Gm Mode 22 Scan Tool By Terry -

Terry’s classic case: “Customer says RPM jumps 200-300 RPM on the highway. No codes.”

Step 1 – Connect & Enter Enhanced Mode
On your scanner, select “GM Enhanced” or “Manufacturer Specific.” Choose the PCM (engine/transmission module).

Step 2 – Select Mode 22 PIDs
Do not use “All Data.” Hand-pick:

Step 3 – Road Test with Graphing
Drive at steady 50-55 mph in 4th gear. Command TCC lockup via scanner (bi-directional function). Watch the TCC Slip PID.

Step 4 – Verify with Gear Ratio
Using Input Shaft RPM / Output Shaft RPM:

“That’s the power of Mode 22,” says Terry. “Generic data would show RPM and VSS, but you’d never see the slip. You’d replace the torque converter unnecessarily.”

This is where Terry’s tool beats any generic scanner. You can command:

Example 1 – Rough idle on Duramax

Example 2 – Poor boost / turbo lag

Example 3 – Transmission slipping

Pros

Cons

Once you have the software and cable, follow this guide:

Step 1 – Locate the ALDL Port: Find the 12-pin connector. On most F-bodies and Corvettes, it is under the dash left of the steering column. On trucks, it may be above the gas pedal.

Step 2 – Connect the Cable: Plug your USB ALDL cable into the port. Pins A and B (top two rightmost pins) are typically the ground and data lines, but quality cables auto-detect.

Step 3 – Install Drivers (Windows only): Terry’s original tool was written for Windows 95/98/XP. However, modern versions or community updates run on Windows 10/11 in compatibility mode. Install the FTDI or Prolific drivers for your USB-to-serial adapter.

Step 4 – Launch the Software: Open “GM Mode 22 Scanner by Terry.exe”. Set the correct COM port and baud rate (8192).

Step 5 – Power Cycle: Turn the ignition to ON (engine off for sensor checks) or start the engine. Click “Connect” or “Scan Mode 22”.

Step 6 – Interpret Data: The software will populate fields. Pay special attention to:

[Screen: HVAC Smart Mode]

LIVE:

CONTROLS:

STATUS: Ready to Command.

This feature transforms the tool from a "code reader" into a "repair tool," perfectly aligning with the persona of Terry.

The GM Mode 22 Scan Tool by Terry is a specialized freeware diagnostic utility designed primarily for Windows and legacy mobile platforms. It is frequently bundled as a complementary software option for ELM327-based Bluetooth and USB scan tools. Key Features and Capabilities

While modern professional scanners like the FOXWELL NT510 Elite offer comprehensive bi-directional controls, this tool serves as a lightweight alternative for specific GM diagnostics:

Platform Support: The software is compatible with Windows, OSX, and older mobile operating systems such as PalmOS and Pocket PC.

Enhanced Mode 22 Access: It is designed to utilize the "Mode 22" diagnostic request, a proprietary GM protocol used to retrieve specific data parameters (PIDs) that standard generic OBD-II Mode 01 requests may not cover.

Core Diagnostic Functions: Like most ELM327-compatible software, it allows users to: Read and clear diagnostic trouble codes (DTCs). Monitor real-time engine data and sensor values. Detect specific system pressures and status indicators.

Portability: Due to its small file size (approx. 1.49Mb), it is easy to install and deploy on older hardware. Limitations ELM327 Software - Interfuse LLC.

In the mid-2000s, the "GM Mode 22 Scan Tool by Terry" was a small but legendary piece of the early OBD-II hobbyist scene. At the time, General Motors used a proprietary diagnostic method called Mode 22, which allowed for deeper data extraction than the standard OBD-II protocols required by law. gm mode 22 scan tool by terry

While professional dealership tools like the GM Tech2 cost thousands of dollars, a developer known simply as Terry released a free, lightweight software utility that could "talk" to GM vehicles using this specialized mode. The Story of a DIY Legend

Terry’s tool was born out of the "Wild West" era of car hacking. During this time, enthusiasts were transitioning from the old ALDL systems of the '80s to the more complex computer networks of modern vehicles.

The Problem: Standard scanners could only see "Generic" data like engine RPM or basic trouble codes. To see "Enhanced" data—like individual wheel speeds for ABS or transmission oil temperature—you needed a tool that understood GM's proprietary Mode $22.

Terry's Solution: Terry wrote a program that ran on simple PC-based interfaces (like the ELM327). It was one of the first times a regular car owner could plug in a cheap laptop and see the same high-level data a dealership mechanic saw.

The Legacy: Terry's tool became a staple on forums like Scantool.net and early car-tuning communities. It paved the way for modern, affordable bi-directional scanners from brands like Launch and Autel that we use today.

Though largely superseded by modern apps and GDS2 software, "GM Mode 22 Scan Tool by Terry" remains a nostalgic name for anyone who spent their weekends in a garage with a bulky laptop and a dream of fixing their own Chevy or Buick.

Unlike Mode 01, Mode 22 uses two-byte hexadecimal PIDs. Here are Terry’s “most-saved-my-butt” PIDs for a typical V8 truck (P01/P59 PCM):

| PID Hex | Description | Why It Matters | | --- | --- | --- | | 0x1654 | Transmission Turbine Shaft Speed | Detects TCC slip or internal clutch failure | | 0x1655 | Transmission Output Shaft Speed | Calculate true gear ratio | | 0x1668 | Line Pressure (PSI) | Low pressure = worn pump or cross leaks | | 0x16B0 | Knock Retard (degrees) | False knock from loose converters vs real detonation | | 0x16C1 | Left & Right Fuel Trim Cell | Long-term cell correction (not seen in Mode 01) | | 0x16D3 | EGR Position Sensor (actual vs commanded) | Sticky EGR valves |

While Terry’s original tool remains beloved for its simplicity and low cost (often free), modern alternatives exist:

| Tool | Protocol | Price | Best For | |------|----------|-------|-----------| | GM Mode 22 by Terry (legacy) | ALDL Mode 22 | Free | Budget, simple diagnostics | | Scan9495 | ALDL Mode 22 | $30 shareware | LT1/LT4 specific (94-95) | | TunerPro RT | ALDL + OBD-II | Free (donation) | Advanced tuning & logging | | ALDLLogger | ALDL Mode 22 | Free | Mac/Linux users | | Tech 1 w/ GM cartridge | All GM modes | $500+ used | Professional restoration | Terry’s classic case: “Customer says RPM jumps 200-300

TunerPro RT is the most powerful modern equivalent. It uses the same Mode 22 principle but adds a visual dashboard, custom definition files (XDFs), and live checksums. However, many old-school mechanics still reach for Terry’s tool because it requires no configuration—plug and play for most $8D and $EE masks.