Series 3 xj6 AC only has defrost

So my 85 xj6 ac/heater only has defrost. With the Texas heat I was hoping it’s fixable here’s some information. The ac temperature knob seems to not do anything is there a way to manually operate that like in my rx8 I can switch it from hot to cold manually at the blend door. Also one time while driving the heater valve hose connection broke so I installed a bypass to keep everything going. I got the car with supposed working heater but never tested I have been given 12a by the previous owner aswell. Fan speed control works it’s just coming from the defrost vents all the time. Honestly all I want is ac so that the car is usable here in the Texas heat

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Do you mean that you only have fans running in ‘Def’, Jabraan - ie it is a fan control problem?

The function control ‘Lo’ and ‘Hi’ manually controls fan speeds - lack of fan control may have a variety of reasons…so…?

Frank
xj6 85 Sov Europe (UK/NZ)
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The fan control works I can switch between 0ff low auto high and def. but in all of those setting the fan does change speed but air only comes from the def vents

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Ah - that means that you have lost vacuum to the defrost vents, Jabraan; they are kept closed by vacuum applied in all positions but ‘Def’. (And vacuum opens the centre vents). However, system operation is very reliant on vacuum for general functioning…

The first step is to check vacuum supply. Locate the vacuum reservoir, disconnect hose and measure vacuum with the engine idling. It should show manifold vacuum - if not; check vacuum connections through various connections to the throttle body…

If you do have vacuum but the defrost vents stays open - it gets more complicated. Next step is to verify that the servo is moving - you should hear it buzzing when temp setting is set to '65’F then to ‘85’. If it is not; your AC amplifier has likely failed…

This is a separate problem, but the system is stuck in one position, not reacting to temp variations, which may mask other problems. Which cannot be resolved without a turning servo…

…which can be run manually - described in archives. But you still need working vacuum…

Frank
xj6 85 Sov Europe (UK/NZ)
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So the first step to getting working ac is to verify vacuum? If so could you explain it better. Or to refer me to vacuum thread as I really don’t know much about vacuum systems, tho not completely lost either. when I last tested the servo I didn’t hear anything moving behind the rh side panel but figured I need to fix the air only coming from def vents before that

Jabraan,

Frank has explained all first steps of fault diagnosis as good as it can be done. It would be more helpful - and appropriate - though if you now started checking the archives as indicated instead of demanding nanny treatment for free.

Good luck with your problem at any rate - though it seems your defrost works well at least as you don’t encounter ice on the windscreen, do you;-)

Jochen

75 XJ6L 4.2 auto (UK spec)

The first step is to get the engine above 50°C.

Then, turn the AC to defrost and see the clutch of the AC compressor (engine bay, large hoses to firewall) engage.

Set the AC to high and fully cold. The heater valve should close (plunger should go up) (also on the firewall, vacuum hose on top and a large hose that goes inside the cabin).

I suspect that you have no vacuum or a large leak if the blower speeds respond to temperature inputs in auto (in that case the servo works, and whirs). Defrost setting will work most of the time, and with all vacuum gone the defrost vents default to being open.

David

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As Jochen mentioned, Jabraam - the archives may contain what you need…

However; the first step is to locate the vacuum reservoir a flattened round tin, usually black, mounted to the inside fender wall near the brake booster. It has one vacuum hose…

Following that hose, you come to ‘T’; one hose goes from there into the cabin, providing vacuum to the AC system. The other hose from the ‘T’ goes to a one-way valve - which opens only one way, retaining vacuum in the vacuum reservoir when the manifold vacuum varies or the engine is shut down…

From the one-way valve; a vacuum hose goes to ‘T’ or ‘X’ connections. With vacuum hoses from there to various vacuum users. Which bewilderingly varies with market,equipment levels and model year - and sometimes PO whims…:slight_smile:

B one hose invariably ends at the throttle body/inlet manifold - where vacuum is supplied by the running engine…

Your problem may just be a disconnected or failed vacuum hose - or a broken ‘T’ connection, or blocked hoses/one-way valve. Easily rectified by reconnecting/replacement - and if so; you are good to go.

Or restoring vacuum may reveal other problems - to be addressed in due course…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)
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Jabraan,

I suspect you have two problems.

First let me say, it is not unusual for previous owners to modify a system to deal with an unresponsive system. Often modifying the hot water valve vacuum line to provide heat in the winter. These undocumented modifications can make diagnosis more difficult…but we will check one system at a time.

As Frank described…Defrost flaps are controlled via vacuum, as well as the hot water valve, recirc flaps and center dash flaps. It is not uncommon for the vacuum lines in the engine bay to be switched moving the system vacuum supply to the hot water valve. You need to confirm these line placements.

Main vacuum comes thru the fire wall to the right hand vent panel and is color coded as white. Then connects to a tee and then to the Mode Control switch ( right hand switch)… the Mode Control vacuum switch controls the defrost flaps ( green pipe)

From the main supply tee…vacuum is directed to the servo to its vacuum valves for hot water control and center dash flaps. Review image.

Once we confirm vacuum into the cars climate control system is present…we can move on to checking the operation of the servo and amplifier by checking all 5 fuses, servo motor and feedback potentiometer.

Cheers

Gary

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