Reverse-Engineered and Rebuilt: Joakim Karlsson Recreates the Saab 900ng / 9-3og ACC Fan Controller

The ACC Fan Controller: A Known Weak Link in 1994–2002 Saabs

Owners of the Saab 900ng (1994–1998) and Saab 9-3og (1998–2002) equipped with ACC are already familiar with the pattern: erratic radiator fan behavior, intermittent operation, or complete failure. Diagnostic routines often confirm what experience has already suggested – the fan control module mounted in the shroud becomes unreliable after two decades of heat cycles.

The original controller integrates power electronics and filtering components on a compact PCB housed in an aluminum heat sink assembly. The location exposes it to sustained thermal stress. Over time, solder joints fatigue, components drift out of tolerance, and filtering circuits degrade.

The failure mode is not random. It is structural. And for years, the only realistic solution has been used modules of unknown condition, partial refurbishments, or improvised transistor swaps.

Joakim Karlsson, known in Saab tuning and electronics circles through roffe.nu and txlogger.com, has now completed a full reverse engineering of the NG900/OG9-3 ACC fan controller – from schematics to PCB layout – and is preparing production units.

This is not a patch repair. It is a recreated ACC controller board.

Reverse Engineering the OG9-3 Variant – and Why That Matters

Karlsson did not replicate the earliest NG900 version directly. Instead, he reverse engineered the later OG9-3 controller variant.

The reason is technical. The OG9-3 module incorporates additional signal filtering components compared to the original NG900 design. Saab introduced these refinements to stabilize control signals and reduce electrical noise within the ACC system.

Rather than simplifying the circuit to the lowest common denominator, Karlsson chose the more complex version as the foundation. The result:

• A universal PCB design
• Compatible with both NG900 and OG9-3
• Includes solder provision for the NG900 blue wire (BL) used for blower motor current draw monitoring.

The universal PCB includes a solder point for the NG900 blue wire (BL), which is used by the ACC system to monitor blower motor current draw as part of its protection and diagnostic strategy.

This approach eliminates the need for separate board variants and reflects an understanding of Saab’s incremental electrical revisions during the late 1990s. The decision is engineering-driven, not commercial.

Designed for Refurbishment – Not Reinventing the Housing

The reproduction board does not attempt to replicate the full mechanical assembly. The original housing, heat sink, and cable with connector must be reused. That is intentional.

The aluminum heat sink assembly used by Saab was dimensioned specifically for the fan’s current handling and thermal load. Replacing it with a generic enclosure would introduce unknown variables in thermal dissipation.

Karlsson’s solution focuses strictly on the PCB – the most failure-prone element – while retaining:

• OEM housing
• OEM heat sink
• Original connector harness

Two service paths are planned:

1. Refurb kit – For technically competent owners who can desolder and reassemble.
2. Mail-in replacement service – Owners send their module; the PCB is replaced and returned.

This structure acknowledges the Saab community’s diversity: some prefer bench-level repair; others require turnkey service.

Why This Controller Fails – and Why Used Units Are Not a Solution

The ACC fan controller operates in a high-thermal, high-current environment. Over time, repeated heating and cooling cycles cause:

• Solder joint fatigue
• Component drift in filtering circuits
• Semiconductor degradation

Many “working” used modules are simply closer to failure. The root issue is not mileage. It is time and heat exposure. A proper solution requires:

• New PCB substrate
• New components
• Clean soldering
• Verified filtering layout

Karlsson’s board addresses exactly that – by recreating the circuit rather than repairing aged traces. For restorers aiming at long-term reliability rather than short-term operability, this distinction is critical.

Not a One-Off Project: A Pattern of Saab-Specific Engineering

Karlsson’s ACC controller reproduction is consistent with his broader body of Saab-focused technical work.

TXLogger – Trionic-Level Control and Logging

Through txlogger.com, he developed tuning and logging tools for:

• Trionic 5
• Trionic 7
• Trionic 8

The system supports multiple CAN bus adapters and provides data logging and calibration capabilities beyond factory tools. It is widely used in performance and calibration circles for Saab turbo engines.

This background explains his ability to accurately reconstruct signal filtering and power circuits. Reverse engineering Saab electronics is not new territory for him.

Saab 9-3 2003-2012 Halogen Check Disabler for LED conversions

Via Hirschmann-Koxha, he provides a solution to disable halogen bulb checks in NG9-3 models.

The issue is well known: LED conversions trigger bulb-out warnings and visible flicker due to Saab’s monitoring pulses. His hardware/software tool disables the check logic without introducing resistive load hacks. It addresses the system at the control level rather than masking symptoms.

Again, the theme is consistent: correct the logic, not the surface behavior.

CIM Tool Nano – Reusing NG9-3 CIM Units

Another development is the CIM Tool Nano, designed to “virginize” NG9-3 Column Integration Modules without requiring original keys.

For NG9-3 owners and independent workshops, this is significant. CIM failures are expensive and often lead to scrapping otherwise viable vehicles due to immobilizer pairing complexity.

By allowing modules to be reset and reused, Karlsson’s tool directly reduces part obsolescence pressure within the Saab ecosystem.

The Broader Context: Saab Electronic Obsolescence

As Saab moves further into post-production decades, mechanical components are still reproducible. Electronics are not.

Specific challenges include:

• Custom Saab firmware
• Proprietary communication protocols
• Limited component documentation
• Heat-stressed automotive PCBs

Official suppliers are gone. Remaining stock is finite. Used modules degrade. Projects like this ACC controller reproduction represent a structural shift in Saab preservation: Not salvaging, but rebuilding.

It mirrors similar efforts in the community:

• SID display refurbishments
• ABS module rebuild programs
• T7 ECU repairs
• SAHR seat mechanisms reproduction

Each addresses a known failure node. The ACC fan controller belongs in that category.

Engineering Integrity vs. Aftermarket Simplification

What distinguishes this project is restraint. Karlsson did not redesign the control logic. He did not attempt to “improve” fan behavior. He did not modify Saab’s thermal thresholds.

He replicated the original logic – including the more robust OG9-3 filtering stage – while modernizing the PCB manufacturing process. That distinction matters. Many aftermarket solutions introduce undocumented behavioral changes. This one does not. It restores the controller to OEM-equivalent functionality.

Implications for NG900 and OG9-3 Collectors

The NG900 and OG9-3 occupy a transitional position in Saab history:

• First GM-era platforms
• Still heavily Saab-engineered mechanically
• Increasingly recognized as analog-era daily drivers

However, electronic parts scarcity is already affecting usability. An inoperative fan controller in an ACC-equipped car is not cosmetic. It compromises cooling logic and climate system functionality. For collectors maintaining low-mileage examples, having a reproducible, verified PCB solution:

• Preserves drivability
• Reduces dependence on used modules
• Extends service life predictability

For workshops specializing in 1990s Saabs, this enables stock-level repair rather than part cannibalization.

Current Status: PCB Production in China

Karlsson has completed:

• Full schematic reconstruction
• PCB layout design
• Universal board adaptation

The first production batch is currently in manufacturing in China. If validation testing confirms electrical stability and fitment, commercial availability will follow. No speculative release dates have been announced. That cautious approach aligns with the engineering profile of the project: test first, release second.

A Quiet but Structural Contribution

There is no spectacle in a fan controller PCB. It does not change horsepower figures. It does not alter exterior appearance. It does not modernize infotainment. Yet it solves a defined and recurring failure in two entire Saab model generations. In the long-term survival of NG900 and OG9-3 cars, these quiet components matter more than cosmetic upgrades.

Joakim Karlsson’s work sits within a growing class of Saab specialists who are not restoring nostalgia but reconstructing functionality at circuit level. That distinction defines the next phase of Saab preservation. And it is increasingly being built not in factories – but on independent workbenches.