Troubleshooting Common Issues For Twin Disc MG-5090A Marine Gear

April 3, 2025
Overhaul plate kit for Twin Disc marine gear MG5090, MG5090A
Overhaul plate kit for Twin Disc Marine Gear MG5090, MG5090A

Parts catalog for Twin Disc MG50590A Marine Transmissions
Overhaul Plate Kit For Twin Disc MG5090A Marine Transmission
Gasket Kits For Twin Disc MG5090A Marine Transmission

Introduction: Diagnosing Problems in the MG-5090A Marine Transmission

The Twin Disc MG-5090A marine gear is a highly reliable reverse and reduction transmission used in commercial vessels, patrol boats, and recreational craft worldwide. Like any precision mechanical system, it is subject to wear, oil-related problems, and external control issues over time. Fortunately, the MG-5090A is designed to be diagnostic-friendly, allowing marine technicians and vessel operators to pinpoint most problems quickly—if they know what to look for.

This comprehensive troubleshooting guide analyzes the most common symptoms, explores their possible root causes, and outlines the corrective actions necessary to restore smooth, dependable performance. Each section is supported by practical testing steps, repair tips, and advice on when to consider professional assistance or transmission replacement.

Whether you’re dockside with a slipping transmission or facing a high-load overheating issue, this resource can help get your MG-5090A back in action quickly.

1. Slipping in Gear

Clutch plate for Twin Disc marine transmission MG5090 and MG MG5091
Clutch plate for Twin Disc marine transmission MG5090 and MG MG5091

Symptom: Engine revs increase, but boat speed does not. The propeller feels like it loses bite.

Possible Causes:

  • Low clutch pressure due to pump wear, internal leaks, or blocked hydraulic passage 
  • Worn or burned clutch discs with insufficient friction material 
  • Incorrect shift linkage or actuator not fully engaging the clutch valve 
  • Contaminated or broken-down oil reduces clutch engagement effectiveness

Diagnostic Steps:

  1. Measure clutch engagement pressure with a hydraulic gauge at the forward or reverse test port 
  2. Inspect oil color and smell: Burnt odor = clutch overheating 
  3. Use a borescope or remove the inspection cover to inspect the clutch stack visually 
  4. Check shift linkage travel or electronic actuator function (EC300, etc.)

Corrective Actions:

  • If hydraulic pressure is below 250 PSI: 
    • Check oil filter 
    • Inspect oil pump and pressure regulator 
    • Verify no air is trapped in the system
  • If clutch pack is worn or scorched: 
    • Rebuild the clutch using a kit with new friction discs and steel plates 
    • Replace piston seals and return springs 
    • Confirm proper piston movement during reassembly
  • If mechanical linkage is misaligned: 
    • Adjust cable end fittings and ensure lever reaches detent position 
    • Lubricate all linkage points 
  • Replace oil with clean, Twin Disc-spec SAE 30 transmission oil

2. Harsh or Jerky Shifting

Symptom: Abrupt engagement or jolt when shifting between neutral and gear

Possible Causes:

  • Sticky or dirty shift valve inside control block 
  • Hydraulic oil contaminated with sludge or metal particles 
  • Actuator delay or overshoot in electronic control system 
  • Improper cable tension or control arm misalignment

Diagnostic Steps:

  1. Inspect oil quality—look for milky, dark, or debris-laden fluid 
  2. Cycle the shift lever manually and observe valve movement 
  3. If electronic, plug into control module (EC300) and run diagnostics 
  4. Remove and inspect control valve spool for sticking or galling

Corrective Actions:

  • Flush and replace transmission oil 
    • Include new filter and clean oil cooler
  • Remove shift valve assembly 
    • Clean with solvent and compressed air 
    • Reinstall with new seals if available
  • Calibrate electronic control systems 
    • Reset shift delay parameters via EC300 or compatible controller 
  • Replace sticky linkage bushings or cable mounts

3. No Response When Shifted into Gear

Symptom: Lever moves but transmission remains in neutral or doesn’t engage propeller

Possible Causes:

  • Control linkage disconnected or misaligned 
  • Low or empty oil level 
  • Actuator failure in electronic systems 
  • Broken internal clutch piston seal or failed spring pack

Diagnostic Steps:

  1. Verify oil level using dipstick (engine idling in neutral) 
  2. Manually shift the transmission at the clutch lever and listen for engagement 
  3. Check actuator movement (electronic) for full travel 
  4. Test hydraulic pressure at forward/reverse ports

Corrective Actions:

  • If oil is low:

     

    • Top off with correct transmission oil 
    • Inspect for leaks at seals, drain plugs, and cooler hoses
  • If cable is broken or actuator fails:

     

    • Replace mechanical cable with marine-grade stainless type 
    • Replace EC300 actuator or calibrate sensor feedback
  • If no pressure at test ports:

     

    • Check for airlock or failed oil pump 
    • Inspect for blocked pressure path or stuck spool valve
  • If piston seals are blown:

     

    • Disassemble clutch housing 
    • Replace seals, springs, and clutch stack if contaminated

4. Noise Under Load

Symptom: Whining, grinding, or knocking when transmission is under forward or reverse power

Possible Causes:

  • Worn or damaged gears (input, output, planetary) 
  • Failed or pitted bearings 
  • Improper gear lash or shaft alignment 
  • Debris or contamination inside housing

Diagnostic Steps:

  1. Run engine at different loads and listen to gearbox 
  2. Use stethoscope or sound probe to isolate noise source 
  3. Drain oil and inspect for metal fragments 
  4. Remove inspection cover and check gear teeth visually

Corrective Actions:

  • If gears show pitting or scoring:

     

    • Replace individual gear or complete gearset 
    • Match gear pairing to maintain alignment and contact pattern
  • If bearings are noisy or loose:

     

    • Remove and press in new roller or needle bearings 
    • Inspect shaft journals for scoring
  • Use proper shimming and torque specs during reassembly 
  • Consider full rebuild or replacement unit if damage is extensive

5. Overheating Transmission

Symptom: Transmission oil temperature rises above 190°F during normal operation

Possible Causes:

  • Clogged or fouled oil cooler 
  • Low oil level or poor oil quality 
  • Clutch slippage under load 
  • Restriction in oil flow (filter or passage)

Diagnostic Steps:

  1. Use infrared thermometer to measure cooler and housing temps 
  2. Check for oil flow at cooler inlet/outlet 
  3. Inspect oil cooler for marine growth or scale buildup 
  4. Check clutch pressure for signs of slippage

Corrective Actions:

  • Flush and descale the oil cooler 
    • Backflush with descaler and fresh water 
    • Replace if pressure test fails
  • Top off or replace oil 
    • Use SAE 30 transmission oil with high heat tolerance
  • Inspect filter and replace if clogged 
    • Install new filter from Diesel Pro Power
  • Inspect and rebuild clutch if slipping occurs 
    • Heat during slippage degrades plates and oil alike

Additional Issues and Corrective Actions

6. Leaking Seals or Gaskets

Gasket and Seal Kit for MG5090A.
Gasket and Seal Kit for MG5090A.

Cause:

  • Worn shaft seals 
  • Improperly torqued covers 
  • Old or reused gaskets 
  • Cracked housing

Solution:

  • Replace all seals during service 
  • Apply sealant sparingly where specified 
  • Torque bolts in crisscross pattern 
  • Replace cracked components or use reman unit

7. Transmission Stuck in Gear

Cause:

  • Seized control valve 

  • Detent mechanism failure 
  • Debris in hydraulic circuit

Solution:

  • Disassemble valve block and clean 
  • Replace detent spring and ball 
  • Flush hydraulic oil and test pressure

8. Shudder on Engagement

Cause:

  • Warped clutch plates 
  • Glazed friction material 
  • Low pressure spike 
  • Air in oil

Solution:

  • Replace friction and steel plates 
  • Check plate stack height 
  • Bleed air from system 
  • Use fresh oil with proper viscosity

9. Noisy Start-Up After Rebuild

Cause:

  • Dry bearings or improper preload 
  • Loose hardware or incorrect reassembly 
  • Clutch misalignment

Solution:

  • Re-check torque settings 
  • Use assembly lube 
  • Inspect for missing shims or reverse gear misalignment

10. Control Lever Feels Loose or Slack

Cause:

  • Worn linkage bushings 
  • Detached control cable 
  • Failed actuator

Solution:

  • Replace bushings and adjust end fittings 
  • Calibrate EC300 system 
  • Lubricate mechanical joints

Preventive Maintenance to Avoid Common Problems

Routine Checks:

Task Interval
Check oil level and clarity Every 50 hours
Inspect shift linkage Monthly
Flush oil and replace filter Every 1,000 hours
Cooler flushing Annually
Clutch pressure test Every 1,000 hours
Full teardown inspection Every 3,000 hours

Tips to Prevent Failures:

  1. Use only approved SAE 30 marine transmission oil 
  2. Do not idle long periods in gear 
  3. Install zincs and strainers to protect oil cooler 
  4. Log all maintenance and oil changes 
  5. Replace shift cables every 3–5 years 
  6. Use only OEM-quality or Diesel Pro Power aftermarket parts

When to Replace Instead of Repair

Consider ordering a remanufactured MG-5090A if:

  • Multiple internal components show wear 
  • Gear case is cracked or warped 
  • Transmission has history of repeated failures 
  • Downtime is unacceptable

Diesel Pro Power offers:

  • Fully rebuilt MG-5090A units 
  • Pre-tested under load 
  • Warranty and fast global delivery

Summary: Identify Symptoms Early to Keep Your MG-5090A in Top Shape

The Twin Disc MG-5090A is a workhorse transmission, but even the toughest marine gear systems experience wear, oil-related degradation, or control issues over time. Understanding how to recognize and respond to early warning signs—like slipping, overheating, or odd noises—can prevent costly breakdowns, unplanned haul-outs, and extended downtime.

This troubleshooting guide provides a practical roadmap for resolving common problems quickly and effectively. With proper diagnostics, use of high-quality replacement parts, and professional support when needed, your MG-5090A can deliver many more hours of dependable propulsion.

Troubleshooting Summary Table

Symptom

Possible Cause

Corrective Action
Slipping in gear Low pressure, worn clutches Test pressure, rebuild clutch
Harsh shifting Dirty oil, sticky valve Flush oil, clean/replace control valve
No response in gear Control fault, low oil Check linkage, actuator, top off oil
Noise under load Worn gears or bearings Inspect, replace with rebuild kit
Overheating Dirty cooler, slipping clutch Clean cooler, test pressure, replace oil
Leaking seals Aged or damaged components Replace gaskets, seals, or housing
Shuddering engagement Warped clutch plates Replace clutch stack, confirm pressure
Stuck in gear Valve or detent failure Clean or replace valve block, rebuild detent
Parts catalog for Twin Disc MG50590A
Parts catalog for Twin Disc MG50590A Marine Transmissions

 

Overhaul Plate Kit For Twin Disc MG5090A Marine Transmission
Gasket Kits For Twin Disc MG5090A Marine Transmission

Videos About Twin Disc Transmissions

6 Reasons Your Twin Disc Transmission Has Low Oil Pressure

7 Reasons Your Twin Disc Transmission Is Overheating

3 Reasons Your Clutch Plates in Your Twin Disc Transmission Are Making Excessive Noise

 

Bull Gear On A Twin Disc Transmission 

Rebuilt Twin Disc Transmissions

 

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