STARTER BRUSHES FOR 1978 MERCURY 90 HP OUTBOARD MOTOR

Starter Brushes for 1978 Mercury 90 HP Outboard Motor

Starter Brushes for 1978 Mercury 90 HP Outboard Motor

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Starter brushes for a 1978 Mercury 90 HP outboard motor are critical components of the starter motor system, responsible for conducting electrical current from the stationary part of the motor (field windings) to the rotating armature. These carbon brushes wear over time and require replacement to maintain starter functionality, especially in marine environments where corrosion and vibration accelerate degradation.

Design and Functionality


The starter brushes for the 1978 Mercury 90 HP typically feature:

 

  • Carbon-Graphite Construction: A blend of carbon and graphite materials to withstand friction and electrical arcing during operation.

  • OEM Specifications: Dimensions matching Mercury’s original design (e.g., length: 1.25 inches, width: 0.5 inches, thickness: 0.375 inches) for precise fitment.

  • Spring Tension: Attached to coiled springs that maintain constant pressure against the commutator, ensuring reliable electrical contact.

  • Insulated Leads: Wire leads coated with heat-resistant insulation to prevent short circuits.


Role in the Starter Motor



  1. Current Conduction: Brushes transfer electrical current from the starter’s solenoid to the armature windings, creating the magnetic field that drives motor rotation.

  2. Wear Compensation: Springs adjust for brush wear, maintaining contact with the commutator until the brushes are worn beyond service limits.

  3. Arcing Protection: Carbon-graphite material minimizes electrical arcing, which can damage the commutator if left unaddressed.


Signs of Wear and Replacement Needs



  • Excessive Wear: Brushes worn below 50% of their original length (e.g., less than 0.625 inches for a 1.25-inch brush).

  • Fractures or Cracks: Broken brush bodies that disrupt current flow.

  • Glazing: Shiny, smooth surfaces on brushes due to overheating, reducing conductivity.

  • Starter Performance Issues: Intermittent starting, slow cranking, or arcing noises from the starter housing.


Replacement Procedure



  1. Safety Steps:

    • Disconnect the boat’s battery and ensure the starter is cool to the touch.



  2. Starter Removal:

    • Locate the starter on the lower starboard side of the powerhead, remove mounting bolts, and disconnect wiring.



  3. Brush Assembly Access:

    • Open the starter’s end cap (held by screws or clips) to expose the brush holder and commutator.



  4. Brush Replacement:

    • Remove worn brushes by releasing the spring tension and detaching wire leads.

    • Install new brushes, ensuring the wire leads are securely connected and springs are properly seated.



  5. Reassembly:

    • Clean the commutator with fine sandpaper (600-grit) to remove oxidation, then reassemble the starter and mount it back on the engine.




Marine-Specific Maintenance Tips



  • Routine Inspection: Check brushes annually or every 100 operating hours, especially in saltwater applications.

  • Corrosion Prevention: Apply dielectric grease to brush leads and connections to combat saltwater intrusion.

  • Commutator Care: After replacing brushes, inspect the commutator for grooves or pitting; minor issues can be filed smooth, severe damage requires armature replacement.


Troubleshooting Brush-Related Issues



  • Slow Cranking: Worn brushes reducing current flow; test brush length and replace if below service limit.

  • Arcing Noises: Glazed or misaligned brushes; clean or replace brushes and dress the commutator.

  • Starter Failure to Engage: Broken brush leads or seized springs; inspect the brush assembly for physical damage.


 
For OEM-style starter brush kits, replacement springs, or detailed service manuals for the 1978 Mercury 90 HP outboard, visit starter motor for specialized marine engine components and resources.

Related Website


https://www.starterstock.com/

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