1985 MerCruiser MCM TR / MCM TRS Stern Drive Units Service Manual #5 (P/N 90-12935 1085)

The 1985 MerCruiser MCM TR/TRS Stern Drive Units Service Manual #5 is Mercury Marine publication 90-12935, with printing code 1085. It covers left- and right-hand drives and transom assemblies with or without power steering.

• Stern drive repair: Drive shaft and gear housing disassembly, bearing and seal replacement, gear shimming, preload adjustment and backlash checks.
• Transom assemblies: Bell housing, U-joint bellows, gimbal ring, swivel shaft and steering lever service, plus drive removal, installation and shift linkage adjustment.
• Power Trim pumps: Oildyne and both Prestolite systems, including electrical troubleshooting, hydraulic pressure testing, motor service and component inspection.
• Trim cylinders: Internal- and external-return-line designs, with disassembly, piston and seal inspection, reassembly and hydraulic bleeding procedures.
• Power steering: Pump pressure testing, control valve balancing, booster cylinder repair and vane pump overhaul, supported by hydraulic flow illustrations.
• Maintenance and protection: Lubrication schedules, propeller servicing, continuity circuit checks, MerCathode testing, lay-up preparation and recommissioning.

Written for dealer mechanics and service personnel, the manual assumes familiarity with marine power systems and the relevant tools and repair procedures.

Drive, U-Joint and Transom Service

Drive shaft and gear housing chapters develop the repair work through inspection, measurement and reassembly. Exploded gear housing drawings show component order, while separate instructions address standard TR/TRS upper gears and the heavy-duty TR Commercial arrangement.

Gear shimming procedures identify dedicated tools, including driven-gear tool 91-98171 and drive-gear tool 91-99924. The instructions also specify lubricants, sealants and checks needed during assembly.

U-joint coverage includes Mercury Marine and Borg Warner transmission-type yokes, cross bearings and splines. Transom service covers bellows, seals and gimbal ring assemblies, including differing swivel shaft connections and the early MCM228TR exhaust-bellows configuration.

Engine installation and alignment refer to the appropriate engine documentation. The transmission output bearing serves as the gimbal bearing; its replacement is referred to the appropriate engine service manual.

Power Trim Diagnosis and Component Matching

Pump and cylinder pressure tests distinguish a pump fault from internal cylinder leakage when the drive loses position. Electrical diagnosis includes solenoid, relay, diode and switch checks, supported by Oildyne and Prestolite wiring diagrams and the Dual Trim Control Panel Wiring Diagram.

Oildyne and low-pressure Prestolite systems use external-return-line cylinders. High-pressure Prestolite systems use internal-return-line cylinders and an external reverse lock; mixing these systems can damage components.

Oildyne motor and adapter service is covered, while its pump unit is replaced as an assembly. Trim procedures warn that releasing hydraulic pressure can allow an unsupported drive to fall.

Steering Tests and Hydraulic Repairs

Power steering coverage proceeds from pressure testing and control valve balancing to booster cylinder seals, flow control components and vane pump overhaul. Cutaway illustrations trace neutral, right-turn and left-turn fluid flow, helping connect steering symptoms with valve and cylinder operation.

Maintenance, Propellers and Corrosion Protection

Maintenance charts distinguish fresh- and saltwater operation and cover shift pivots, U-joint lubrication and propeller shaft care. Lay-up and recommissioning instructions accompany the 10-hour break-in guidance for new or replacement gears and MerCathode output testing.

Continuity circuit checks and trim-cylinder grounding springs receive specific attention: missing grounding springs can expose piston rods and pivot points to galvanic corrosion. Propeller material includes pitch, rake, cavitation, ventilation, hydrodynamic vector diagrams, cutaway illustrations and a propeller-selection chart example.

Sample Trim Pressures and Transom Tolerances

For the TR/TRS Power Trim up circuit, the specifications give 2,200–2,600 psi for Oildyne and low-pressure Prestolite pumps, versus 3,000–4,000 psi for high-pressure Prestolite pumps. These separate ranges support the system-specific hydraulic tests.

The transom installation drawing distinguishes mounting surfaces: the inner transom plate area must be flat within 1/8 in. (3.2 mm), while the gimbal housing footprint must be flat within 1/16 in. (1.6 mm).

PDF Navigation and Included Pages

Section bookmarks and diagram viewing: PDF bookmarks provide access to model coverage and the main service sections. PDF text search is not available. Small wiring labels require enlargement; some terminal numbers in the dual trim control diagram remain soft at high zoom.

Supplied as one printable, English-language 326-page PDF. This total includes the original cover and section dividers, a repeated 24-page block of troubleshooting and installation material, and 14 other blank pages.