Dash air Vacum control How does it work

[RickNB]

I have noticed that when my dash air is running and the motor is pulling harder than normal and causing less vacum then I loose the air coming from the vents. There is a small vacum actuator located on the AC/Heater box on the front of the fire wall with a small blue vacum hose running back to the AC controls. The actuator appears to control a air control flap on the side of the AC/Heater box. Does anybody know when or what triggers this actuator and what it is for? Should I loose cold air flow from the AC vents when the motor is under load? I have checked for vacum leaks in the hoses that I can easily see but have found none.
Thanks
Rick

[RustyJC]

The fail-safe setting for the AC/heater vacuum doors in a low/no vacuum condition is full defrost. So, yes, under low vacuum conditions, loss of airflow at the dash vents can happen. It still sounds like you may have a vacuum leak, though, since even under load your vacuum accumulator should hold the doors in position until you back off the throttle.

Rusty

[jdsr]

I didnt think I had any leaks either but I found very small cracks(almost haieline) in mine. I replaced and has worked great every since. BTW, 2 shops couldnt find problem and tried to blame it on comp, evap and so forth. good luck

[Luv2go]

These systems have a check valve going from the accumulator to manifold vacuum. When they go bad they can cause the same symptom (happened to me on an '84 T-bird where everything was run by vacuum). If you can't find any cracks in the lines like jdsr suggested I'd look there.

Good Luck!
Stewart

[wa8yxm]

While I do not know everyone's system.. GM and Workhorse share a common trait. The "Default" for the HEVAC is "DEFROST"

If you loose vacuum it defaults to defrost

This is normal.. However.. the loss of vacuum.. Might not be

There are two cures for this

1: Search for the cause of the lost vacuum and replace it.. (Too small an engine and a long uphill grade.. Well, that will do it in many cases)

2: if it's "Too small/long-grade" then add a vacuum tank in-line.. You can make one yourself, or get one at NAPA stores. Sorry do not have the part number, but it's sold as an accessory (or was) for a cruise control.


One side comment: In the good old days cars often had vacuum operated windshield wipers.. You went up a long hill with a heavy load in the rain.. NO WIPERS

[Oemtech]

I would be looking at the vacuum accumulator, the vacuum line that feeds this from the engine and the output line that goes to the dash control.

[kjt92840]

I have the same problem on my mh, OEMTECH can you give more detail on how to check for a vacume leak at the accumulatur. I lose vent air at 60 mph and on up hill grades, if you can post more pictures that would be great. I have a 02 Winn on workhorse chassie W22. Thank you....

[Oemtech]

Other that the obvious of a physical crack or a bad hose you may have resort to something like soapy water in spray bottle.

[TrekPR]

I had critters eat through the vacuum line on our 06, it took me a little while to find but there was only one vacuum line attached to the engine and it went directly to the pot. A couple of bucks would replace the whole thing. Odds of the leak being in the pot itself are longer than in the vacuum lines and as it is sucking and not blowing it will be harder to find with soapy water.

[jdsr]

I found mine by putting alittle bend and twist in line. It didnt take much of a bend. I could see the hairline cracks when I did this. These lines go back to dash ac switch. I replaced both the green and blue line( these 2 lines are made of plastic and not rubber) . Ac has worked great. Piece of cake. BTW, the lines that go to and from vac ball are rubber(on my mh) and didnt need replacing .

[kjt92840]

JDSR. Thank you, I think you just saved me a trip to the shop and probably $150.00 to $200.00 in repairs for a simple hose replacement. Thanks again.
________________________
2002 adventurer 35U workhorse W22

[Oemtech]

Back in the day... we use a propane bottle with a small diameter hose attached to it to check for vacuum leaks. Of course you have to have the engine running and if you hear an increase in speed, TaDa, you found the leak!! Not sure that it is appropriate for this type of vacuum leak.

[jdsr]

Originally Posted by kjt92840

JDSR. Thank you, I think you just saved me a trip to the shop and probably $150.00 to $200.00 in repairs for a simple hose replacement. Thanks again.
________________________
2002 adventurer 35U workhorse W22

Your welcome. I hope thats your problem because its an easy fix. I dont remember but I probly was told about this fix by someone here.

[DriVer]

Originally Posted by jdsr

Your welcome.

jdsr!