When evaluating the efficiency of an automatic gearbox in a four-wheel drive SUV with a large engine, a professional mechanic follows a systematic and thorough approach. Here are the detailed steps involved in the evaluation, including the rationale for each step and what specific findings may indicate.

Step 1: Preliminary Visual Inspection

What to Do: Start by visually inspecting the vehicle, focusing on the transmission area. Look for any obvious signs of damage, leaks, or wear.

Know How:

• Check for fluid leaks around the transmission pan, cooler lines, and seals. A bright red or brown fluid can indicate various issues.
• Look for any physical damage to the transmission casing; cracks or dents can lead to failure.

Findings and Meanings:

• Leaks suggest that the seals or gaskets might be faulty, which can lead to a drop in fluid pressure, impairing performance.
• Visible damage can indicate prior problems that need further investigation.

Step 2: Check Transmission Fluid Level and Condition

What to Do: Ensure that the transmission is at the correct fluid level. Check the fluid’s color and smell.

Know How:

• With the engine running and the vehicle in park, use the dipstick to check fluid levels. The fluid should be within the recommended range.
• Inspect the fluid’s color; fresh fluid is typically red, while burnt fluid will appear brown or dark.

Findings and Meanings:

• Low fluid levels can lead to slipping and overheating. If levels are low, it may be due to leaks or consumption.
• Burnt or dark fluid indicates overheating or contamination, suggesting that a transmission service may be required.

Step 3: Transmission Function Test

What to Do: Conduct an operational test of the transmission by driving the vehicle.

Know How:

• While driving, evaluate how the transmission shifts through each gear. Accelerate smoothly and listen for any unusual sounds.
• Note if the vehicle hesitates during gear shifts or if it shifts roughly.

Findings and Meanings:

• Smooth shifts indicate a healthy transmission, whereas hesitations or hard shifts might suggest a problem with the valve body or solenoids.
• Unusual sounds like grinding or clunking while shifting indicate potential internal wear or damage.

Step 4: Diagnostic Scan

What to Do: Use a diagnostic scanner to check for any stored trouble codes related to the transmission.

Know How:

• Plug the scanner into the vehicle’s OBD-II port and retrieve any diagnostic trouble codes related to the transmission control module (TCM).
• Pay special attention to codes regarding shift solenoids, speed sensors, and transmission overheating.

Findings and Meanings:

• Codes pointing to solenoid failures suggest issues with gear engagement, while speed sensor errors might affect shift timing.
• Overheating codes could indicate a problem with the cooling system or low fluid levels.

Step 5: Perform Road Test Under Varying Conditions

What to Do: Take the vehicle on a road test that simulates various driving conditions, including stop-and-go traffic, highway driving, and hill climbing.

Know How:

• Evaluate the transmission’s responsiveness and smoothness under different speeds and loads, including engaging in 4WD.
• Note any slipping, delayed engagement, or unusual noises while driving uphill or during acceleration.

Findings and Meanings:

• Inconsistent performance during demanding conditions indicates potential wear in the clutches or bands.
• If the transmission struggles under load, it may signify impending failure or inadequate cooling.

Step 6: Check Transmission Temperature

What to Do: Assess the operating temperature of the transmission during the road test.

Know How:

• Use a transmission temperature gauge if available, or monitor for signs of overheating, like fluid discoloration or smells.
• The optimal operating temperature typically ranges between 175 to 220 degrees Fahrenheit.

Findings and Meanings:

• An overheated transmission could lead to severe damage and indicates the need for immediate service.
• Consistent high temperatures can be attributed to low fluid levels or issues with the cooling system.

Step 7: Inspect Transmission Filter and Pan

What to Do: If accessible, remove the transmission pan to check for debris and inspect the filter.

Know How:

• Remove the transmission pan bolts and carefully lower the pan to avoid spilling fluid everywhere.
• Inspect the filter for clogs and assess the amount of metal shavings or clutch material in the pan.

Findings and Meanings:

• A clean filter suggests good maintenance, while heavy debris may indicate internal wear or failure.
• Excessive metal shavings can signal serious internal damage, necessitating a more detailed inspection or rebuild.

Step 8: Fluid Pressure Test

What to Do: Conduct a fluid pressure test using a pressure gauge.

Know How:

• Install the pressure gauge into the transmission test port to measure the line pressure during various operational conditions, including idle and acceleration.
• Compare the values against manufacturer specifications.

Findings and Meanings:

• Low pressure readings indicate problems with the pump or leaks in the system, affecting shifting performance.
• Extremely high pressure may point to blockage or issues with shift solenoids.

Step 9: Inspect Electrical Connections and Wiring

What to Do: Examine the wiring and connectors to the transmission control unit and other related components.

Know How:

• Look for any frayed wires, loose connections, or signs of corrosion, particularly in high-moisture environments.
• Ensure that all connectors are clean, secure, and free from dirt.

Findings and Meanings:

• Poor connections can lead to erratic shifting or fault codes indicating malfunction.
• Corrosion can hinder signal transmission and may require replacement of connectors or wiring.

Step 10: Visual Inspection of Underlying Components

What to Do: Inspect surrounding components such as the transfer case, driveshafts, and axles for related issues.

Know How:

• Look for play in the driveshafts and inspect for unusual wear patterns.
• Check the transfer case fluid levels and condition as well.

Findings and Meanings:

• Excessive play in the driveshaft can result in poor shifting and may indicate wear or bearing failure.
• Issues in the transfer case can also lead to performance problems in the drivetrain.

In conclusion, a thorough evaluation of an automatic gearbox in a 4WD SUV involves systematic steps that encompass visual inspections, operational tests, and diagnostic assessments aimed at determining the health and efficiency of the transmission system. Each step provides vital information that guides the mechanic in diagnosing potential issues, ensuring optimal performance and reliability.