Mechanics may advise against changing transmission fluid, particularly in older vehicles with high mileage that have never had their transmission fluid serviced, because introducing fresh fluid can paradoxically cause the transmission to fail.
Here's a detailed explanation:
Why Changing Transmission Fluid Can Be Risky in Certain Cases
While regular transmission fluid changes are crucial for the longevity of a transmission, there's a specific scenario where mechanics might caution against it: when the fluid is very old, highly degraded, and has never been replaced.
The Role of Degraded Fluid
Over time and thousands of miles, transmission fluid degrades. It loses its lubricating properties, its detergents break down, and it accumulates tiny metal particles from the normal wear and tear of internal components like clutches and gears.
- "Sticky" Qualities: This old, degraded fluid can develop a "sticky" quality.
- Metal Flecks: Accumulations of tiny metal flecks from wear are suspended within it.
- Aiding Friction: Surprisingly, in a transmission that's already experiencing significant internal wear, these sticky qualities and metal flecks can act to aid friction. They might help the worn clutch packs and bands to grip, compensating for their diminished original friction material.
The Risk of New Fluid
When this old, "sticky," and metal-laden fluid is replaced with fresh, clean fluid, it can strip away these compensating deposits and introduce fluid with different friction characteristics.
- Washing Away Deposits: The new fluid, designed for optimal performance in a healthy transmission, might wash away the beneficial "gunk" that was helping the worn components engage.
- Exposing Wear: Without the aid of the old fluid's altered properties, the underlying wear of the clutch plates or bands can be fully exposed.
- Causing Slipping: This can lead to the transmission starting to slip, shudder, or even fail completely, as the worn components can no longer create enough friction to engage properly with the new, clean fluid.
When This Advice Applies
This cautionary advice primarily applies to:
- Neglected Transmissions: Vehicles where the transmission fluid has gone well beyond its recommended service interval (e.g., 100,000+ miles with original fluid).
- Already Showing Symptoms: Transmissions that are already exhibiting minor issues like delayed shifts or rough engagement, where the old fluid might be subtly masking bigger problems.
Transmission Fluid Maintenance: A Balanced Perspective
It's crucial to understand that this advice does not mean you should never change your transmission fluid. Regular, preventative maintenance is vital.
Best Practices for Transmission Fluid:
- Follow Manufacturer Recommendations: Adhere to your vehicle manufacturer's recommended service intervals for transmission fluid changes, typically every 30,000 to 60,000 miles, depending on the vehicle and driving conditions.
- Regular Fluid Checks: Periodically check your transmission fluid level and condition. It should be red or dark red, clear, and not smell burnt.
- Preventative vs. Reactive: Change the fluid as preventative maintenance before it becomes heavily degraded and causes issues.
In summary, mechanics advise caution with changing transmission fluid in very old, neglected systems because the worn components may have become reliant on the "sticky" nature and metal particles in the old fluid to maintain friction. Replacing it with fresh fluid can remove this artificial aid, potentially leading to immediate slipping or failure.
