A Detailed Overview of Porsche 911, 964, and 993 Electric Convertible

In this blog, we provide a detailed explanation and description of the operation and logic behind the electric convertible tops in Porsche 911, 964, and 993 models.

Introduction to Porsche Electric Convertible Tops

In 1987, Porsche introduced its first fully motorized electric convertible roof, utilizing durable and waterproof canvas. Before this, earlier versions required the operator to step outside the vehicle to manually open or close the top, making it both physically demanding and inconvenient.

The first generation of the electric convertible roof was installed in the 1987 Carrera 3.2 and the 1989 911 Turbo. From this early version through to the last of the 993 models in 1998, the basic concept and mechanical parts remained largely unchanged.

How the Electric Convertible Top System Works

For the electric convertible top to function, a Top Control Unit was essential. This electronic unit would register inputs from the driver and determine the position of the top to safely open and close it. The system was designed with key checkpoints at every stage of operation to ensure the integrity of the roof’s mechanical structure.

The system featured a push-button switch located on the dashboard beneath the gauges. By pressing the switch forward or backward, the driver could initiate the opening or closing of the convertible top.

A light on the dashboard illuminated to acknowledge that the top was in operation.

Mechanical Components and Operation

The electric components of the system included a main motor. This motor was responsible for spinning a cable that powered the movement of the crank gearbox, which in turn moved the top forward or backward.

There was a designated Home Position microswitch that registered when the top was fully open. The role of this microswitch was to inform the Top Control Unit to stop the motor, thereby preventing mechanical damage to the roof system.

Additionally, two other microswitches were installed on the top frame bar of the roof. These switches were activated when the top approached the closed position, where the frame bar met the window frame. These microswitches signaled the control unit to stop the forward movement of the top to avoid overextension.

The Locking System and Microswitches

The front frame bar also housed two locking motors. These motors were designed to secure the frame bar to the window frame, completing the locking process of the top. By securing the front frame bar, the motors ensured a tight seal around the rubber seals, preventing water from leaking into the cabin when the top was closed.

One of the most crucial components in this system is the Limit Microswitch, which served as a safety feature. If the front microswitches failed to register the closed position, the limit microswitch would shut down the main motor, preventing further movement. This feature was crucial to avoiding serious damage to the top’s canvas or mechanical parts.

Safety Features in Different Generations

In the 911 Carrera and 911 Turbo models, the system was designed with the car’s oil pressure in mind. Porsche intended for the top to be operated without the engine running, with the ignition switch in the "Radio Contact" position (one step before the ignition or accessory position).

In the 964 generation (1989-1993), Porsche introduced the ability to operate the top while the engine was running. However, the system required recognition of the parking brake signal and speed signal, preventing the top from being operated while the vehicle was in motion.

The 993 generation (1994-1998) followed similar safety protocols, allowing the top to operate with the engine running but only if the parking brake was engaged.

The Evolution of Top Control Units

Over the 11 years and three vehicle generations, Porsche developed three distinct part numbers for the Top Control Units, each one tailored to the specific chassis and wiring harness of the car:

• 911.618.313.00: Compatible only with 911 Carrera and Turbo models
• 911.618.313.01: Designed specifically for 964 models
• 993.618.313.00: Works exclusively with 993 models

Each generation of Top Control Units had unique input and signal requirements, making these units non-interchangeable. Porsche no longer sells the parts numbered 911.618.313.00 or 911.618.313.01 and now only offers 993.618.313.00. Installing this part in earlier 911 or 964 models requires modifications to the wiring harness, adding time and complexity to the repair process.

Challenges of Original Control Units and Our Solution

The original Top Control Units were designed using 1980s technology, and their basic design was never updated. As a result, they became prone to failure after several years of operation due to various factors:

• High electrical current demands from the motors
• Environmental influences, including high heat and humidity
• Vehicle vibrations transferring to the control unit
• Voltage fluctuations from the battery, particularly in early 911 models

These conditions made the original units unreliable over time, leading to frequent internal electronic failures.

At Specialized ECU Repair, we have redesigned and upgraded the Top Control Unit to provide improved reliability and longevity. Our upgraded units are designed to be drop-in replacements, requiring no modifications to the wiring harness, making installation much easier and hassle-free.

Tips for Maintaining the Convertible Top

It’s important to note that if your convertible top canvas needs replacing, it’s essential to find a reputable and experienced technician. Incorrect calibration of key components like the tension, height, throw, or microswitch placement can cause significant problems and make restoring the system to proper operation challenging.

Additionally, after any significant work, the convertible top system must be reset to its Home Position for the Top Control Unit to function correctly. This is a commonly overlooked step that can lead to frustrating diagnosis issues.

Conclusion

As with any complex mechanical and electronic system, understanding how the electric convertible top works and adhering to proper maintenance protocols is critical to ensuring its longevity and reliability.