Blower keeps switching between dash vents and defrost when taking off..

[sattech2000]

My explorer has a mind of its own it seems.... When I have it set on ac or max ac (havnt tried other settings) it blowes through the dash vents but it seems like when I take off from a stop it switches to defrost for a minute then back to the dash vents once I get to crushing speed. Kind of a huge pain since I stop/go about 500 times a day delivering mail! Lol.

I'm guessing a vacuum issue somewhere but it's been a really long, and very hot, day!! Im completely brain dead at the moment and hoping to figure this out before work tomorow.

I recently did a ton of work on the top end of engine and replaced the entire AC system including the evaporator and condensor.
I looked under the hood but not seeing anything visually.

 

[donalds]

Mine did this I put a one way check valve $2 in line with the vacuum ball ....the ball has one but mine went bad

You may have a small vac leak

 

[sattech2000]

Perfect! Thank you. Do you remember if you found that locally or did you have to mail order it?

 

[donalds]

Autozone in the help section

 

[1998Exp]

Nice catch, donalds!
Also, the ball may have a hole (it's glued from two parts). Or the line is disconnected/broken/cracked. The system defaults to defrost when vacuum supply is lost -- the ball provides it when you step harder on the accelerator.
If it only happens in MAX A/C, here is a possible culprit: in MAX A/C the system goes to recirculate, closing the fresh air door behind the glove compartment. In my Crown Vic the tube to the vacuum motor that operates it disconnected, and each time I selected MAX A/C, that created a huge leak. The ball is there to only supply vacuum for a short period, so it couldn't help in this situation.

 

[Gary Crist]

Warning fun follows
AAAHHHHH my age will show here!! All you "young pups"
have never driven autos with vacuum windshield wipers!

Let's put a cam in that will not idle below 2 K RPM, and accelerate
in a Texas downpour, the wipers for some reason, glue to that place
on the windscreen till you crash or let up. Now do this trying to
get on an on-ramp.

Fun fun fun till daddy takes the T-Bird awayyy.

 

[Mr. Alligator]

Used to have a 1966 VW Beetle that had windshield wiper sprayer powered by air from the spare tire.

Hmmmmm.... Genius Level simplicity.? Dumb priority? Maybe you could look it at it either way, and you would have to keep track of things.

Keep safe.

 

[3xowner]

Mine did this I put a one way check valve $2 in line with the vacuum ball ....the ball has one but mine went bad

You may have a small vac leak

do you have a part number for that check valve you used?

 

[3xowner]

In the '02 Expedition I had, it diod this too. I took out the EATC control unit and replaced the orings for the selection solenoids which fixed it.

However I now am having this same issue on my 00 Explorer with EATC. I stripped out a solenoid assembly from an Explorer at the junk yard for $11 and replaced the orings… but seems that did not fix it.

Last year while doing some work on the right side, spark plugs / heater control valve … I broken the vacuum lines for the vacuum reservoir (ball) and replaced that from the junk yard. Soon afterwards my issues started of going to defrost mode under load. Not only that, it does seem that even with the EATC off, in the mountains under load uphill that I lose vacuum in the system as the floor vents blow outside air sometimes and then other times it blow out hot from the floor too.

Although I bought a motorcraft heater control valve, it was not exactly the same has the original one. I do not like the way the vacuum line fits onto it.

I have not done any other diagnostics to trace this down.

I wanted to write this on this thread instead of a new one as possibly we can figure out what diagnostics can be done to find the root issue.

 

[FordGuy78]

Or the line is disconnected/broken/cracked. The system defaults to defrost when vacuum supply is lost

Don't discount this possibility. Check every line, not just the valves. I had the same issue on an 02 Taurus. The vacuum line supplying the selector switch had cracked ever so slightly. During heavy acceleration the engine torqued a bit and in doing so it stretched the line enough to temporarily widen the crack. Sufficient vacuum was lost and it defaulted to defrost.

In a fit of frustration I replaced the whole line. It wasn't until I had the old line in hand that I found the crack. Best I could tell it was right around where the line entered the firewall, so perhaps a combination of heat, bend, and flex did its magic over time. Impossible to see on visual inspection under the hood and in the cabin.

 

[donalds]

do you have a part number for that check valve you used?

Won't let me post a link .....Idk... just Google doorman check valve

 

[koda2000]

You have a small vacuum leak somewhere. You'd be surprised how small of a leak will cause this dash vent/defrost vent switch to happen. The default when low/no vacuum occurs is to blow air out the defroster vents. This typically happens when engine vacuum is low (like when accelerating or climbing a hill).

 

[Old289Guy]

My explorer has a mind of its own it seems.... When I have it set on ac or max ac (havnt tried other settings) it blowes through the dash vents but it seems like when I take off from a stop it switches to defrost for a minute then back to the dash vents once I get to crushing speed. Kind of a huge pain since I stop/go about 500 times a day delivering mail! Lol.

I'm guessing a vacuum issue somewhere but it's been a really long, and very hot, day!! Im completely brain dead at the moment and hoping to figure this out before work tomorow.

I recently did a ton of work on the top end of engine and replaced the entire AC system including the evaporator and condensor.
I looked under the hood but not seeing anything visually.

Had the same issue with a 2010. Thought I was gonna have to replace with dash control unit, but then I started looking at the problem. Had a couple older Explorers that all had a nice vacuum canister; newer ones didn't. Seems FoMoCo decided to save a few bucks by NOT installing a vacuum reservoir on some models. Check to see if your vacuum hose simply has an in - line coupler running from your vacuum source to a port running thru the firewall. That's a vacuum check valve, and sometimes they can start leaking. Checking & replacing that would be the simplest repair. Didn't quite do it with mine so I had to replace the in-line check valve with a "T" ( a three way fitting) and install a vacuum reservoir and a hose running to it. Solved the problem.

 

[Alex7512]

My explorer has a mind of its own it seems.... When I have it set on ac or max ac (havnt tried other settings) it blowes through the dash vents but it seems like when I take off from a stop it switches to defrost for a minute then back to the dash vents once I get to crushing speed. Kind of a huge pain since I stop/go about 500 times a day delivering mail! Lol.

I'm guessing a vacuum issue somewhere but it's been a really long, and very hot, day!! Im completely brain dead at the moment and hoping to figure this out before work tomorow.

I recently did a ton of work on the top end of engine and replaced the entire AC system including the evaporator and condensor.
I looked under the hood but not seeing anything visually.

Vacuum issue my friend. When I are in the throttle vacuum changes from that at idle. Telling the valve to do silly things.

 

[Panduh]

My explorer has a mind of its own it seems.... When I have it set on ac or max ac (havnt tried other settings) it blowes through the dash vents but it seems like when I take off from a stop it switches to defrost for a minute then back to the dash vents once I get to crushing speed. Kind of a huge pain since I stop/go about 500 times a day delivering mail! Lol.

I'm guessing a vacuum issue somewhere but it's been a really long, and very hot, day!! Im completely brain dead at the moment and hoping to figure this out before work tomorow.

I recently did a ton of work on the top end of engine and replaced the entire AC system including the evaporator and condensor.
I looked under the hood but not seeing anything visually.

Are you sure it's switching vents or is it maybe just blowing warm while you accelerate? If it's like the old 5.0 Stangs, the AC was designed to cut out under hard acceleration in order to maximize power to the wheels. Quite a few cars have this feature. IDK if it can be defeated.

 

[McFudden]

My explorer has a mind of its own it seems.... When I have it set on ac or max ac (havnt tried other settings) it blowes through the dash vents but it seems like when I take off from a stop it switches to defrost for a minute then back to the dash vents once I get to crushing speed. Kind of a huge pain since I stop/go about 500 times a day delivering mail! Lol.

I'm guessing a vacuum issue somewhere but it's been a really long, and very hot, day!! Im completely brain dead at the moment and hoping to figure this out before work tomorow.

I recently did a ton of work on the top end of engine and replaced the entire AC system including the evaporator and condensor.
I looked under the hood but not seeing anything visually.

Okay so there’s a vacuum line that goes to your HVAC switch. If that has a leak or is disconnected it will go out your defrost vents regardless of what setting you have selected on the dial. The fact that it’s intermittent and depends on the speed of the fan, it’s more likely that the HVAC door is actually broken and hanging loose. You have two fixes:
One. Replace the HVAC gate. If you’ve ever wanted to tear your dash apart and remove the center console as well, well boy are you in luck!
Two. Take a thin wooden dowel and jam it in the bottom corner of the driver’s side center vent under the radio. The wooden dowel will force the HVAC gate open and the air will come out the vents. When you want defrost, remove the dowel.

 

[koda2000]

A 1999 Explorer will most certainly have a vacuum reservoir ball, which stores vacuum to be used when engine vacuum is low. Our very first Gen II, a 2001 Explorer, would switch from the dash vents to the defrost vents every time I climbed a fairly long hill near my house. I knew this was likely a vacuum leak somewhere, but I also knew it must be a small leak, as this vent switching only occurred when engine vacuum was at a minimum (like when climbing a hill, or when accelerating hard). I looked at all the vacuum lines in the engine compartment and everything looked okay. Without a smoke test it is difficult the locate the source of a small vacuum leak, and because modern vehicles use engine vacuum for a lot of things. One day I noticed that there was a rubber "T" fitting in a vacuum line leading to the vacuum reservoir ball located in the front of passenger side inner fender liner. I knew that was no reason for this 3-way "T" to be there as one of the legs was plugged with a metal screw. I removed the screw and found it to be a course-thread sheet metal screw. I removed the "T"(apparently someone's broken vacuum line repair splice) and replaced it with a short length of appropriately sized rubber vacuum hose. Problem solved. The course threads of the sheet metal screw allowed enough of a vac leak to cause my problem.

No one on this forum will be able to pin point the source of your vacuum leak because there are too many possible places that a leak can occur (and not only in the engine compartment). If you can't locate it visually, take it to a mechanic with a smoke machine. He should be able to locate the source of you vacuum leak in less than a 5 minutes. Yes, you'll have to pay him for the diagnosis, but the repair itself will probably cost you next to nothing if you do it yourself.

Some possible sources for vacuum leaks:
In-cabin HVAC controls and vacuum motors/solenoids
Heater control valve
Bad power brake booster
Fuel vapor purge valve or hose
Fuel tank/vapor canister emissions sensors
EGR valve
Bad DPFE
PCV valve hoses, gromet, elbows, lines
Cracked/broken vacuum lines under the hood
Intake manifold related gaskets/O-rings, or even a crack
Cracked vacuum reservoir ball

 

[Gary Crist]

2 things, Now that "everyone" vapes, a smoke machine is right
around the corner, cheap and easy.

When our '67 Galaxie 500 XL was new (only 289 2V) it had a funny looking
vacuum canister. It looked familiar and when I took it
to the car wash the paint came off. Under the paint was
a 1 quart Donald Duck orange juice can, Factory!

To fix cars to go up the grades from L.A,so the A/C
didn't drop out, I soldered a short copper tube to a
"tin can" and teed it w/ a 1 way to the vac supply
to A/C controls..

 

[z13]

I was losing AC out of main vents going up hills in my 2007 Mountaineer v6.

The plastic vacuum line was cracked at the check valve. I cut off about an inch from the end and re-seated it in the valve.
Problem solved

 

[koda2000]

2 things, Now that "everyone" vapes, a smoke machine is right
around the corner, cheap and easy.

When our '67 Galaxie 500 XL was new (only 289 2V) it had a funny looking
vacuum canister. It looked familiar and when I took it
to the car wash the paint came off. Under the paint was
a 1 quart Donald Duck orange juice can, Factory!

To fix cars to go up the grades from L.A,so the A/C
didn't drop out, I soldered a short copper tube to a
"tin can" and teed it w/ a 1 way to the vac supply
to A/C controls..

Yep, my dad bought a new Thunderbird Landau 390/4bbl in '65. I remember seeing that vacuum can on the firewall and thinking... that's a quart tomato juice can.