Air Conditioning, Uncategorized

Turn Of The Screw – Adjusting Tecumseh Or York Compressor Service Valves On Your Car’s AC

Working with vintage auto air conditioning systems is a labor of love for us and we like to share tips and tricks-of-the-trade for maintaining these old systems.

A recent customer of ours purchased a compressor and installed it on his 70 Ford Maverick and brought the car to his a/c mechanic to have the compressor charged with refrigerant.  Shortly after, he called us in a panic when the pressure relief valve on the Maverick’s compressor opened during the charging of the air-conditioning system, spraying oil & refrigerant throughout the car’s engine compartment.

Our customer and his a/c mechanic were initially upset that we sent a defectively rebuilt product, suspecting a faulty pressure relief valve. The valve is designed to vent high pressures whenever there is a severe spike in pressure or blockage in the air-conditioning system, in an effort to protect against major component failure.  (This relief valve system was used long before various types of pressure cut-off switches were automatically installed in factory air-conditioning systems; back when the old R12 refrigerant cost about 50 cents a can.)

As soon as a Classic Auto Air troubleshooting technician spoke with our customer’s A/C mechanic, we realized he had unknowingly left one of the service valves in the front-seated (off) position, (see figure 1.) This position is incorrect for evacuating & charging a system.  This caused a blockage in the system, which forced the pressure relief valve to do what it was designed to do – blow.

Since service valves have not been in regular use for approximately 30 years, many certified A/C technicians have never worked on an air conditioning system that has service valves. Even technicians who have seen these older systems are out of practice, so let’s take a moment here to give some inside scoop on a compressor service valve system.

There are two valves on most York or Tecumseh compressors. One on the suction “low” side and one on the discharge “high” side of the compressor.  (The system should always be charged on the SUCTION side.)

Compressor Service Valve 1

Figure 1: Overview of York / Tecumseh Compressor

Although the car in question was a 1970 Ford Maverick, these rules apply to most Ford and Mercury vehicles using a York or Tecumseh style compressor from the mid 1950’s and into the early 1980’s, and some late 50’s to early 60’s Mopar products.

Once the system is ready to be charged, the valve must be adjusted to the center seated position.  (See figure 2)  This position allows refrigerant to flow past the valves and through the service port. Set to this position, pressure gauge readings can be monitored and the system can be properly evacuated & charged.  The best way to achieve the proper center seated position is to turn the valve clockwise until it stops in the front-seated position, then counterclockwise approximately 4 complete revolutions.

Compressor Service Valve 3

Figure 2: Compressor Valve Set For Charging Or Evacuation

Once the system has been evacuated and properly charged, the valve should be back-seated for normal operating conditions.  (See figure 3)  This allows for full flow through the valves and blocks off the service valve port to prevent refrigerant loss.  (A service valve cap is also used to further insure against leaks.)  This setting is achieved by turning the valve counterclockwise until it stops.

Figure 3: Compressor Valve in Normal (Back-Seated) Position

Figure 3: Compressor Valve in Normal (Back-Seated) Position

A final tip and illustration related to these vintage auto a/c compressors; the valve should be slightly opened in a “cracked” position when checking system pressure readings.  (See figure 4)

Figure 4: Compressor Gauge Reading Valve Position "Cracked" Open

Figure 4: Compressor Gauge Reading Valve Position “Cracked” Open

Here at Classic Auto Air’s Original Air Group, we rebuild and refurbish these York & Tecumseh compressors (as well as many nearly every other part in the air-conditioning system of most Ford, GM & Mopars from the 1980’s & older) on a regular basis.  And by the way, we always include a set of instructions that include the diagrams shown above – to help educate an installer or classic car owner in how to properly set the service relief valves.)  If you need a compressor or any other part for your classic car rebuilt, or even just some advice on the proper way to charge the system, give us a call. We’re here to help.



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5 thoughts on “Turn Of The Screw – Adjusting Tecumseh Or York Compressor Service Valves On Your Car’s AC

  1. Charles Davidson says:

    I have a 68 Ranchero…I tried to charge the system(didn’t cool)It too a little freon..very little…I opened the valve to charge…and closed when finished..I dould not tell any cooling..The compressor was working..Its on a toogle is factory air….what next???

  2. Jörg Zimmermann says:

    I search for a 1967 chrysler 300 a original York Compressor please quote

    1. Steven R says:


      Thanks for the email.I have a Original Air group in Tampa, Florida that would be able to help you out with that.
      If you like please give them a call at 877-371-0570 and they will be more than happy to help you.

  3. Richard Peacock says:

    I have a 1963 Thunderbird Roadster, can’t open either service valve of the York air cond system, tried 1/4 socket,vice grips, nut driver they seem frozen wi)l post picture later today

    1. Steven R says:

      Hello Richard

      I have a factory air division in Florida that would actually be able to help you out with that. Here at the Texas location a only handle
      complete new aftermarket systems. Here is their number if you would like to give them a call 877-371-0570. Thanks again

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