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The chart below shows the areas boat owners have most common questions on when trying to troubleshoot why their Allison marine transmission MH, models MH20, MH20L, MH20R, MH45, MH45L and MH45R 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 Allison 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 Allison marine transmission Allison marine transmission MH,models MH20, MH20L, MH20R, MH45, MH45L and MH45R Owner's operating or service manual.

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AREA

PROBLEM

POSSIBLE REASON

RECOMMENDED ACTION

 CHECK

POWER

A. No power is transmitted when selector is in forward position.

 Worn forward clutch facings Replace clutch plate.  
Damaged forward clutch plate Replace clutch plate.  
Improper dump valve operation Clear dump valve thoroughly and replace damaged parts.  
Low oil pressure Refer to D, E, or F.  

B. No power is transmitted when selector is in reverse position.

 Worn reverse clutch facings. Replace clutch plate.  
Damaged reverse clutch plate. Replace clutch plate.  
Damaged planetary assembly Repair or replace planetary assembly  
Low oil pressure Refer to D, E, or F.  

C. No power is transmitted regardless of selector position.

 lnternal mechanical failure Overhaul transmission  
Driverline failure Check input and output of transmission  
Low oil level Restore proper oil level.  
Low oil pressure Refer to D, E, or F.  
Manual selector valve not positioned properly. Check linkage.  

OIL PRESSURE

D.Low oil pressure regardless of selector position.

 Low oil level Restore oil level.  
Improper operation of oil gage Replace oil gage or, if the oil gage is slow to register, bleed the oil gage line to remove entrapped air or obstruction  
Improper oil viscosity Check the oil temperature and if found to be abnormally high, remove the transmission oil cooler and clean internally and externally. Replace the cooler if necessary.  
Improper pump adjustment Shirn the pump. Refer to engine service manual.  
Restrieted or damaged oil lines Check oil lines.  

E.Low oil pressure when selector is in forward position.

 Broken or worn forward piston sealrings Replace forward piston sealrings  
Leaking pilot bearing oil seal Remove and replace pilot bearing oil seal  
lmproper operation of dump valves Clean dump valves thoroughly and replace damaged parts  
Faulty flywheel. Replace loose or missing oil passage plugs. Check emergency engagement bolts for tightness. Tighten loose bolts.  
Damaged output shaft sealrings Replace sealrings.  

F.Low oil pressure only when selector valve is in reverse position.

 Broken or worn reverse piston sealrings Replace sealrings  
Damaged driveshaft sealrings Replace sealrings  

DRIVE SHAFT

G. Drive shaft rotates with selector valve in neutral after shifting from forward to neutral.

 Sticking forward clutch plate Replace clutch plate  
Damaged forward clutch plate Replace clutch plate  
Damaged forward piston drive pins Replace damaged drive pins.  
Damaged forward piston Replace piston.  
Damaged planetary assembly Repair or replace planetary assembly.  
Improper dump valve operation Clean dump valve thoroughly and replace damaged parts.  
Faulty drive shaft sealrings Replace sealrings.  
Control linkage out of adjustment Check linkage adjustment  

H. Drive shaft rotates with selector valve in neutral after shifting from reverse to neutral.

 Sticking reverse clutch plate Replace clutch plate  
Damaged reverse clutch plate Replace clutch plate  
Damaged reverse piston drive pins Replace damaged drive pins.  
Damaged reverse piston Replace piston.

 

  
Damaged planetary assembly Repair or replace planetary assembly.  
Control linkage out of adjustment Check linkage adjustment  

CLUTCH

I. Forward clutch slipping or slow to engage.

 Low oil pressure Refer to B,D or F  
Damaged forward piston or clutch plate Replace piston or clutch plate.  
lmproper dump valve operation Clean dump valve thoroughly and replace damaged parts.  
Worn piston sealrings Replace sealrings.  
Low oil temperature Use recommended oil. Preheat if required.  
Worn driveshaft sealrings Replace sealrings.  
Leaking pilot bearing oil seal Replace oil seal.  

J. Reverse clutch slipping or slow to engage.

 Low oil pressure Refer to D or E.  
Damaged reverse piston or clutch plate Replace piston or clutch plate.  
Worn reverse clutch facings Replace clutch plate  
Worn piston sealrings Replace sealrings  
Low oil temperature Use recommended oil. Preheat if required.  
Worn driveshaft, sealrings. Replace sealrings  

POWERPACK

K. Powerpack lacks power and acceleration.

 Engine malfunction Refer to engine service manual.  
Output  shaft (propeller) restricted Clear away obstruction  
Boat not properly propped Change transmission ratios or re-prop boat  

TRANSMISSION

L. Transmission overheats

 High or low oil level Restore proper oil level  
Aerated (foaming) oil Refert to M.  
Craft overloaded Reduce load.  
Engine coolant overheated Correct engine overheating.  
Cooler or iines kinked or clogged Clean or replace cooler or line.  
Low coolant level Add coolant, check for leaks  
Oil leaking Correct leaks  
Low oil pressure Refer to D or K.  
Clutch slipping Refer to I or J.  
Output  shaft (propeller) restricted Clear away obstruction  

OIL

M. Oil is aerated (foaming)

 Incorrect type oil used Change oil; use proper type  
High or low oil level Restore proper oil level  
Air entering suction side of oil pump Check oil pump bolts and gasket  
Water in oil Check for source; clean system  
Gear baffle not installed Rebuild reverse components, install baffle.  

N. Heavy metal debris found in oil

 Transmission internal failure Completely disassemble, clean, and repair transmission. Replace filters and transmission oil cooler. Clean all external lines.  

O. Coolant is found in oil

 Cooler leaking Completely disassemble and thoroughly clean transmission. Replace both foward and reverse clutch plates. Repair or replace cooler. Replace filters. Clean all external lines  
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