Community Forums

Mike44 · 05-03-2025, 02:05 PM

I have only defrost vents on heat or a/c ..Winnebago 35 vista

d23haynes57 · 05-03-2025, 02:11 PM

All the dampers are controlled by vacuum. You have a failed vacuum pump or one of the actuators have failed.

Vintage Iron · 05-03-2025, 02:29 PM

If you lose vacuum in the AC/HEATER system, the system will default to defrost mode. You can lose vacuum at a connect point, an actuator, a line or from the system's source. Check the connections that you can see for lines that have fallen off or are damaged. Old lines can become brittle and break. A good tool for testing lines is a brake bleeder tool. You can apply vacuum to individual lines and check for leaks.

Mike44 · 05-03-2025, 02:48 PM

I have checked vacuum lines that I can see and trace. Where is the mode door actuator on my rig…2014 vista

Mike44 · 05-03-2025, 04:43 PM

The motorhome has from time to time switched to the front vents but for only a short time during coasting or at idle ?

d23haynes57 · 05-03-2025, 04:48 PM

Quote:

Originally Posted by d23haynes57

All the dampers are controlled by vacuum. You have a failed vacuum pump or one of the actuators have failed.

I missed that this is a gasser. No vacuum pump. Vacuum comes from engine. There should be a small vacuum tank and check valve in the engine compartment or near the air box.

StarkNaked · 05-03-2025, 04:59 PM

Here is the best "fix" I have found. Note that this is a generic Ford problem.

https://www.ford-trucks.com/forums/1...-problems.html

As others have said, it's a vacuum leak somewhere between the engine and the system that controls the dampers in the AC/heater. If the system does not have enough vacuum, it defaults to the defrost vents.

P.S. Yes, my Class C ford has this problem. I just live with it.

Skip426 · 05-03-2025, 05:12 PM

Here's a vacuum system schematic for an Evans/Tempcon system , poor connection or a crack in the control head cannot be ruled out.

Garlic_Rudy · 05-03-2025, 09:12 PM

Under the hood check to see if there is a heater control valve / vacuum diaphragm plumbed into the heater hoses going to the heater core. The line going to that diaphragm commonly falls off or becomes brittle and cracks. The rubber diaphragm inside the control valve may also have failed. As others have stated any vacuum leak in the control circuit will result in the system defaulting to defrost only.

Mike44 · 05-04-2025, 01:46 PM

So on my vista 35 gas the vacuum reservoir was under the hood in plain site. After a lot of contortion under the dash I followed the small line out of the reservoir to the back of the control switch on the dash. The black line was attached somehow with adhesive and I could not pull it out. So on a long shot I was able to run a new line to the black line on the back of the control switch …..and it WORKED !!!!!! Everything good now!

Skip426 · 05-04-2025, 03:58 PM

A vacuum reservoir is in place to keep the dash controls set when engine vacuum drops on hills and throttle increases to full ; if you can't use the one that's already there , consider adding a new reservoir where you can access it.

You can look for one on Amazon or at a local auto wrecker ; any number of vehicles use them.

MTranch · 05-04-2025, 04:20 PM

Quote:

Originally Posted by StarkNaked

Here is the best "fix" I have found. Note that this is a generic Ford problem.

https://www.ford-trucks.com/forums/1...-problems.html

As others have said, it's a vacuum leak somewhere between the engine and the system that controls the dampers in the AC/heater. If the system does not have enough vacuum, it defaults to the defrost vents.

P.S. Yes, my Class C ford has this problem. I just live with it.

If your vacuum is coming from the intake manifold aren't you pulling unfiltered air into your engine through the leak?

It would also be air that isn't taken into account going through the air flow sensor on an EFI, or through the carb, causing the engine to run a little bit lean. Depending on how big the leak is of course.

StarkNaked · 05-04-2025, 08:05 PM

Quote:

Originally Posted by MTranch

If your vacuum is coming from the intake manifold aren't you pulling unfiltered air into your engine through the leak?

It would also be air that isn't taken into account going through the air flow sensor on an EFI, or through the carb, causing the engine to run a little bit lean. Depending on how big the leak is of course.

There is a check valve. If the check valve is working properly, you won't have a problem. The leak area is after the check valve.

MTranch · 05-05-2025, 05:29 AM

I think that the check valve is there to prevent the vacuum can from sucking air out of the manifold. I think that dirt that is pulled through in through a leak in the vacuum system is still going to end up in the engine, regardless of which side of the check valve the leak is on.