Friday, April 10, 2015

Suction Throttle Valves and other vintage Rolls-Royce AC troubles

If you have a 1965-1976 Rolls-Royce or Bentley, and you have weak air conditioning the problem may be in your suction throttling valve.  If your air conditioning can’t be adjusted – whether it’s on weak or full blast – the suction throttle valve is likely at fault.  If your AC system won’t make normal pressures, and it’s full with a good compressor, the suction throttle valve or expansion valve are almost certainly stuck.

Both valves are located under the brake reservoir.  The suction throttle valve’s job is to regulate the amount of refrigerant delivered to the expansion valve. By doing so it regulates the capacity of the refrigeration system.

Look for the suction throttle and expansion valves on the left fender well, under the brake reservoir 
All the valves in the Shadow climate control system are operated by servo motors.  This one is no exception.  The servo pulls the valve open, and springs on the body of the valve pull it closed.  You can see those pieces in the photo of the valve, removed from the car.

Suction throttling valve with the operating servo attached, removed from car

Several things go wrong with the suction throttle valves.  All the problems are illustrated in this series of images, of a valve from a 1972 car.

Deformation of the valve body is obvious in this top view of the suction throttle valve
The first problem comes from the heavy springs used to pull the valve back to its resting position.  Over time, the strain from those springs warps the housing and when it warps enough, it jams.  That is a recipe for failure on most of these.  Albers sells a machined aluminum replacement that is less likely to deform in that manner, as shown in the photos.


A new machined housing next to the old suction throttle valve
When the housing warps the piston can't move in the housing.  If this state of affairs is sustained over several years or decades (as is often the case) the valve may be held quite tightly by corrosion.  We soak then in penetrant and knock them free.  The diaphragm on the other side is replaceable.






The next thing that happens is that the valves clog with debris.  The desiccant leaks from the receiver drier and small particles clog the screens on this valve and the expansion valve.  When the screens clog the air conditioner quits working, but not before the compressor has been ruined by trying to pull a vacuum too long (in most cases.)
Sediment from a clogged throttle valve
Clogging is a problem in every vintage Rolls.  In my opinion, the screens don't serve much purpose and you're better off taking them out.  If the screens are in place, the clog and the system fails.  The clogged part has to be found, removed, and cleaned or rebuilt.  Without screens, the valves might eventually clog, but it would take longer and it might never happen.  

Rebuilt Rolls-Royce suction throttling valve with attached servo
Suction throttle valves have not been used in cars for 40 years, and they are unknown to younger technicians.  But they are essential to the good functioning of your car's AC system.

Till next time
John Elder Robison


John Elder Robison is the general manager of J E Robison Service Company, independent Rolls-Royce and Bentley restoration and repair specialists in Springfield, Massachusetts.  John is a longtime technical consultant to the RROC and other car clubs, and he’s owned and restored many fine vehicles.  Find him online at www.robisonservice.com or in the real world at 413-785-1665

Reading this article will make you smarter, especially when it comes to car stuff.  So it's good for you.  But don't take that too far - printing and eating it will probably make you sick.


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