Dash air moves to defrost under load

John Mo · 06-24-2019, 11:57 AM

My dash A/C has begun showing its age and Ford-ness. I've seen several Fords that switch to windshield defrost under heavy engine load and switching back to dash vents with the load reduced. With a Ford chassis turning 11 years old, I'm seeing that problem now. After I shut it down I can hear a prolonged hissing noise in the dash behind the controls. Seems to me that it's vacuum bleeding off somewhere that it shouldn't be.

There are a few videos on YouTube for fixing this issue in Econoline vans, but I think the anatomy is a little different on an F53 with a motorhome vendor in the mix.

If anybody's dealt with this successfully and can point me to the common failure point, that would be great, but I've searched the issue a lot and haven't found the magic bullet forum post or even some good breadcrumbs to follow. I'm looking at tracing it down myself, but some diagrams would be a nice place to start.

I've found this from Winnebago in 2005-06, but I don't know if my 2008 chassis/2009 body Sightseer still uses the SCS/FRIGETTE system. Maybe since there's no superseding document in the Winnebago resources, this is what I'm looking for. If I'm reading it right, it does appear that loss of vacuum in the system will result in air being directed to the defrost position.

I've opened up the dash from the top and started going through some of the maze. I see some check valves that would be a cheap, easy swap. I may start with those and see if my shutdown noise and under-load symptoms resolve.

Any other tips, suggestions, or resources?

Thanks

al1florida · 06-24-2019, 09:03 PM

I think you know you have a vacuum leak, finding it is a problem.

The Ford van cut away chassis is just a Ford van w/o the back part on it. So looking at the econoline is useless.

The Ford F53 chassis comes with a steering wheel and an instrument panel, very little else. The stuff under the dash, such as vacuum hoses for the heating A/C ducts are all installed by the MH mfg.

Winnie has a bunch of docs online that may help. I don't know if or where the docs are located for the things you need. I have only used the wiring diagrams.