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Troubleshooting Guide

Troubleshooting Guide

Troubleshooting Checklist for Twin Disc MG520 Marine Transmission

The chart below shows the areas boat owners have most common questions on when trying to troubleshoot why their Twin Disc ® marine transmission MG520 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 MG520 Owner's operating or service manual.  
For your convinience in looking up parts and pricing for your marine transmission please click on the following link click here. You can also call us at 1-888-433-4735 or 305-545-5588.
Print out our checklist and post it in your boat engine room or Marine work station.







 Low oil pressure

Partially clogged oil strainerRemove and clean. 
Stuck pressure regulation piston in selector valve assemblyRemove selector valve assembly. Disassemble the valve and clean piston. 
Broken piston rings on clutchesRemove the manifold and dissemble the clutches. Replace piston rings. 
Come-home setscrew loose or missingRemove 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 assemblyAdjust linkage so that selector valve stem is indexed properly by detent. 
Clogged or plugged orifice in orifice plate of selector valve assemblyRemove 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 strainerRemove 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. 


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 highCorrect oil level. 
Improper oil sump.Drain marine gear, and fill with proper oil. 
Clutch plates warpedReplace Clutch plates . 

Excessive noise

Worn splines between reverse driving and driven gears and clutch shaftsRemove marine gear, dissemble marine gear, and replace worn splined parts. 
Bearing failureRemove marine gear. Dissemble marine gear and replace damaged or worn parts  
Worn or damaged rubber blocksRemove marine gear and replace worn or damaged rubber blocks  

No neutral

Clutch plates warpedReplace Clutch plates. 

Harsh engagement

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


Clutch slips

Improper adjustmentAdjust Clutch. 

Clutch will not remain engaged

Clutch adjustment too tightAdjust 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 platesRemove power take-off and replace clutch plates . 
Bearing failureRemove power take-off and replace clutch plates . 

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