bundles with protection wrapping.
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Old 03-30-2005, 03:18 PM   #7
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You might consider running a new and separate vacuum line for test purposes. Just go from the pump to the dash valve controller and temporarily tie it up with wire ties here and there. If that solves the problem, you can do a neater job later. You have to buy a lot of vacuum hose, but it probably beats tearing into wiring bundles looking for a defective hose.
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Old 03-31-2005, 09:48 AM   #8
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Thanks, Gary. I agree with your suggestion and may try to pick up some hose to try it this weekend. I remain concerned that the vacum problem exists between the dash control and the operational valve and, if I'm understanding you correctly, you are suggesting bypassing the pump to dash control valve line. In either case, I still believe that I'm going to have to find the dash valve controller - something that I've not been able to do yet.

I'll be sure to keep you posted about my progress and do appreciate your help.

Charlie
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Old 05-14-2005, 02:07 PM   #9
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Here is an update. I finally figured out(duh) how the dash control works. The vacum line comes from the pump mounted in the engine bay directly to the back of rotary control knob on the dash. The hoses all plug into that and go directly to the respective valves. I traced the white line that I believe is the source of the failure and found it wrapped up tightly with electrical tape to the other hoses, It was done is such a way that the white hose could have been collapsed but, with the tape on, I couldn't tell for sure. I removed all of the tape and loosely placed the hoses behind the dash. I measured the vacum at the valve and found 15 inches so I know what the "working" level is. I can feel that valve pulsing a little and suspect that it might have a leak.

I'm going to the hardware or auto parts store to pick up some small hose fittings to allow me to adapt my vacum gauge to fit easily onto the vacum line. When the air re-direct malfunction occures again, I'm going to stick the vacum gauge on the line immediately. If I don't have 15 inches, I'll know that it is a flow problem. If I do, I'll know it is a leaking valve (that leaks more when it gets hot).

I'm getting closer........
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Old 08-14-2005, 04:46 PM   #10
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I'm declaring this problem closed. This one was an engineering problem. THe vacum pump that drives the dash air controller was located in the engine compartment and had 34 feet out output hose run from it to the controller. My dealer's tech relocated the pump the electrical compartment in the front of the coach, signficantly shorting the output hose and it hasn't failed since.

BTW, I used the vacum gauge to isolate this one. When it failed, I had no vacum at the control valve but did have vacum at the pump. I originally suspected a hole in the vacum line that only opened up when it got hot. While I had my coach in for A/C service, the tech told me that he could fix this problem, too. I'm glad I told him to go ahead. It was worth the hour's labor charge that I paid.
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