Troubleshooting Common Problems For Allison M & MH Marine Transmissions

April 8, 2025

Allison MH Transmission) — Allison MH Transmission

Allison M Rebuilt Marine Transmission
Overhaul Kit & Related Components For Allison M
Allison M Seals
Allison M Clutch Plates
Allison M Hydraulic Pump
Allison M Selector Valve & Related Components
Allison M Bearings & Related Components

Allison MH Rebuilt Transmission
Allison MH Overhaul Kit & Related Components
Allison MH Seals
Allison MH Clutch Plates & Related Components
Allison MH Hydraulic Pump
Allison MH Selector Valve
Allison MH Bearings & Related Components

Slipping in Gear

Slipping in gear—whether in forward or reverse—is one of the most commonly reported transmission issues in marine vessels equipped with Allison M & MH Series marine transmissions. This condition usually manifests as an inability to maintain consistent propulsion or engine RPMs increasing without a corresponding increase in vessel speed. Diagnosing this condition effectively starts with understanding its root causes:

Low Oil Pressure: The Allison M & MH transmissions rely heavily on hydraulic pressure to engage the clutches that drive the vessel. If oil pressure drops, the clutches may not engage fully, resulting in slippage. This can occur due to aerated oil, clogged filters, or leaking seal rings. Pressure should be checked with the selector in both forward and reverse to determine the fault.
Clutch Plate Wear: Forward and reverse clutch facings are wear components that degrade over time. Worn clutch plates are a direct cause of slippage. Visual inspection is required, and if friction material is thin, glazed, or burned, the clutch should be replaced.
Dump Valve Malfunction: The dump valve facilitates rapid oil evacuation during gear changes. If it malfunctions, it may restrict pressure buildup or delay engagement. Disassemble and clean the dump valve, ensuring no scoring or debris is present.
Oil Viscosity and Temperature: Operating with incorrect oil viscosity or at suboptimal temperatures (below 180°F) can delay clutch engagement. Allison recommends matching engine oil viscosity and avoiding C-3 oil types. For marine applications, the correct warm-up procedure is critical.
Sealring Deterioration: Sealrings on pistons and shafts degrade with age and heat, especially in vessels operating in tropical climates or those frequently switching between gears. These components should be replaced during overhaul.

No Engagement in Forward or Reverse

A total lack of engagement in either gear direction can render the vessel immobile, often requiring tow-in or emergency clutch engagement. This issue can be mechanical, hydraulic, or electrical depending on how the selector linkage is integrated into the boat’s control systems.

Hydraulic System Failure: The oil pump, driven off the engine, must generate sufficient pressure to apply the clutches. A failed pump, improper adjustment, or air leaks on the suction side can result in zero pressure delivery. If oil pressures in both forward and reverse are low or absent, the pump or drive may have failed.
Selector Valve Malfunction: The manual selector valve may not move into position due to internal obstruction or linkage misalignment. Always inspect linkage for full range motion and ensure it reaches detent positions.
Clutch Pack Assembly: Severely worn or misassembled clutch components can cause failure to engage. If this is suspected, disassemble and inspect forward and reverse clutch packs, including piston pins and spring assemblies.
Planetary Gear Failure: The internal planetary gear system can seize or break teeth. If both forward and reverse functions fail and there’s noise during cranking or unusual resistance, internal mechanical failure is probable.
Fluid Level Issues: Always check fluid level first. The transmission should be checked hot, at operating temperature, with the engine running. A low or overfilled sump causes aeration, preventing engagement.

Overheating

Transmission overheating is a critical failure mode that can cause long-term damage if left unaddressed. Allison M & MH units are designed to operate within a 180°F–200°F range. Exceeding these temperatures repeatedly can damage seals, clutches, and internal passages.

Incorrect Fluid Level: Either too much or too little oil can cause foaming (aeration), leading to heat buildup. Always check oil per specification and correct for angular installation.
Clutch Slippage: Prolonged slipping causes frictional heat. If slipping is observed, stop the vessel, allow the system to cool, and investigate the root cause.
Restricted Oil Cooler Flow: Marine environments often result in fouled oil coolers from salt, sediment, or growth. The oil cooler should be removed, backflushed, and tested annually. Replace if internal damage is suspected.
High Load or Poor Prop Selection: Excessive load, incorrect pitch or diameter of the propeller, or marine growth can overload the powerpack, leading to excessive torque demands and overheating. Verify vessel trim and prop match.
Coolant System Malfunction: If the engine cooling water is shared with the transmission cooler and the engine is overheating, transmission temperatures will rise in tandem. Always monitor both coolant and oil temps on the control panel.

Noise Under Load

Unusual noises—such as whining, grinding, or clunking—can signal anything from minor wear to impending failure.

Worn Bearings: The input and output shaft bearings wear over time. A high-pitched whining sound that increases with RPMs is often indicative of bearing wear. Inspect using a stethoscope, and confirm by checking endplay.
Damaged Gears: Pitting or broken teeth on planetary gears, reduction gears, or pinion gears create grinding or knocking sounds. Disassemble and visually inspect all gearing if noise persists.
Loose Components: If bolts on the output flange or coupling are loose, it may produce a knocking sound especially when under torque. Always torque all bolts to specification and inspect during routine maintenance.
Misaligned Shafts: Misalignment between the transmission and propeller shaft introduces vibrations and cyclic noise. Check alignment during every seasonal haul-out using feeler gauges at the coupling.
Pump Cavitation: Aerated oil or suction-side leaks in the oil pump cause cavitation, which sounds like gravel in the transmission. Bleed and inspect the oil circuit to eliminate air ingress.

Oil Contamination or Leakage

Oil issues not only affect performance but also indicate deeper system health problems. The oil in an Allison M & MH transmission should be golden-clear, without milky or metallic appearances.

Coolant Ingress: A leaking transmission oil cooler can allow engine coolant (glycol) into the transmission. Glycol destroys clutch facings and forms varnish. If coolant is suspected, fully disassemble, clean, and replace both forward and reverse friction plates.
Metal Debris: If heavy metal debris is found on the magnetic drain plug or in oil samples, an internal failure—such as gear damage or bearing failure—is probable. Filter the fluid, inspect internal parts, and flush the system.
External Leaks: Common leak points include the breather, fill plug, selector valve shaft seals, and oil lines. Clean the case and use UV dye to trace leak origin if the source isn’t obvious.
Breather Blockage: A blocked breather can cause pressure to build inside the transmission, forcing oil past seals. Inspect and clean the breather at regular intervals.
Oil Sample Testing: Periodic oil sampling and lab analysis can identify contaminants like metal, glycol, or water before they cause major issues. Diesel Pro Power recommends sampling every 500 hours or annually.

Additional Problem Scenarios and Remedies

Drive Shaft Rotates in Neutral
- Cause: Sticking clutch plate, worn piston, improper dump valve operation.
- Fix: Replace or rebuild clutch assembly, check dump valve, adjust linkage.
Low Oil Pressure (Only in Reverse or Forward)
- Cause: Sealring leaks, pilot bearing failure, loose engagement bolts.
- Fix: Replace seals, tighten bolts, inspect pistons and gaskets.
Heavy Vibration Under Load
- Cause: Bent shaft, misalignment, damaged gears.
- Fix: Recheck shaft run-out, verify alignment within 0.005” TIR.
Transmission Stuck in One Gear
- Cause: Selector valve jammed or mechanical failure.
- Fix: Disassemble selector valve assembly, inspect detents and bushings.
Emergency “Get Home” Engagement Required
- Use: Only for short distances if the hydraulic circuit fails.
- Method: Remove plug, engage bolts manually, add extra gallon of oil.

Prevention Tips and Best Practices

Routine Fluid Checks: Always check oil hot, engine running. Maintain between FULL and ADD mark. Correct for angled installations.
Torque Specs: Use torque wrench and reference OEM specs—critical for output flanges, pump covers, and gear nuts.
Use Correct Oil: Only use engine oil recommended for your Detroit Diesel engine. Avoid C-3 oil formulations.
Maintain Cooling System: Clean and test oil coolers, ensure adequate water flow, and use strainers to prevent sediment intrusion.
Log Problems: Use a maintenance logbook. Note oil changes, unusual sounds, and any signs of performance loss.

Conclusion: When to Seek Professional Help

While many issues can be diagnosed and corrected dockside with a few tools and careful inspection, some problems—especially internal ones—may require specialized tools, test stands, or part fabrication. Diesel Pro Power recommends contacting a trained technician or utilizing professional rebuild services if:

Gear replacement or clutch pack disassembly is required.
The transmission shows signs of coolant contamination.
Oil pressure is not within 130 psi (forward) or 110 psi (reverse) at rated RPM.

Disclaimer

This article provides advanced-level service and troubleshooting insights for Allison M & MH Marine Transmissions. For deeper technical procedures, part tolerances, and rebuilding protocols, always refer to the original Allison Transmission Service Manual or consult with a certified marine transmission specialist.

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Troubleshooting Common Problems For Allison M & MH Marine Transmissions