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The chart below shows the areas boat owners have most common questions on when trying to troubleshoot why their Twin Disc ® marine transmission MG521 underperform. It categorizes areas of possible malfunction, likely cause and recommended action to bring your new, rebuilt or used Twin Disc ® marine transmission back to proper operation. The chart will give you confidence when speaking with your Twin Disc ® marine transmission mechanic or ordering parts. As a quick reference guide only, the chart must not be thought of as a substitute for the use of your Twin Disc ® marine transmission MG521 Owner's operating or service manual. AREA PROBLEM POSSIBLE REASON RECOMMENDED ACTION CHECK OIL PRESSURE Low oil pressure No oil pressure, High main oil pressure Overheating Excessive noise No neutral Harsh engagement CLUTCH Clutch slips Clutch will not remain engaged Power Take-off continues 
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Partially clogged oil strainer
Remove and clean.
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Stuck pressure regulation piston in selector valve assembly
Remove selector valve assembly. Disassemble the valve and clean piston.
Broken piston rings on clutches
Remove the manifold and dissemble the clutches. Replace piston rings.
Come-home setscrew loose or missing
Remove the some-home access pipe plugs. Tighten a loose come-home setscrew by turning counterclockwise, or replace setscrew.
Damaged or worn oil pump assembly.
Remove and replace oil pump .
Incorrect linkage installed on selector valve assembly
Adjust linkage so that selector valve stem is indexed properly by detent.
Clogged or plugged orifice in orifice plate of selector valve assembly
Remove orifice plate cover. Clean parts.
or erratic low pressure
at control valve tapLow oil level or empty sump.
Check gaskets and seals for leakage. Find parts causing leakage fill marine gear and replace them.
Fully clogged oil strainer
Remove and clean oil strainer.
Damaged or worn oil pump assembly.
Remove and replace oil pump .
Stuck pressure regulation piston in selector valve assembly.
Remove selector valve assembly. Disassemble the valve and clean piston.
Insufficient heat exchanger capacity.
Find a heat exchanger of sufficient capacity and replace it.
Insufficient cooling water flow.
Inside diameter of water lines too small. Replace lines with larger inside diameter hoses.
Clutches Slipping.
Low oil pressure will cause a clutch to slip. Adjust clutch.
Oil levels too high
Correct oil level.
Improper oil sump.
Drain marine gear, and fill with proper oil.
Clutch plates warped
Replace Clutch plates .
Worn splines between reverse driving and driven gears and clutch shafts
Remove marine gear, dissemble marine gear, and replace worn splined parts.
Bearing failure
Remove marine gear. Dissemble marine gear and replace damaged or worn parts
Worn or damaged rubber blocks
Remove marine gear and replace worn or damaged rubber blocks
Clutch plates warped
Replace Clutch plates.
Steel ball in selector valve not seating properly.
Remove orifice plate cover clean parts. Replace parts if necessary .
Improper adjustment
Adjust Clutch.
Clutch adjustment too tight
Adjust Clutch.
Improper remote engaging linkage.
Adjust linkage as required to assure proper travel of operating lever. Engaging force must be removed after complete engagement of the clutch.
to drive when clutch is desengagedWarped or welded clutch plates
Remove power take-off and replace clutch plates .
Bearing failure
Remove power take-off and replace clutch plates .
