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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.

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AREA

PROBLEM

POSSIBLE REASON

RECOMMENDED ACTION

CHECK

OIL PRESSURE

 Low oil pressure

 Partially clogged oil strainer Remove and clean.  √
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.  
 

No oil pressure,
or erratic low pressure
at control valve tap

 Low 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 .  

High main oil pressure

 Stuck pressure regulation piston in selector valve assembly. Remove selector valve assembly. Disassemble the valve and clean piston.  

Overheating

 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 .  

Excessive noise

 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   

No neutral

 Clutch plates warped Replace Clutch plates.  

Harsh engagement

 Steel ball in selector valve not seating properly. Remove orifice plate cover clean parts. Replace parts if necessary .  

CLUTCH

Clutch slips

 Improper adjustment Adjust Clutch.  

Clutch will not remain engaged

 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.  

Power Take-off continues
to drive when clutch is desengaged

 Warped or welded clutch plates Remove power take-off and replace clutch plates .  
Bearing failure Remove power take-off and replace clutch plates .  
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