Center Vent Not Opening/Defrost Vents Not Closing

After reading for about 2 hours previous posts, I need help!

On my 85 XJ6, the servo seems to be working correctly in that when I change from fresh air to heat, the lower vents open and I have heat out the side vents. When I change to fresh air, the bottom vents close and the blower fans increase speed & I get fresh air out the side vents. The problem is the center vent does not open for the fresh air and the defrost vents do not close. The defrost vents stay open all the time. Could this be the amplifier?

Any help much appreciated.


Your system seems to be modified (‘Gardener’ mod?), Richard - there is no original ‘fresh air’ alternative…

Modifications do not usually involve the servo/amp - which likely work as they should. Basically; the symptoms described indicates that vacuum is removed from the centre and bottom vents (opened by vacuum) and the defrost (closed by vacuum)

To pursue the matter; you must identify possible modifications. What do you do to get ‘fresh air’ setting…?

xj6 85 Sov Europe (UK/NZ)


I am also confused as to what you mean by “fresh air”…
The Gardener mode, if there, does not involve either the vents mentioned, either fan speeds.

When the system calls for cooling the center fascia vent will open, and in Max cooling it goes to Recirc. mode.
In Defrost mode the bottom vents will close and at the same time the windscreen vents will open, they are both controlled by the same (green) vacuum line, accessible if you remove the left side lower center console cover.
I think that first you should check if the lines are intact and if the actuators work (easy with a large syringe).



It would appear that you have a vacuum circuit issue. Center dash flaps are controlled via a vacuum switch on the servo. The defrost vents are controlled via a vacuum switch located on the end of the mode control switch assembly ( right side switch knob marked OFF- LOW-AUTO-HIGH-DEFROST).

I suggest start with the vacuum circuit white feed line where it connects via a tee to the mode control unit.

None of these symptoms suggest a failed amplifier. In fact the fact that the servo responds suggest the amplifier and servo are actually working.



Had time today to get back to the vent problems. I applied vacuum to the defrost and center vent actuators, and they are working fine. The servo is working by going to heat to cool air (and cool to heat). The problem remains with the defrost vents not closing and the center not opening when cool is set. I do not have vacuum on the white or green line. The foot vents stay open all the time with cooling and defrost. Gary, you mentioned the switch behind the mode switch. I looked but couldn’t find anything. As you know, it’s nothing but wires with no space. I left it with frustration! HELP!


The mode switch is electric + vacuum.
You have to dismount the fascia to gain access, no way to see anything from behind.

Different colours are obviously used, Richard - your colours tells me nothing…:slight_smile:


  1. Connect gauge to the (disconnected) vacuum hose at the reservoir. Idle engine; gauge should read manifold vacuum.

  2. Stop engine - gauge should retain vacuum…if not;

  3. Idle engine, then clamp hose going into the cabin from the ‘T’ connection. Stop engine - vacuum should be retained…

If so, vacuum is lost inside - and apart from a plain leak; a ‘dump’ valve is activated by a microswitch on the function switch in ‘Def’ - but may have malfunctioned open. This removes all vacuum, and all vacuum operated flaps etc go to ‘default’ positions…

Plastic 'T’s are used extensively, they get frail over time and fracture easily - causing leaks. As do the plastic actuators - which, when failing, may cause single or multiple flap malfunctions…

Apart from visual inspection; selective clamping of hoses is the usual diagnostic tool. (Cannot help you there; my diagrams have different colours). A vacuum source is very useful during testing - eliminate constant start and stopping of engine…

The servo is reset by using the temp control; all vacuum valves are operated either mechanically or electrically by the servo cams. Heating and cooling is controlled by routing air flow through or past the heater core…

As an aside; if cabin temp is below ‘65’ the system stays to ‘heat’. If above ‘85’ it stays to ‘cool’…irrespective of temp control position…

xj6 85 Sov Europe (UK/NZ)

Understand, Frank. Any idea where the “dump valve” is and/or how to get to it?

I do have vacuum going into the car;therefore, the lost is somewhere internally.



Check for ‘vacuum control solenoid’ in Archives, Richard…there was something about it fairly recently.

In my diagram it is connected to the blue vacuum supply line - which may be your white? It has electric wires - normally activated by from a microswitch at the function control in ‘Def’…

Finding an internal leak in the AC set-up is like looking for one needle in several haystacks - you just have to apply vacuum…and go looking…:slight_smile:

xj6 85 Sov Europe (UK/NZ)