What is a marine gearbox cooler and why does it need cleaning?

A marine gearbox cooler regulates transmission oil temperature using seawater. Over time, salt, debris, and marine growth can restrict flow, leading to overheating. Regular cleaning ensures proper cooling and protects the gearbox.

What problems can a clogged gearbox cooler cause?

A clogged cooler can cause high transmission temperatures, slipping gears, contaminated oil, and premature gearbox failure. Early cleaning helps avoid expensive transmission repairs.

What does gearbox cooler cleaning include?

The service includes removing the cooler, flushing internal passages, clearing salt and debris, inspecting fittings, and reinstalling the unit to restore proper cooling performance.

Why is pressure testing the gearbox cooler important?

Pressure testing checks for internal leaks that could allow seawater into the transmission oil. Detecting leaks early prevents oil contamination and serious gearbox damage.

How often should a gearbox cooler be cleaned and tested?

We recommend inspection and cleaning every 100–200 engine hours or annually, especially for boats operating in saltwater or warm environments.

Can this service extend the life of my marine transmission?

Yes, maintaining clean cooling passages and leak-free operation reduces heat stress and oil contamination, significantly extending gearbox and transmission lifespan.

Do you inspect transmission oil during the service?

Absolutely. We inspect transmission oil for signs of water intrusion or overheating and provide recommendations if fluid replacement or further service is needed.

Breeze Marine technician performing a hydrostatic pressure test

Marine Gearbox Cooler Cleaning & Pressure Test

SKU

2182

Rating:

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$150.00

Heat kills transmissions. We remove your gearbox oil cooler, ultrasonic clean and acid flush the raw water channels, and perform a hydrostatic pressure test to ensure that water and oil never mix.

Details

Transmission Protection

Cool Under
Pressure.

Your gearbox cooler is the most critical component in your drivetrain. Neglecting it leads to overheating or catastrophic transmission fluid contamination. We prove its integrity.

Heat Kills Clutches

Marine transmissions rely on friction plates. If your cooler is clogged with scale or debris, fluid temperatures spike, glazing the plates and leading to transmission slipping or total failure.

The "Strawberry Milkshake"

If the internal core corrodes, high-pressure raw water pushes into the low-pressure oil, turning your transmission fluid into a pink sludge that destroys the gearbox instantly.

Hydrostatic Testing

Cleaning isn't enough. After marine transmission cooler cleaning, we bench-test every cooler under pressure (PSI) to verify that the internal tubes are sound and that no cross-contamination is possible.

Bench Service Protocol.

From clogged and risky to clean and pressure-tested.

Remove & Inspect

We extract the cooler from the engine, capping hydraulic lines to prevent leaks, and inspect the end caps for corrosion.

Acid Flush

We circulate a descaling solution (like Barnacle Buster) through the raw water side to dissolve calcium, salt, and marine growth.

Pressure Test

We cap the water ports and pressurize the oil side (or vice versa) with air, submerging the unit to check for even the tiniest bubbles.

Paint & Install

Once passed, we prime and paint the exterior (if metal) and reinstall with new crush washers and hose clamps.

System Integrity.

A small service that saves thousands.

Pressure testing a marine gearbox oil cooler

Gearbox cooler core before and after cleaning

Reassembling cooler lines on a marine transmission

Gearboxes Serviced.

We handle cooling systems for all major marine transmissions, from a Velvet Drive cooler to ZF and Twin Disc units.

Velvet Drive 71 / 72 / 5000 Series

ZF Marine Hurth & ZF Geared Units

Twin Disc Large Capacity Coolers

Common Questions

How do I know if it's clogged?

You often won't know until you notice boat transmission overheating or poor shifting. If the cooler inlet hose is cold but the outlet is hot, flow is restricted.

Can you repair a leaking cooler?

Generally, no. If the internal copper/nickel tubes are perforated, the cooler must be replaced. A patch repair is too risky for the transmission.

Save Your Trans.

Don't wait for the "milkshake." Test it now.

Service My Cooler

Cooler Service vs Marine Transmission Repair

Many boat gearbox repairs trace back to the cooler: scale restricts water flow, the oil runs hot and the clutch plates glaze. A cooler that fails the pressure test can also let seawater into the oil. Cleaning and testing the cooler is a bench job, while marine transmission repair after overheating or water contamination means taking the gearbox apart. If shifting feels slow or the fluid looks darker or milky, have the cooler checked before the damage reaches the clutches.

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