How Dash Air Works

Topic: How Dash Air Works - U320 (Read 2315 times) previous topic - next topic

How Dash Air Works - U320

December 25, 2020, 09:49:38 pm

I'm getting ready to replace the vacuum pump for the dash air (behind entry step). I understand how a vacuum pump works, and I understand that when there is a failure, the air blows out of the defroster vents only, as a default. So when the vacuum pump quits working, I get the symptoms I'm seeing. What I don't understand is what a working vacuum pump is actually doing that allows the knobs on the dash to direct air to different vents. I know what I need to do to fix my problem, I just don't actually understand how the system works. Google and FF searches have yielded little results.

Could someone please give me a 1000ft explanation of how the system works? I was under the impression that a vacuum pump is often used to remove excess moisture from an HVAC system but I don't see how that would affect whether or not the system responds to the dash controls.

Bonus points if you can explain why, when installing the new vacuum pump, you simply cap off that line that comes from the right side of the unit.

Re: How Dash Air Works - U320

Reply #1 – December 25, 2020, 10:05:47 pm

The dash registers have diaphragms that move levers that control where the hvac air is directed. The vacuum pump pulls vacuum on the diaphragms. With no vacuum the default is defroster for safety. My system uses a venturi effect vacuum generator. Air passing by a line going to the vacuum reservoir creates vacuum. If a vacuum pump were to replace it the line would not be needed that the air passes through and would be capped off.

Re: How Dash Air Works - U320

Reply #2 – December 25, 2020, 10:27:03 pm

Elliot, here's how to replace the dash vacuum generator with a diesel vacuum pump:

Dash Air Vacuum Generator

Re: How Dash Air Works - U320

Reply #3 – December 25, 2020, 10:33:12 pm

1 verify power to vacuum pump prior to replacement
2 If you are receiving power verify old vacuum pump is or is not working, finger over open line is enough to tell tone change in pump and actual vacuum
3 If good vacuum at pump it is a control issue, no vacuum and you have power or bad pump.

The system works by having a vacuum source and valves switching were the vacuum goes to open or close diaphragms. Diaphragms are basically like motor actuators using vacuum for power.

System needs to be in good condition because of low volume pump, leaks will cause system to cycle back and never fully operate/close. Old lines, diaphragms, pumps, valves can be problematic Start T/S at power then pump to thru system. I single failure anywhere will cause poor or no operation of the system this is not the kind of vacuum pump used to evacuate your Freon system. This is control of your HVAC controls
Scott

Re: How Dash Air Works - U320

Reply #4 – December 25, 2020, 10:33:35 pm

Or you can repair the black box for a few bucks.

obsolete vacuum generator

Re: How Dash Air Works - U320

Reply #5 – December 25, 2020, 10:50:19 pm

Thanks guys. The pump isn't turning on at all so I'll check that it's getting power before I go replacing the whole thing.

To that note, when inspecting the two electrical terminals coming from the new pump, there is nothing indicating whether or not one is negative and one is positive. In order for the compressor to work in the pump, I thought the motor needed to spin the rotor in the right direction to create a low pressure system that moves air from the suction port to the exhaust valve. Is it not important to apply the positive and negative wires to the correct terminals to achieve this?

If yes, how do I determine which wire goes to which terminal? Both wires are black.

Re: How Dash Air Works - U320

Reply #6 – December 25, 2020, 11:15:46 pm

I would think they would be polarity sensitive. My dash was also not working and it was not getting power. Found a crushed wire under coach. I seem to recall they were different colors and like a number 12 wire.
Scott

Re: How Dash Air Works - U320

Reply #7 – December 26, 2020, 12:02:14 am

I had to change the vacuum hose that came out of the vacuum motor as the ends of the hose wouldn't seal any more.

Re: How Dash Air Works - U320

Reply #8 – December 26, 2020, 12:15:28 am

Quote from: Elliott – December 25, 2020, 10:50:19 pm

Thanks guys. The pump isn't turning on at all so I'll check that it's getting power before I go replacing the whole thing.

To that note, when inspecting the two electrical terminals coming from the new pump, there is nothing indicating whether or not one is negative and one is positive. In order for the compressor to work in the pump, I thought the motor needed to spin the rotor in the right direction to create a low pressure system that moves air from the suction port to the exhaust valve. Is it not important to apply the positive and negative wires to the correct terminals to achieve this?

If yes, how do I determine which wire goes to which terminal? Both wires are black.

Did you look closely at the pump to see if the + and - are marked?
Curious if all the vacuum motors that I see posted were originally the same vacuum generator I have and have been upgraded.

Re: How Dash Air Works - U320

Reply #9 – December 27, 2020, 02:03:14 am

Here is a pic of the vac pump assembly that I built to replace the vacuum generator. Worked well.
Amazon.com: Dorman 904-214 Electrical Vacuum Pump for Select Ford/Dodge...

Amazon.com: Dorman 47076 Vacuum Amplifier: Automotive

Keith

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Re: How Dash Air Works - U320

Reply #10 – December 27, 2020, 06:15:30 am

Search for a 85 Mercedes SL,has the vacuum controlled vents,then fill out your signature so we know what you have for your coach.

Re: How Dash Air Works - U320

Reply #11 – December 27, 2020, 07:34:57 am

Elliott, it may be just a bad vacuum switch,,,,,,,,,,,,,,,,
here is a link to my post when I replaced mine

Vacuum switch for dash AC controls.

Re: How Dash Air Works - U320

Reply #12 – December 27, 2020, 11:15:52 am

There is no vacuum switch in a 2001 U320.

Quote from: FourTravelers – December 27, 2020, 07:34:57 am

Elliott, it may be just a bad vacuum switch,,,,,,,,,,,,,,,,
here is a link to my post when I replaced mine

Vacuum switch for dash AC controls.

Re: How Dash Air Works - U320

Reply #13 – December 27, 2020, 03:12:25 pm

Thanks guys, this is all great stuff. The HOA we're at right now is rather up tight so I've had to be sneaky when working on the rig. I was able to confirm that I'm getting 12v at the wires, and I was able to source a 1/4" plug to cap the air line off at the T behind the step. Now I just need to find a place to get it done and test it.

One thing I have noticed is that despite setting the knob to "AC" or "Max AC", it still only blows hot. Could this also be a symptom of the bad pump or does it indicate I also have an issue with the air conditioning unit on the M11?

Re: How Dash Air Works - U320

Reply #14 – December 27, 2020, 03:22:59 pm

Turn on the A/C and see if the A/C compressor is working. Probably not and that would be the problem.

Re: How Dash Air Works - U320

Reply #15 – December 27, 2020, 03:30:28 pm

If low on Freon the compressor won't work. That is the first thing to check. If the compressor is seized the belt would be broken,

Re: How Dash Air Works - U320

Reply #16 – December 27, 2020, 03:39:02 pm

On the 96 you have to turn the switch to AC,turn the fan on and turn the temperature controller all the way to the left for the coldest.

Re: How Dash Air Works - U320

Reply #17 – December 28, 2020, 01:10:54 pm

I've been experiencing AC problems for a while. When I was at Xtreme last month I was told that my vacuum pump was rusted out and probably no good. I have a dorman vacuum pump being delivered in the next few days and when replace it when I get out to Arizona. I'm not sure if that will fix the problem but that will be step one. after that I'll take it to an AC place and let them fiddle around with it. I also think I need to replace the dash controls but have done little to research that one yet I figure I'll try the vacuum pump first.
Good luck in your troubleshooting I feel your pain when it comes to the homeowners association. best thing we ever did was when we sold the house. Got rid of the homeowners association may they rot in hell... Well maybe not that but I would never buy a place with a home association again.

Re: How Dash Air Works - U320

Reply #18 – May 03, 2021, 03:44:58 pm

What were the original issues/symptoms that led you to the vacuum pump as the culprit?

On my last trip, the dash A/C had some issues. When I turned it on, I heard some clicking sounds, like a relay that was clicking on and off. After sitting for a few minutes, I tried it again and it came back on. Now it's intermittent. While I had the back of the dash off to replace the 2000 Ford radio (haha), I looked at the switches and they aren't in the greatest of shape. I figured given the condition of them, I'd replace these first. Anyone have a source?

Re: How Dash Air Works - U320

Reply #19 – May 03, 2021, 04:14:32 pm

Inadequate vacuum to the dash HVAC results in air coming out only the defrost vents-- the default position.

Could be a bad vacuum pump, could be something as simple as a vacuum line that has come off.

Certainly, start with the cheap, easy things-- check lines, vacuum canister, etc.

Re: How Dash Air Works - U320

Reply #20 – May 03, 2021, 04:24:41 pm

VCS should know where to get the switches;

Victory Climate Systems - Climate Control Products for a World in Motion
Victory Climate Systems - Climate Control Products for a World in Motion

Re: How Dash Air Works - U320

Reply #21 – May 03, 2021, 08:17:10 pm

Thanks T-man. Need new switches on mine. I'll call them tomorrow.

Re: How Dash Air Works - U320

Reply #22 – June 28, 2021, 03:22:31 pm

I replaced the switches in mine but that didn't solve the issue. When turning the mode switch, I do hear the actuators moving in the dash, but the fan motor is intermittent at best. Turning to Max AC, the engine loads down (AC compressor) and the fan on the condenser starts running, so I know that part is working, but the fan comes on if/when it feels like it. Maybe it's a bad connection or it's a fan.

Anyone have pictures of where the fan is in the dash? I see where all the ducting comes from and it sure looks like if there's a fan in there, it's buried and a bear to get to. I hope I'm wrong.

Re: How Dash Air Works - U320

Reply #23 – June 28, 2021, 03:41:34 pm

Quote from: WS6_Keith link=msg=427614 date=1624908151

Anyone have pictures of where the fan is in the dash?
[/quote

I have a U320. My dash HVAC fan is located at the back of the unit, facing towards the front of the coach, in a plastic housing.

Tim

Re: How Dash Air Works - U320

Reply #24 – June 28, 2021, 05:55:10 pm

To administrators:

When I want to print a post for future reference I go to the top and click "PDF" in the right corner.

This works fine for presently posted posts, except this one, "How dash air works."

There I receive an error .... 16 bit depth not supported ....

I assume it has something to do with the picture quality in the post.

Advice, please

Regards

Klaus

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