Dash AC blows out of the defrost vents

MHancock wrote:

Krusty wrote:
Yes, its a vacuum leak. Have a look under the hood for a vacuum reservoir. Test it for leakage. You will need a vacuum pump for testing. Also have a look at any vacuum lines for cracking etc. You may have a vacuum check valve near the reservoir. Test it as well, when they get worn, they don't hold vacuum well.

thanks, will do.

Yes, this is where you need to start. These HVAC systems are very easy to diagnose.

Last, Diesels have the 12 volt pump. One of the complaints years ago and rarely now is, customers come in and state they hear this motor buzzing occasionally when driving. It is the 12 volt pump, replacing the lost vacuum that leaks out. Since NO engine vacuum, it is the inherent leaks in the AC system. Doug

dougrainer wrote:

wolfe10 wrote:
Doug,

Have you experimented with a vacuum reservoir along with check valve to maintain adequate vacuum to the HVAC system under high throttle/low vacuum situations?

Yes. I have experimented with dozens of attempted fixes over the years, including dealing with the OEM motorhome makers and Evans/tempcon. NOTHING works. The only thing that works and is consistent is adding a 12 volt electric vacuum pump. My statement about inherent vacuum leaks came from the makers of the Dash AC evap systems. I have replaced various vacuum motors and dash switches per the OEM's instructions and none fixed the vacuum leak problem. When the engine goes to zero vacuum, it does NOT pull Vacuum from the reservoir ball. The system just uses up what little vacuum it has. Doug

I took Doug's advice on this and this worked perfectly.
Was fairly easy to do, wasn't expensive and worked perfect.
I put a switch on my dashboard to manually turn the pump on/off.
Tried everything else and nothing else worked. Mine was doing exactly what you describe.

I’m just glad they don’t run the wipers off vacuum anymore.

I remember when they would quit when you accelerated.

Probably the best thing you can do is to add an external vacuum pump as suggested. The ones on Amazon that are built by Dorman seem to have a poor performance and reliability rating. You might have to either salvage yard the part, or buy a new Ford one off ebay. Being ebay the link below might expire soon, but it will give you and idea what you can get.
Ebay vacuum pump

You will also need an electrical pig tail which you can get off Amazon.
Link

old hose going flat with vacuum applied? Hose attached to vacuum chamber (had that happen on a pusher we owned once)

Thanks, Doug for the answer.

From dealing with diesel pushers for years, very familiar with 12 VDC vacuum pumps. Most had vacuum reservoirs "T'ed" into the system.

I didn't know ta reservior would help enough on a gas rigs to make their systems functional under high throttle/low vacuum conditions.

wolfe10 wrote:
Doug,

Have you experimented with a vacuum reservoir along with check valve to maintain adequate vacuum to the HVAC system under high throttle/low vacuum situations?

Yes. I have experimented with dozens of attempted fixes over the years, including dealing with the OEM motorhome makers and Evans/tempcon. NOTHING works. The only thing that works and is consistent is adding a 12 volt electric vacuum pump. My statement about inherent vacuum leaks came from the makers of the Dash AC evap systems. I have replaced various vacuum motors and dash switches per the OEM's instructions and none fixed the vacuum leak problem. When the engine goes to zero vacuum, it does NOT pull Vacuum from the reservoir ball. The system just uses up what little vacuum it has. Doug

Doug,

Have you experimented with a vacuum reservoir along with check valve to maintain adequate vacuum to the HVAC system under high throttle/low vacuum situations?

23hotrodr wrote:
You should make sure there is a check valve installed between the vacuum reservoir and the port it is connected to on the engine. This will hold vacuum in the system when engine vacuum is low. You can purchase one at any parts store.

The vacuum line that goes to the vacuum servo on the HVAC box under the hood and controls the outside/inside air door at the passenger footwell is prone to cracks and pinholes. It is easily replaced. You definitely have a vacuum leak somewhere. Good luck.

The problem is, the Duct system has built in leaks at the Vacuum actuator/motors. So, when the Engine goes to zero vacuum, the inherent leaks in the system drain the Reservoir and the default is defrost. Having that check valve will not fix anything. Doug