Air Conditioning

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When does the Air Conditioning operate?

The A/C is originally designed to run when the interior fan runs, regardless of any other control settings.

Install a trinary switch

A trinary switch is a combination High and Low limit switch and fan control switch. The OEM SM AC design lacked an overpressure limit switch of any kind. Thus failure in the aircondition system, such as a failed fan relay or blocked condenser, can cause an overpressure condition, which causes the pump to stall. Similarly, loss of refrigerant and with it the lubriating oil can cause the compressor to sieze. Since the compressor is driven by the main timing chain, either of these failures are worth avoiding as the failed system load is very high indeed (the drive belt has to be melted or broken) and often damages the tensioner, leading if ignored to possibly a broken timing chain.

All modern cars with airconditioning have this safety switch as standard.

The Trinary switch is simply wired in series with the AC compressor electric clutch power lead. If the gas pressure in the system drops below the low limit threshold, or if the compressor produces dangerous levels of high pressure (for example if the fans have failed), the switch disengages the clutch.

An additional two wires on the switch are designed to be wired into the cooling fan circuit to turn on the fans when the pressure rises above a certain level. This feature can be incorporated in the SM by making it an alternative ground path to the radiator coolant temperature switch (thus bypassing the mano contact speed senser and mano contact relay in the OEM system). Simply wire the Trinary Switch into a "T" with the positive lead to the thermal switch on the radiator. The other lead of the Trinary Switch fan loop is grounded.

The switch can be compressor mounted or installed on the reciever/drier or even into the high-pressure line of the system.

Trinary Switch Installed on Compressor Service Port

Install an A/C defeat switch control

Because the airconditioning runs whenever the interior fan is running (even when the heat is on) it is sensible to fit a defeat switch. This is best done by inserting it into the airconditioning thermostat feed wire. See Air Conditioner Cutout

There is often a space (for an original '70's Citroen 'cold fan' or CX airconditioning switch) in the row of buttons below the three subsidiary guages for oil, water temperature & fuel level.

Simply run two short wires to the 'yellow' male/female bullitt electrical connector to the thermostat which is mounted on the side of the evapourator box. To do this remove the passenger centre console side panel & find the wires, then bridge in the wires to the switch.


  • York Compressor background:
    • The York, being designed to be bolted to the engine, is not internally balanced as there was no need since engines are normally isolated from the car. Thus the accessory tray on the SM is mounted on rubber as an isolator. This doesnot prevent all vibration however, and the alternative compressors suggested do remove this annoyance.
York Compressor

Most SM's are equipped with air conditioning systems. Whether installation of such a system was a standard feature or optional depended probably on the market were the car was sold. In Holland and Belgium airco's were an option. We guess that this was the case in all countries with a mild climate. In warmer climates, or in markets were most cars were fitted with airco's (USA), it might have been a standard feature.

The SM's air conditioning installation has been criticized a lot. It may ruin the engine. See above about loss of gas & oil. This is one subject we'll deal with here. Ger van Rootselaar in Bolsward Holland gave us important information on this problem.

The second item discussed here are the changes required to meet environmental law. R12, the gas inside air conditioning systems in the time the SM was built or serviced, has been out of production for a number of years because of its contribution to the greenhouse effect. It needs to be replaced by R134a. We won't go into air conditioning theory here.

Airco compressor breaking chains?

The SM's airco installation has been criticized a lot. The York compressor, like the hydraulic pump and generator, is driven by the intermediate shaft which primary job it is to drive the camshafts. The compressor is said to be the cause of fatal engine failures. When 'switched on' by the electromagnetic clutch, the sudden load breaks the camshaft chains. Depending on engine speed valves will meet the pistons, and thus another Maserati engine may meet it's end!

At low engine revs, in villages and cities, the risk seems largest. In the past we talked with SM owners who fitted a master switch on the dashboard which allowed them to disconnect the airco compressor (in villages) while still being able to have fresh air inside the car.

The culprits in this horror scenario, apart from the compressor which does its job without problems in other applications, are the electromechanical clutch (more a switch than a clutch; see [# #11 illustration 642]) and the camshaft chains.

Retrofitting a system from R12 to R134a

Back to the air conditioning system itself. The R12 gas inside airco system was outlawed on wide scale in the 1990's because of environmental problems. Meant is the contribution of R12 to the destruction of the ozone layer. Production of CFC-based refrigerants, like R12, stopped (in the USA) in 1994. Details around maintenance of systems still filled with R12 may differ according to local regulations. Basically however, it is impractical to continue use of R12.

In locations where hydrocarbon refrigerants are legal, hydrocarbon refrigerants are the most practical substitute refrigerant. See "Retrofit with drop-in refrigerants HC12 and Freeze-12" below

If HC12 is not permitted in your area, it may be necessary to convert to R134a.

To get the picture of what exactly is involved when going from R12 to R134a we talked with Han Wandel and Ger van Rootselaar in Holland.

First thing to realize is that working on airco systems is a job for licensed specialists. Certainly when vacuuming and refilling the system. Not only is the loss of CFC-based gasses into the atmosphere punished by law, there is also a safety risk. Refrigerants will freeze your skin and R12 may turn into poisonous phosgene: a killing gas! Also airco systems need the right amounts of refrigerant and oil, which requires the right equipment.

Rebuilding the mechanical part of the system however may be done by yourself, but only when no chemical residues are left. Our point of departure here is an airco system which has been maintained like it should. Of course we realize a lot haven't. When condensers or hoses are leaking they should be replaced, but in itself this has nothing to do with a R134a retrofit!

Of course fitting new equipment like a condenser adds to the costs.

Literature on retrofitting airco's always mention theses items:

  • Vacuum system to remove and recycle R12 gas, then flush with special solvent to remove all traces of R12 lubricating oil
  • Change connections, to fill and vacuum the system, on the compressor
  • Change all hoses for neoprene hoses with a built-in gas barrier ([# #9, 10 in illustrations 6-641])
  • Hoses should have O-rings to seal the system
  • Change the receiver-dryer ([# # 4 in illustration 6-641])

In the past it was sometimes also suggested to rebuild the compressor and replace the condenser. Rebuilding the compressor was related to the need to remove all traces of the mineral lubricant used with R12. The new refrigerant R134a needs a synthetic ester type oil which is called PAG.

Replacement of the condenser ([# #5 in illustration 6-641]) is necessary only in very hot climates such as the United States desert southwest. Due to reduced thermodynamic efficiency of R134a versus R12, a more efficient condenser is required to reach the same performance as R12 systems. In less severe climates, the original condenser is adequate. Rebuilding the compressor is not necessary, however, it should be fully drained and flushed of all R12 lubricant.

  • EDITORS'S NOTE - Because of its high greenhouse potential, R134a has been phased out as a refrigerant in the EU since 2011 for new vehicles and will be phased out for all vehicles in 2017 in favor of HFO refrigerants such as HFO-1234yf (tetrafluoropropene) - except in Germany, where Daimler, Volkswagen, Porsche, Audi, and BMW have refused to adopt HFO-1234yf because of its potential inflammability and the possibility of releasing toxic hydrogen fluoride gas as a combustion product. These manufacturers have opted for their own carbon dioxide based refrigeration systems. HFO-1234yf has approximately the same thermodynamic efficiency as R134a so it will run with approximately the same condenser pressures as R134a. Anyone contemplating R134a retrofit should strongly consider HFO-1234yf as an alternative, in spite of the high cost of the refrigerant itself.

Jobs to be done

  • Vacuum the old system.
  • Fit new receiver-dryer.
    • R134a needs a receiver-dryer with a larger capacity.
    • Receiver-dryers by the way need replacing every 2-3 years.
  • Fit new connections on compressor to fill and empty system.
  • Check quality of hoses. When they are fine, you don't need to replace them according to Ger van Rootselaar "they are 'saturated' and won't leak." Others insist that the R12 oil will leech out of the hoses and contaminate the R134a regfrigerant so recomend installation of new barrier hoses.
  • Refill the system with R134a and PAG oil and check for leaks.

Retrofit with drop-in refrigerants HC12 and Freeze-12

HC12 (hydrocarbon blend B) is a mixture of propane and isobutane. Approximately 6 ounces of HC12 is sufficient to charge the air conditioning system of the SM. HC12 is actually a more efficient refrigerant than R12, so it operates at lower condenser pressures. This means there is less strain on the compressor and, therefore, less strain on the main timing chain. HC12 is compatible with R12 oil, so it can be used without flushing the system or changing components. Because HC12 is inflammable, however, it is not legal to use in many places, despite its obvious advantages and despite the fact that there is only a minimal amount of inflammable refrigerant present as compared with the many gallons of inflammable gasoline in the car. In the United States federal regulations make it illegal to use hydrocarbon blend B in mobile applications. In Australia, HC12 is widely used with few, if any, reported incidents of injuries relating to the use of inflammable refrigerants.

Another drop-in gas is Freeze-12, which is basically R134a mixed with an emulsifier to make it compatible with R12 lubricating oil. Because the primary constituent is R134a, the performance of Freeze-12 will be similar to R134a

Fit another compressor?

The York may or may not be responsible for catastrophic engine failures, but there is no dispute that the York (which is designed to be mounted to an engine block and, therefore, has no internal balancing) causes an unpleasant shaking of the accessory tray. The most common solution for this problem is to change to a 5 piston Sanden or Sankyo swash plate compressor instead of the reciprocating piston York. The SD-508 is available with a 6 inch diameter pulley (152mm rotor) which fit among other vehicles a 1984 Volvo 244DL. For R134a the equivalent compressor is a Sanden Model 4531. The 4531, like most R134a compressors does not have service ports, so you may have to have new refrigerant lines made up with the service ports in the hose fittings (or add service valves as shown below) or, if you have a friendly A/C parts supplier, they can swap the output head for one with service ports. The pulley from the model 4531 can be machined to accept the OE polyflex belt, which has a 60 degree vee. Below is a link to the drawing showing the modification. Any competent automotive machine shop should be able to do the job.

File:SM AC Pulley Modification.pdf

Note that if the polyflex belts are not aligned perfectly, they can become quite noisy to the point of sounding like someone is hammering on the HP pump. One solution is to use a pivot bracket such as a Nostalgic Air Parts Model 2500 bracket or the virtually identical Ranshu Model 5023 bracket (shown below) so that the compressor can be aligned with the belt removed. Once aligned it will stay aligned. Another trick is to hold a candle against the side of the belt while the engine is running. The wax deposited on the belt helps to quiet the hammering.

SD508 with pulley machined to accept polyflex belt. Compressor Mounted with Ranshu Model 5023 pivot bracket, Sanden Model 4531 with service ports and service valves added - John Titus installation

The more common 5 inch diamter SD-508 pulley cannot be machined to accept the polyflex belt, so you are left with the option of running a conventional 38 degree belt, which does not fit the 60 degree groove in the drive pulley properly, or a polyflex belt which does not fit the 38 degree groove in the A/C pulley. As an alternative to machining the A/C pulley, Mike Oberle marketplace:OOTFAB has produced a drive pulley for the HP pump with a 60 degree groove for the alternator belt and a 38 degree groove for the A/C belt.

To keep the authentic looks underneath the SM bonnet however Rootselaar is researching other ways, like changing the orifice size in the expansion valve on the evaporator. So less work has to be done by the compressor. Adaptations to the existing system may be available in the future. Contact Ger for further info.

Need Part numbers and supplier...

Sanden Compressor installed
Installation by Kerry Stiff

Sanden Compressor

  • Sanden 508
  • Note that compressor is ordered with the correct fitting type (R12 or R134a)
Sankyo Compressor installed

Depending on where you buy this compressor, it will be labelled as either a Sankyo or a Sanden: it is the same, but sold as a different brand in different countries!


  • Remus, T. and Chisenhall, J, How to air condition your car. MBI publishing, Osceola, 1993, ISBN 0-87938-765-3
  • Traister, J. Automotive air conditioning handbook, 2nd edition, Blue Ridge Summit, 1988, TAB. ISBN 0-8306-9050

-- Thanks to Ger van Rootselaar and Han Wandel. Originally migrated from

See also