Why Convertible Tops Fail

Hydraulic Cylinder Seal and Hose Failure:

Many of today’s convertible top automobiles have automated tops powered by hydraulic systems. Hydraulic cylinders, also known as rams, struts, or actuators, are one of the main components of these hydraulic top systems. Most convertible top malfunctions are caused by leaking rod seals in the convertible top hydraulic cylinders. Hydraulic fluid can also leak from failed seals in the end caps of certain cylinders and from aged or broken hydraulic hoses.

Hydraulic fluid leaks from the failing components and can impact the convertible top operation in two ways:

  1. Leaking hydraulic cylinders cannot build enough pressure to deliver the required force to open or close the convertible top.
  2. Hydraulic fluid level in the reservoir becomes too low and the pump is unable to supply fluid to the cylinders.

Assuming that the recommended hydraulic fluid is used, two factors can accelerate material failure:

  1. Hydraulic fluid contamination. Contamination comes from internal moving parts that wear, causing particles, mostly metal, to enter into the fluid. Water contamination is more prevalent in humid environments. Water decomposes the organic hydraulic fluid and the seals and corrodes the metal. Water molecules are picked up by the exposed cylinder rod and introduced into the fluid through the seal/rod interface. Water molecules also diffuse through the thin wall plastic hydraulic hoses. Small cylinders with long thin hoses do not exchange fluid with the reservoir very well, so the fluid stagnates and ages the seals. For these reasons, frequent hydraulic fluid changes are vital in maintaining a healthy hydraulic system.
  2. Heat. Heat reduces the shelf life of many compounds. Accelerated age studies are frequently conducted by introducing a known quantity of heat to predict the natural life span of a material. In many of these studies, higher heat results in a shorter life span. For convertible automobiles sitting out in the sun, heat radiates from the hot outer surfaces and into the often closed-in compartments where the hydraulic cylinders reside, consequently aging the seals. Hydraulic hose materials are affected by heat as well. For example, in the Jaguar XK8/XKR, Bentley Azure, and Rolls Royce Corniche, the most common hose failure occurs above the windshield. The sun heats up the confined space where the hydraulic locking mechanism resides, resulting in high temperatures.  With these hot conditions, the hose materials fail. As a result, the hoses blow out under operational pressure.

Visual Indicators of Hydraulic Cylinder Seal and Hose Failure:

Some of the visual signs of leaky cylinders and hoses are oil-stained headliners and oil drips from above the windshield in automobiles with hydraulic cylinder-actuated front top locks. Other indicators of leaking hydraulic cylinders are oil leaks from the front and back of the rear wheels. Look for oil spots on the driveway or garage floor.