Dash Airflow at Highway Speed?

[Chuck and Rita]

Any of you diesel pusher folks have dash air vent airflow dropping off at highway speeds? With our 2008 Itasca Horizon, we get good airflow out of the dash vents when stopped or slow speeds for either heat or A/C settings. By the time we hit highway speed, that flow drops way off. It acts as if the heat/AC air inlet is starving for air.

[smlranger]

I'm not sure how they connect the 'vents' to outside air on DP's but in mine, I don't get much air flow in the vent (or the AC) positions unless I turn on the fan. Air flow certainly does not seem related to speed of the vehicle.

[robbstein]

We've always used the fans (duct fans) when using the dash air vents. Haven't had any problems with airflow. I have noticed that sometimes I have to switch from a/c to max a/c and back to get the proper flow - probably due to a valve malfunction, but more of a minor annoyance than a major problem. I understand this is a vaccum problem probably due to a cracked vaccum hose, but I haven't taken time to check it out and repair/replace.

[jdsr]

Quote:

Originally Posted by robbstein I understand this is a vaccum problem probably due to a cracked vaccum hose, but I haven't taken time to check it out and repair/replace.

Thats what was wrong with mine. easy fix on my mh

[wa8yxm]

I drive a Gasser, specifically a Workhorse, I also drive a Chevy towed.

I found out that on GM products, and I very strongly suspect other brands too, the air flow is controlled by vacuum motors or solenoids. ON a DP it might be pressure that does it.. Just so you know.

If for any reason I loose vacuum on the brake booster line the HVAC system switches to DEFROST cause of all the directions it can blow air, DEFROST is the most important.

I'm not suggesting this is your problem, as I said, you may be using pressure (Technically I am too but let's not get technical) but it sounds like a control pressure issue to me and it's defaulting on you