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· Fresh Water Areas - 11.5 volts minimum
· Seawater Areas - 3.55 volts minimum
If reading is low, replace controller.

14. Additional corrosion protection required. Boats which are equipped with a sizable amount of underwater metal (stainless steel prop, after planes, etc.), or that are moored in an area with warm or rapid flowing water, may require additional protection.
Install additional zinc anodes or a MerCathode System 88334A2. If unit is already equipped with a MerCathode System, a second system may be required.
====== HIGH READING
1. Stray current corrosion. If an electrical current flowing along a metal conductor leaves the metal for a water path, it will cause ionization of the  metal, and an area of rapid corrosion.
Observe reading while disconnecting electrical components (one at a time) until high reading is eliminated. Correct source of stray current.

2. *Poor connection between MerCathode reference electrode (brown) lead and "R" terminal on controller.
Clean and/or tighten connection. Repair wiring  as required.

3. *Faulty MerCathode reference electrode.
Disconnect reference electrode (brown) lead from "R" terminal on controller. Connect lead to positive (f) terminal of digital multi-meter (set on 0-2000 millivolt scale). Connect negative meter lead to negative (-) battery terminal. Note meter reading; then, repeat test using MerCathode Reference Electrode Tester (76675A1). The same reading should be obtained in both cases. If not, replace reference electrode.

4. *Faulty MerCathode controller.
Replace controller.
==== NORMAL READING BUT CORROSION IS EVIDENT

Corrosion on Entire Drive Unit Drive?
Unit raised so far that sacrificial zinc trim tab is out of the water. Drive unit must be left "in" ("Down") position when boat is moored for trim tab to provide protection.

Corrosion Problem Developed after Drive Unit was Refinished?
Steel wire brush used to clean aluminum casting. Steel particles become entrapped and set up a small galvanic cell. Use a nylon or bristle brush only.

Paint Blistering on Drive Unit?
Battery charger, which uses 110 volt shore power, improperly connected to battery. Make sure charger is connected correctly.

Trim Cylinder Corroding?
Loss of continuity between trim cylinders and drive unit. Install proper continuity devices.

Corrosion isolated to one or two components (bell housing, gimbal ring, etc)?  Loss of continuity between drive unit components. If not already done, install Continuity Circuit Kit 99940A1. If unit is already equipped with continuity circuit, test circuit as outlined under Stern Drive Continuity Test.

Corrosion in Area of Exhaust Outlets?
Accumulation of exhaust gas deposits on the drive exterior may result in paint blistering and corrosion. Deposits can be removed with marine or automotive wax which also will help protect finish.

Corrosion Occurs After Unit is Removed From the Water?
Salt crystals remain on the surface of drive components and high humidity causes electrolyte to form with subsequent corrosion resulting. Wash exterior and flush interior with fresh water.

Corrosion Between Surfaces?
Salt buildup between surfaces. Exclude moisture from between mating parts with Quicksilver Special Lubricant 101. 2-4-C Multi-Lube or Perfect Seal.

Aluminum Corroding in Areas Where Lubricant Was Applied?
Lubricant used contained graphite. Never use lubricants which contain graphite as they will accelerate corrosion.

Stainless Steel Components Corroding?
Foreign matter (fish line, marine growth, etc.) covering steel and starving it of oxygen. This causes a breakdown of the protective oxide film and subsequent corrosion (known as oxygen starvation corrosion). Burying stainless steel in sand or silt also can cause this problem. Remove foreign matter and prevent surfaces from being covered by sand and silt.

Stainless Steel Propeller Corroding?
Loss of continuity between propeller and prop shaft. Clean mating surfaces on propeller, propeller shaft and attaching parts. If applicable, install continuity washer. Torque propeller nut to 55 lbs. ft. (75 N.M.).

Paint blistering - metal under blistered paint is not pitted?
Surface was not properly prepared before paint was applied. Sand surface down to bare metal, prime and repaint with Quicksilver Phantom Black Spray Paint.
========= PREVENTATIVE MAINTENANCE

You can do a few simple service procedures to maintain the quality of corrosion resistance built into your Mercury Drive Unit. Here's what you can do:

Maintain a complete paint-covering on the lower unit. Check finish regularly and prime and paint nicks and scratches. If bare metal is exposed, protective Zinc tabs will be eaten away rapidly, and corrosion can occur quickly. Use only tin anti fouling paint on or near aluminum surfaces below the waterline. Never use paints containing copper or mercury. It's a good idea to check the lower unit frequently for sand abrasion, nicks and scratches. If bare metal is showing, apply two coats of paint to prevent corrosion.

Spray the entire powerhead and all electrical connections and everything under the cowl with Quicksilver Corrosion and Rust Preventive Type II.

Inspect Sacrificial Zinc Trim Tabs and replace before half the tab is gone. Never paint trim tabs. Additional Zinc or a MerCathode System may be required if a stainless steel prop is installed.

Flush motor with freshwater after each saltwater excursion, and wash down exterior of motor with freshwater as well. Flushing attachments are available for all Mercury Drive Units and are designed for easy access. Use them!

Check the prop shaft for fish line. If a stainless steel shaft is wrapped with line, oxygen is eliminated from the surface, allowing corrosion. Remove the fish line. Fish line cutters are standard on all Mercury, Mariner Outboards. 50 to 115 horse power.

Lubricate according to your owner's manual. In saltwater, lubrication is required more frequently. Do not use lubricants that contain graphite on or near aluminum. Graphite causes rapid corrosion to aluminum when the two are combined in saltwater. Additional grease fittings have recently been added to many Mercury Drive Units in many areas. The more lubrication, the better corrosion resistance and performance you'll realize. Remember, saltwater operation requires lubrication more often!,

The propeller should be removed every 60 days and Quicksilver 2-4-C or type 101 lubrication applied to the prop shaft. When re-installing the prop, be sure the prop nut is tightened sufficiently (see your owner's manual). Have an authorized dealer lubricate the cover nut and bearing carrier spool at least once each session.

Other lubrication points are: thumbscrews, starter motor pinion shaft, upper shift shaft, swivel tube, steering tube, throttle and shift linkage, reverse lock lever and cam, and tiller handle hinges. The lower unit must be checked and lubrication added as necessary.