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Chuck, ASE Certified Technician
Category: Mercury
Satisfied Customers: 2100
Experience:  33 Years experience,Ford Senior Master,ASE Master,L1 Advanced Engine Performance
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2005 Mercury Mountaineer: v6..the ac goes to the defroster vents

Customer Question

2005 Mercury Mountaineer 4.0l v6 AWD. AC is on regular vents, blowing cold as it should, during acceleration, once the car gets to 45 mph the ac goes to the defroster vents, once past 50-55 mph it switches back to the regular vents, if you come back down to below 40 then accelerate again, it will not do this. It seems every time you are at a dead stop then accelerate, this happens. Very WEIRD! I felt the air coming from the defroster, it is air conditioning air. What may be causing this? I have tried taking the ac off of auto and have even turned the defroster on and back to the regular vents. Nothing seems to remedy the issue. I have even sat in park with the accelerator at WOT, it hits the rev limiter at 3.5K but the issue doesn't exist in park.
Submitted: 7 years ago.
Category: Mercury
Expert:  Tim replied 7 years ago.
First check the black vac line that comes in from under the hood and make sure it is fully connected under the dash.Than you will need to check the line from the firewall and trace it to the vac canister and make sure there are no leaks. If the line is good than you will need to vac test the canister to see if it is holding vac under load.The lack of vac is what is causing the vents to go to defrost. Defrost is the default for the system when you loose vac so that the windshield stays clear.
Customer: replied 7 years ago.
Ok jsut got in from troubleshooting, I checked and reseated all lines and connections under the dash. No trouble found. I opened the hood to find the two lines coming from the firewall. One goes to the plastic plenum (Black), then there is a grey line, that runs the passenger side of the engine. The line is taped onto a heater hose, appears to be factory black tape then there is a butt connector line goes in, then there is nothing on the other end. Started the car, there is vacuum, I hooked up my gauge to it and test drove.

The problem is gone if this line is plugged or has a gauge on the end. Amazingly enough, when i unplugged the line, the problem came back and it shifted a lot harder, I think I may have found a big problem! Please advise, i know a plug wasnt standard so where does this line go?

Expert:  Tim replied 7 years ago.
From the engine the line should have a check valve than run to a tee that runs to the vac canister and to the inside of the truck.
Customer: replied 7 years ago.
Ok so is the vac canister under the dash or under the hood? What does the check valve look like? I shouldn't have an open line correct?
Expert:  Tim replied 7 years ago.
No there should not be any open lines. The canister should be under the hood and the check valve should be a round valve that only allows vac to pull one way you should be able to blow thru the way you should not.That is the piece that actually stops the loss of vac under accel the canister holds the vac so the vents do not switch to defrost but the valve keeps the system from losing the vac.
Customer: replied 7 years ago.
Where is the vac canister located under the hood? Is it up towards the front of the car?
Expert:  Tim replied 7 years ago.

ItemPart NumberDescription
1W701696Vacuum reservoir screw
219A566Vacuum reservoir
3-Vacuum connector (part of 18C581)

Removal and Installation

  1. Remove the vacuum reservoir screw.
  1. Remove the vacuum reservoir.
  1. Disconnect the vacuum connector.
  1. To install, reverse the removal procedure.
Customer: replied 7 years ago.
Relist: Answer quality.
In previous questions I was used to receiving actual diagrams of the part locations, this is a link that doesn't work. I have had to ask too many questions it seems, there is no doubt he knows what he is talking about but lacks the thoroughness that I am used to with JustAnswer. Sorry.
Expert:  Tim replied 7 years ago.

I am sorry if that did not open I will resend another.


Customer: replied 7 years ago.
Okay, I checked all of these connections last night inside the car. They are good. I guess my question is where does the grey line go, it is just sitting there with nothing to plug into? It runs a heater hose going toward the front of the car, is there an climate control component that is up towards the front of the car, that needs vac?
Expert:  Tim replied 7 years ago.

The vac line on the heater control valve located in the heater hose runs to the control head in the dash.

Customer: replied 7 years ago.
Ok heater control valve is next to the engine a little further down that heater line?
Expert:  Tim replied 7 years ago.
Ok I will opt out and see if another expert can help you further. Do not reply to this it will only cause the question to be locked back with me.
Expert:  Chuck replied 7 years ago.



Maybe I can help you with your situation.


The gray line goes to the heater control valve vacuum switch.



ItemHose ColorFunction
1GrayHeater control valve
2BlackVacuum source



Here is the vacuum schematic




ItemPart NumberDescription
118A318Vacuum control motor - panel door
2-Panel airflow
318A318Vacuum control motor - defrost door
4-Panel door (full vacuum position)
5-Defrost door (full vacuum position)
6-Defrost airflow
7-Side window demister airflow
8-Temperature blend door (full heat position)
919860Evaporator core (part of 19850)
10-Air inlet door (full vacuum position)
11-Outside air inlet
1218A318Vacuum control motor - air inlet door
13-Recirculated air inlet
1419805Blower motor
1518495Heater control valve (full vacuum position)
16-Vacuum source
17-A/C vacuum check valve
1819A566Vacuum reservoir
1919B888Function selector switch
20-Heater control valve vacuum switch
2118476Heater core
22-Floor door (full vacuum position)
23-Floor airflow
2418A318Vacuum control motor - floor door


It feeds the heater control valve



ItemPart NumberDescription
18287Inlet and outlet hose clamps (2 required)
218495Heater control valve
39E498Heater control valve vacuum connector



The heater control valve is actuated by the heater control valve vacuum switch (manual climate control) or the heater control valve vacuum control circuit (Electronic Automatic Temperature Control (EATC)). When the heater control valve is closed (full vacuum), coolant flow through the heater core is shut OFF and A/C cooling efficiency is increased.


Here is the lines coming from inside the car





Then they go to these connectors





The vacuum reservoir is located under the dash next to the blower motor on this vehicle.So the black supply line comes from intake manifold vacuum to the reservoir then to the control head and the heater control vacuum switch then back out to the heater control valve in the heater core hose.