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Porsche 928/S A/C & Climate Control , Part 1 , 1980 - 1986

Buchanan Automotive

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928 ( all Models ) Air Conditioning Technical Tips particularly with these cars are now between 28 to 45 years old ( as of January 2023 ) & yes the first thing to let go in old age in all cars ( not just Porsche ) when they get over 10 + years of age ( from new ) is usually Air-conditioning  & thats natural as A/C is not vital at all for the car to run , but in the Australian summer it will somewhat restrict the use of cars where the A/C does not work ( its just way too uncomfortable in the Australian hot and often very humid conditions )

 Some interesting background info }
 The first thing to know is the 928 was the first Porsche to have A/C as a standard fitment unlike the 911 ( except the 911 Turbo ) and even the 964 ( 911, 1990 - 1993 ) ) had no A/C as standard for the UK market as it was an option, same goes for the 924 / 931 / 944 / 951 / 944S2 / 968 through to 1995 , only the 928 had A/C on all versions as standard , yes I know the 1988 year model 928 ClubSport did not get A/C but that was a rare anolmoly & Porsche learnt quickly that it was not a good idea in that price range & the next year ( 1989 ) the 928 GT did get A/C as per normal ( see below )
Note } The exception was the very rare 1988 ( one year only ) 928 Clubsport A/C was deleted to save weight .
Note 2 } For the Australian market , Porsche Cars Australia decided ( wisely ) to have A/C & climate control as standard on all 944 models from 1986 onwards , A/C became standard for the 911 NA from 1984 onwards ( the 911 Turbo 75 onwards / A/C was standard for Australia )
Note 3 } In This century we take for granted decent size air vents blowing air ( forget about A/C for the moment I just mean ambient temp ventilation air via an internal fan ) onto the driver and passenger on all motor cars in this century , but this was just not the case in last century cars , interior ventilation in 95 % of last century cars was very bad by todays standards 
Note 4 }  This one seems basic , but is essential to all vehicles with A/C , with the A/C system in reasonable condition , to get the max efficiency , the system must have 100% of the required refrigerant charge , by comparison if only 10% of the refrigerant has leaked out ( micro leak or leaks ) on a hot humid day the A/C system will be noticably poor in cooling performance & the hotter the weather the worse the result , so apart from everything else the No 1 item on the agenda is to STOP any A/C system from loosing refrigerant gas & as cars get older the REAL possibilities of micro leaks gets more and more each year , so when recommissioning a A/C system on a 928 , particularly one that has not worked for some years or more its important to know to what are the components that are common A/C gas leak points/ areas so the owner can can decide what to do ( knowledge in this area is vital )
Note 5 } Quite understandably a lot of people do not realise that on a hot day “ stuck in traffic “( very poor or no airflow getting to the condenser through low road speed ) with the sun is beating down on you through the windows & you naturally have the A/C on with the interior fan on MAX ( who could blame them ) and thats why the A/C is there ? , well yes BUT the longer you leave the A/C on with no break ( stuck in traffic) the more likely the A/C system will not cycle , the interior temp (with recirculate airflow is on ) evaporator temperature is not getting to the 2 or 3 degrees celsius temp to turn the  evaporator switch off the 12v to the A/C clutch, so hence the compressor is running all the time with NO break at all , if this situation continues for even 20 minutes ( compressor running with no break at all ) plus with the combination of very high engine bay temperature ( radiant heat from the engine saturating A/C compressor and A/C hoses and pipes ) only exasperates the issue & the A/C compressor gets hotter and hotter and hotter , they will get so hot you can not touch the outside of the compressor ( they get alarmingly hot ) , this means you are actually decreasing the life of the compressor , not just the numerous number of seals ( tying to keep the gas in along with its oil ) but the A/C oil Temperature gets in the area where oil shear is starting to take place , with he A/C lubricant life span being reduced & remember the A/C oil runs in situ with the A/C refrigerant & with A/C oils ability to lubricate after being massively overheated numerous times this effects / accelerates wear inside the compressor , being bearings and multiple pistons & cylinders ( scoring in the cylinders ) which creates minute metal particles which then are spread throughout the entire A/C hydraulic system & these minute metal particles will settle in the varnish layers in the old condenser when the old condenser is extremely hot only to be released bit by bit when extremely hot on another occasion doing more wear to the old compressor until one days it replaced but NOT the condenser , 99.9% of all automotive A/C specialists know thats a sure way to reduce the life of the new A/C compressor dramatically ( well before any warranty has expired ) hence why a new condenser is installed with a new compressor on any car, the remainder of the system is flushed out with dedicated A/C flushing fluid ( TX valve removed ) and new receiver drier + if the said last century car is over 15 years old then new A/C hoses are normally required 
The above Note 5 ) is very relavent to the 928 series , in particular the 1978 to 1986 year models , because of the way the condenser ( free standing which is unusual in cars ) but more importantly the very deep last century use of  “mildly” effective ( Not Good) cooling of the A/C condenser , above 60 kmh ( road-speed ) its perfectly fine , above 80 kmh ( road-speed ) its getting even better in relation to cooling the A/C condenser and its A/C pressures , the lower the high side pressure the cooler the charge is being cooled ( shedding heat ) and the better the lower temp results at the interior cabin air vents & conversely, sitting in traffic conditions ( stopped or crawling along & the summer sun beating in all that 928 cabin glass ) with the A/C on , the A/C compressor clutch will eventually NOT switch off and on etc ( cycle ) , this in turn will overheat / over-pressurise the system ( cat chasing its tail ) & even back in the mid 1980’s at the Sydney Porsche dealership I was working at we would see every summer a number of 1 or 2 or 3 year old Porsche 928S cars where they would explode a A/C hose or part of the Reciever drier in Sydney traffic on a hot day & naturally all these cars were on the original R12 refrigerant , all stock standard and these cars were often less than 4 years old & yes driven by Sydney busnessmen everyday as part of their work travels , we knew at the time that the cooling of the A/C charge in these conditions needed improvement .
With the early 928 A/C  from 1980 up to and including 1986 year model , the large round condenser “cooling" electric motor / fan blades assembly( mounted in-front of the condenser )  , being round in shape and close to the condenser ( condenser is larger in a rectangle shape ) , naturally the effect from the electric fan ( when operating ) will be directed onto the center portion of the condenser surface area , the surface area outside receives little no no cooling effect from this large round fan , however all is not lost, on the other side ( engine bay side ) the large single viscous fan assembly which has a large plastic shroud does its best to pull some air but at idle speed ( car stationary ) the small amount of air being pulled is barley enough for the coolant radiator let alone the A/C condenser & as I have mentioned , the A/C condenser is free standing and not mounted to a normal radiator support panel that pretty much all other front engine’d cars have , this contributes to air being pulled by the viscous fan to be sucked in to the readiator from around the perimeter of the condenser ( air will always take the path of least resistance ) 
Note 6 }  The 928/S up to 1986 year model has / had a factory fitted seperate temperature switch mounted on the reciever drier that activates the electric A/C condenser fan , but as the years go by the temperature that it operates at creeps up and up which is the complete oposite from what the A/C system needs , we quickly realised the issue even when these cars were near new , exploding A/C hoses and receiver drierers ,  so using common sense for decades now , we along with other Porsche repairers remove this useless switch & install & seperate Bosch relay & new wiring that will mimick what other car makers were doing in the late 1980’s onwards by having the electric A/C condenser fan switch on when the A/C clutch is engaged , nothing new or brillant here just common sense & helps a lot because the electric fan now helps with the refrigerent pressures under control
Note 7 } The electric fan is switched on via the A/C being turned on ( as above ) or by the coolant radiator ( 2 pin ) fan switch & yes even these switches , as they get old they will switch on later and later and later 
  Explanation some of the main components and what they do }

"Condenser "= its like the radiator ( heat exchanger) for the A/C and on conventional cars its often installed in front of the engine's radiator ( directly in the incoming air flow) & it gets hot with the A/C on ,& its sole job is to shed its heat , but we need to cool it as much as possible & the cooler the condenser the cooler the A/C temps are at the interior vents ( to put it in a basic way )

Note 1 } On a conventional car you will see viscous or electric fan/s behind the radiator ( pull fan/s ) & sometimes another electric fan in front of the A/C condenser ( push ) as well 
Note 2 } The Japanese & Korean car makers were the ones who not only perfected automotive A/C but made it possible ( in super mass production )for very efficient A/C to be installed in even cheap cars from the 1980's onwards ,particularly in the last 25 + years
Note 3 } The Japanese & Korean car makers perfected ( in mass production for cheaper cars ) the use of very efficient "Parallel Flow " A/C condensers with state of the art & quiet "curved blade" twin fan motors in completely rear of coolant radiator double shroud , meaning with vehicle stationary & the twin fans running ,all the air being pulled through has to make contact with the complete surface area of the Parallel Flow A/C condenser & coolant radiator 


"Evaporator"  = its the mini ( usually thick ) radiator looking component that is meant to get very cold & is positioned in the dash area of cars ( usually in the climate control/heater box of conventional cars) directly in the interior ventilation air flow to the air vents , meaning its what creates the cold air as the air( from the interior fan ) passes through it & naturally because its cold ( hopefully around 2 to 3 degrees celsius ) in humid weather , water / condensation pours off it and this water has to be managed to exit the car without spilling into the cabin of the car , naturally in hot dry weather ( very low humidity ) very little water drips from the evaporator 


"Compressor" = its the refrigerant pump , generally / traditionally driven by the engine via a belt , what a lot of people do not know is as the A/C gas amount reduces ( via leakage ) the "A/C on/ pumping" gas pressure reduces which in turn permits the A/C compressor lubricating oil ( that runs in situ with the refrigerant ) will start to drop out and will pool in the bottom of the condenser & or the evaporator ,which in turn starves the A/C compressor of lubricating oil , this is one the main "traditional " causes of A/C compressor seizures , but before it seizes it has the ability to spread tens of thousands of tiny metal particles from the wearing compressor internals all throughout the hydraulic system ( very nice ) 
The expression } A/C Cycle is referring to the A/C system is being as designed and the A/C clutch is being turned off ( hopefully ) in regular time segments , due to the fact that the evaporator has cooled down to reach the 2 or 3 degrees celsius temp which activates the evaporator switch


"TX Valve " ( TXV) Thermostatic Expansion Valve , it regulates the rate at which liquid refrigerant flows into the the evaporator , it maintains the correct supply of refrigerant by matching this flow rate against how quickly the refrigerant evaporates ( boils off ) in the evaporator , TXV was ( in the 1970's , 1980's,1990's ) was used in only high end cars & all Porsche "factory A/C " systems from that era have them

"Receiver drier" , only on TXV systems , located on the high side , it contains desiccant ( this should be replaced every few years or so )


A/C Barrier Hose or A/C hose , over the decades the quality of barrier hose has become better & better & in this I mean not allowing leakage from the hose itself & yes A/C barrier hose from the 1970's & 1980's is by todays standards of "barrier hose “, is just a bit better than junk , then add to that 30 to 40 years of deterioration  & we have completely decrepit barrier hose & will continue to leak 24 hours a day 7 days a week ( waste of time & money to leave as is ), this includes the barrier hose Porsche used , it was all you could buy then , but like I said, its not much use now.
The 928 has three seperate A/C ( barrier ) hoses , the first two are fairly obvious being to and from the compressor , the 3rd hose is the hidden Liquid line hose ( quite difficult to remove to replace as access is poor  )
As the years went by Porsche kept changing the 928 A/C hose / pipe assemblies is their shape / routing and sizes of the threaded ends of the two main A/C hoses and even the materials changed , up to around 1984 the A/C pipes were steel for the main hose/pipe assemblies & copper for the liquid line  & from around 1985 /1986 the materials changed to thin wall aluminium for the main A/C hose /pipe assemlblies & the Liquid line as well 


 "Pressure Safety Switch/s" , from the early to mid 1980's "some" car manufacturers installed with the factory A/C, a pressure safety switch , so if  the A/C "pumping" pressures dropped below a certain point , the switch would cut out the 12v power supply to the A/C clutch & this would save the system from destroying itself through lack of A/C lubricating oil that would have already started to fall out of situ with the refrigerant because if the A/C compressor kept trying to pump with low refrigerant levels the A/C oil will not run & the compressor will go metal to metal internally due to lack of oil supply , thousands of metal particles spreading all through the system until the compressor seizes ( what an expensive mess )

On some Porsche cars ( last century ) the pressure switch will have another function ( two stage switch ) , it will respond to excessive pressure ( beyond a nominal amount ) and instruct the electric condenser fan/s via its relay/s to turn on ( good design ) but can be an issue in old age , meaning not turning on or too late to turn on


" Evaporator Switch " , known also as the deicing switch , it monitors the temp of the evaporator to make sure it does not turn into a frozen ice block ( water cascading off it turning to ice ), remember on a A/C system working correctly ( COLD ) , meaning its cycling ( A/C compressor on & off via deicing switch ) the evaporator is like the outside of a cold beer glass , its dripping with water from the atmosphere & its this water that will freeze if the A/C is too good , if it freezes , the air that was ( before it froze ) going through the evaporator , now just hits a wall of ice & the air from the interior fan just goes around it = air temp at the interior vent/s is no longer cold , hence why a very good system must have a decent & reliable de-icing switch & yes they play up in old age
The Evaporator switch interrupts the 12v power to the A/C compressor clutch 


Now onto types of A/C Condensers & the evolution of condensers from the deep last century Serpentine Condenser to the new "this century" Parallel Flow Condenser designs 

Paramount is the ability of the A/C condenser/s to shed heat as efficiently & as quickly  as possible & when it comes to road cars( limited space) the condenser/s can ( almost) never be BIG enough & then to make matters a bit worse , along came R134A back in 1993 & R12 was phased out , which even I admit was a good thing to get rid of R12 as it was a very harmful ( to the Ozone Layer ) Chlorofluorocarbon, the reason it was a bit worse "refrigerant wise" was that the long gone R12 worked OK with poorly designed A/C systems , even after R12 was long gone , it took A/C component manufacturers ages and ages to come up with a better designed A/C condenser that would shed heat more efficiently than the 1950's designed "Serpentine" A/C condenser 

It wasn't until this century did we start to see the first versions of the "now standard in new cars " Parallel Flow" A/C condensers , size for size comparison , a Parallel Flow condenser is up to 30% more efficient in shedding heat as compared to a NEW last century Serpentine A/C condenser ( New against New )
Now if we take you average 30 year old Serpentine A/C condenser from a last century Porsche ( example only ), the difference will be more like 50 to 70% difference in the ability to shed heat & naturally the higher the outside ambient temp the worse the last century Serpentine A/C condenser will perform ( shedding heat ) as compared to a Parallel Flow ( this century ) A/C condenser 

Question } Why would a 20 or 30 year old Serpentine  A/C condenser be less efficient than say a brand new Serpentine A/C condenser ?

Answer }  The 20 or 30 year old Serpentine A/C condenser has been through thousands & thousands of A/C heat cycles & each high temp heat cycle ( stuck in traffic etc )& few more molecules of burnt A/C oil is deposited as a lacquer/varnish on the inside of its condenser capillary tubes , this builds up in time & becomes a real problem , because it acts as an insulator & the condenser is much less able to shed heat 

Note 5 }  There were NO "Parallel Flow" condensers fitted to last century Porsches by the factory , be it the 944 , 951 , 944S2, 968 , 911 , 930 , 964 ,993 , 928S/GT/GTS ( NONE ) , because the technology was not ready , its all this century when it come to the latest in true Parallel flow condensers 

Note 6 ) Last century "affordable cars " be it Ford , Holden ( GM ) Toyota , Mazda , Nissan etc etc etc used deep last century Serpentine condensers that were built to a price ( cars are built to a price ) the the more common cars had components that were less expensive to make & sell , so we saw Serpentine A/C condensers would pinhole ( spring a leak ) often within the first 3 or 6 years from new and if the said vehicle lives and drives close to( ocean ) salt air they will deteriorate quicker  , thats because they were made from thinner alloy as compared to high end cars , even the grade of alloy was less durable ( more susceptible to salt air corrosion ), but thats fine because those cars had A/C condensers made in very large numbers & were "relatively cheap to buy " once you spring an A/C leak , its all relative 
 One good thing about cheap mass produced thin walled last century serpintine A/C condensers that leak after "say " 6 years is they never really suffered from internal "lacquer " / varnish build up internally because they never( on average ) lasted long enough to get to that stage
The other interesting aspect of the cheap "thin walled" serpentine condensers is ( when new ) shed heat fractionally better than the more expensive higher quality( thicker wall) serpentine condensers 

Now onto normal (1980 to 1986) 928 & 928S A/C overhaul ( if you want reliability and as cold A/C as humanly possible in a 37 to 43 year old European Low Volumn production car ) 
A ) Remove the old compressor & hoses and the liquid line hose/pipe assembly , remove the TX valve & old evaporator switch , remove the old A/C condensor and old reciever drier etc
B ) A/C flushing / clean out of the existing A/C pipes and the old evaporator and blow out old dirty liquid ( the now dirty flushing fluid ) with Nitogen gas  
C  Then remanufacture new A/C liquid line using Lokring fittings/ferrules , this eliminates so many dissimilar metal joining issues , meaning copper to alloy or dissimilar alloy to another alloy 
D )Then make new A/C hose/pipe assemblies to and from the compressor 
E ) Then install a new 928/s ND compressor ( these are sold as reconditioned overseas, but they are completely new including the clutch & its coil )& we always install new A/C oil & install the new( remanufactured ) A/C hoses to and from the compressor & new A/C “V" belt
F ) Reinstall the remanufactured liquid line ( not easy )
G ) Install new TX valve R134 version ( two version , one version from 1978 to around 1984 & a second version / very different from 1985 )
H )Then measure up and make a custom alloy frame for that particular 928 to enable the new Parallel Flow Condenser to fit , along with a new later style Reciever drier and new bracket for that as well
I )  New low pressure safety switch
J ) New Evaporator switch ( adjustable)
K ) New high & low side A/C service ports
L ) Evacuate system and re-gas 
M ) The A/C condenser cooling fan/motor etc , naturally we want this fan to switch on when the A/C clutch is engaged ( just like on all modern cars ) and the relay wiring mod already mentioned is vital , BUT with the electric motor of is at least 37 years old ( 1986 year model ) & 43 years old ( 1980 year model ) & we must have this electric fan to be 100% reliable because if it stops working ( too old ) a few weeks or even months later there is no warning to the driver sitting in the car , but the A/C system ( sitting in traffic etc ) we get damaged , so its just not worth the risk , see below on A / C electrical upgrades
The above takes care of the A/C hydraulics providing the decades old A/C evaporator ( DEEP inside the dash / heater box ) has NO hidden A/C leaks 
Note } From the 1985 year model , Porsche intriduced the 928 only rear A/C option ( expensive option ) for the 1985 & 1986 year models ( and later as well ), unfortunatly as time went by we noticed that the rear A/C evaporator was very prone to micro leaks , meaning constantly loosing A/C regrigerant 24 hours a day 7 days a week , they were not a real issue for the first 10 to 15 years from new, but after that they are one of the most common A/C leakage components , only 2nd in leakage to compressor/s and hoses ( so its fairly common after 37/38 years )
However all is not lost as the rear A/C was always a disappointment from new anyway , thats not to say it was very well implemented ( gived the very limited space available , big tick there ) but there is only one A/C compressor ( not 2 compressors ) , so what that meant is ( when the car was new ) , say the engine is running and the A/C is turned on & the main interior vents ( dash board ) are at 7 degress Celcius = very nice , however the moment you turn the rear A/C on you will notice that The rear A/C vents are naturally cooling , BUT the main dash vents are not getting any colder , in fact they are warming up , from the 7 degrees a few moments age to around 10 or 11 degrees or more , but why is it so ?? 
Answer =  The reason is quite simple , the A/C hydraulic refrigerent branch off ( incorporated into the liquid line ) drops down at the firewall area to go to the rear A/C unit , where as the main in dash A/C unit , the liquid line has to go verticle and then turn 90 degrees to go through the firewall much higher up , naturally if you give liquid a choice of up or down , down is so much easier path to take , so the main A/C ( dash ) where the ocupants are at in the front seats get” less” ( warmer ) direct cold air  , the other thing ( cosmetic ) is the rear A/C air vents basically just point to the roof lining & are prone to damage ( vent slats ) from luggage if the rear seats are folded down for extra luggage , but the standard rear glove box that is there if there is NO rear A/C is handy and looks good , particulary in leather 
If we find that the rear A/C evaporator is leaking , then we give the owner the option of either replace the rear evaporator with a new one from Porsche plus a new rear A/C TX valve ( this gets expensive ) or we just hydraulically blank off the A/C hydraulic pipes under the car to stop the refrigerent getting to the rear evaporator from either side ,( we keep the blanking off fittings in stock if needed )  but we leave all the rear A/C components ( in cabin ) untouched , so the rear A/C fan will still operate if needed & the" in cabin " looks untouched , but the said 928S will operate with the main ( dash A/C only ) to elinate as many A/C leakage areas as possible ( the owner has the choice ) & either way we do not want to have the A/C system loosing A/C gas/refrigerant otherwise there is little point in doing the above work 
Now onto A/C Electrical upgrades , again concetrating on the 1980 to 1986 year models ( for the moment )
N )  As mentioned , the A/C condenser extra relay ( fused ) and wiring loom mod
O ) New large A/C condensor fan/motor assembly with new custom made brackets as like with the condenser , the fan assembly is also free standing( again unusual in the car world ) so the support brackets are vital & it has to look the part , also, we use curve bladed fan assembly , these are much quieter than the old straight bladed fan ( no big deal but a bit more modern in noise control )
P ) With Climate Control there are two versions of the HVAC ( slide control ) unit ,  there is no seperate A/C push button , this early system ( 1980-1983 where climate control is fitted ) was called” Automatic” on its facia panel , with no A/C button anywhere , naturally the A/C clutch is switched on via the micro relay inside the HVAC unit , this will always fail in old age is this undersized relay has to cope with the amperage spike , which over time will fail to switch , we always remove this HVAC ( slide control ) unit and dismantle to inspect , if the electrcal board is not too bad , ( depends on what we find ) we can spend the time to de-solder the old micro relay and we can use a more robust relay that we install externally via a small dia wiring loom , this works quiye well “ if “ the circuit board is not badly knocked about ( burning and corrosion etc ) , if the board is a real mess ( we see a few of these ) then we can give the owner the cheaper option of us installing a separate A/C switch , usually ( like other people do ) we can install a small easy to operate toggle switch under the dash cluster ( easy to reach and operate by the driver ) & then we  install a seperate Bosch relay ( fused ) and corresponding custom wiring loom to take all the amperage spike ( potential damage ) from the A/C clutch ( 3 to 5 amps engagement ) 
P ) Part II , 928S ,1984 to 1986 inclusive , Porsche reinstated a seperate A/C button from the 1984 year model ( good idea )  , the seperate A/C button sits in another more slender panel below the HVAC unit .  The new for 1984 HVAC ( slide control unit ) with the wording on its facia “ Climate Control “ instead of the earlier version  that stated “ Automatic “ , these HVAC ( slide control ) control units also have a circuit board and a micro relay that does a similar job as the ealier unit & yes the amperage spike damage does cause them to become intermittent in operation , meaning they will not switch the A/C clutch on even though the A/C switch button is depressed and illuminated , so the same goes for these unitss , they can either be repaired if feasible or bypassed but this time we can use the A/C button and a seperate Bosch relay to take the ampertage shock away from the HVAC unit ( been doing this for 20 + years for reliability reasons & works perfectly )
Q ) New interior fan relay and fuse 
Note } While we have the HVAC ( slide control unit / both versions  ) apart we also clean its internal brass electrical tracks and install a new globe and if needed new slide knobs and clips ( we keep all this in stock )
Next ( staying with the 1980 to 1986 928/S ) we are going to check for vacuum leaks for the climate control vacuum actuators that operate ( supply vacuum ) to the heater tap , floor vents , demister vents , dash vent defuser , recirculation vent flap etc , if the vacuum is less than 17inhg at idle , particulary under acceleration the heater tap will come on and lots of lovly HOT air ( from the heater core ) will come flodding out onto the driver and passenger and on a HOT day this is not good at all ( but very common in old age )
Note } The vacuum is used as the force to operate flaps and the heater tap , with low or no vacuum the herater tap is spring loaded internally to turn onto FULL HEAT = Not Nice at all in summer 
So we measure the vacuum under a few different conditions and rectify  the vacuum leaks , either by installing new vacuum actuators or we give the owner the choice of bypassing the leaking actuators to resore vacuum to the other actuators that still work & we always install a new heater tap and a new genuine Porsche 928 vacuum one way valve ( this valve maintains vacuum in the vacuum tank for acceleration events )  
Also , all 928 ( 1980-1986 ) Climate Control systems have an interior and exterior tempature sensors , these sensors must be checked for relevant Ohms readings depending on the outside ambient temperature and inside ambient temperature , this is a critical test because the information from these sensors goes to the S1 & S2 climate control unit ( under the dash ) and its this S1 /S2 unit that does the calculations and then operates the bank of vacuum solenoids that operate the vacuum acuators that control the heater tap , floor vents , demister vents , dash vent defuser , recirculation vent flap etc , if the oms readings are wrong ( say with the outside temp sensor ) the system will often not regulate and the heater will come on at the wrong time , so its important to make sure these sensors read correctly to the S1 & S2 control unit 
Once you understand this entire system/s ( as a whole ) its actually quite simple , but then again after working on the 928 series since they came out in 1978 ( 45 years ) it would be a bit strange if I didnt understad the systems ( plural ) well by now & know what modfs these cars respond well to 
That will do for Part 1 ) of the 928 A/C and climate control , later I will do part 2 ) 928S4 / GT / GTS 
Bruce & Sean Buchanan
Buchanan Automotive 
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15 hours ago, Dreamr said:

Thank you for posting this ...

I don't even have a 928, and have no intentions of owning one, but thoroughly enjoyed he read.

Is there something wrong with me?? ...😏

Im hearing ya!

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Bruce I have a 1980 928 imported from Austria and it has a slide type HVAC system and is fitted with a seperate AC button. The belt was not fitted to the compressor ( clutch is noisy ) as I suspect the clutch is bad. The compressor is not seized but has'nt been used for who knows how long. What do you  think a rough estimate of cost will be in incurred to bring the aircon back to life?

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