NOTE: The following corrosion PROTECTION test supersedes all previously issued tests. This test can be used on applications with or without a MerCathode System, and with either the old or the new Mercathode System.

Use the following test to determine if drive unit is being afforded adequate corrosion protection, or if additional corrosion protection is required. If the unit is equipped with a MerCathode System, it is recommended that this test be performed at least once each year (where the boat is moored) to ensure that the system is functioning properly.

Test requires the use of MerCathode Reference Electrode Tester; 6675r\1 and Quicksilver VOA Meter 91-62562r21. (Figure 23) This meter is no longer available from Mercury Marine. If you do not already have this meter, a digital multi-meter (such as a Radio Shack 22-191) must be used. A STANDARD ANALOG (Needle Type) METER CANNOT BE USED AS AN INACCURATE READING WILL RESULT.

IMPORTANT: Quicksilver Volt/Ohm Meter 91-43572 and Multi-Meter DVA/Tester 91-99750 are no longer recommended for testing corrosion protection.

The MerCathode Reference Electrode Tester 76675A1 is equipped with a special jack (containing a resistor) to provide the proper scale reading when used with Quicksilver VOA Meter 91625621. Previously, we stated that this plug could be removed to allow tester to be used with other analog type meters. Further testing has revealed that this could result in inaccurate readings and; therefore, we no Longer recommend the removal of this plug and the use of other analog meters. Resistor jack can be left in place when using digital meters.
IMPORTANT: Be sure to observe the following when performing test:

If unit is equipped with a MerCathode System, make sure that battery is fully charged (12.6 volts or above).

New boats just placed in service usually will produce a reading higher than normal. This is due to the drive unit being protected by a good finish and a new sacrificial zinc (aluminum) anodes. To obtain an accurate diagnosis, the test should be performed after the boat has been in service at least one or two weeks. This will give the paint a chance to "soak" and minor abrasions and scratches will have appeared, which will result in a more accurate reading.

Boats should be moored (without being operated) for at least 8 hours before performing tests. This is necessary to allow the MerCathode System and/or sacrificial zinc anodes to polarize the surrounding water. Be careful not to rock the boat excessively while boarding to perform test as this will alter the test reading.

1. Plug negative meter lead into negative (-) receptacle of meter. Connect other end of lead to negative (-) battery  terminal or other convenient engine ground.
2. Plug Reference Electrode Tester lead into positive (+) receptacle of meter.
3. If using Quicksilver VOA Meter 91-62562A1, set meter on  "AUX TEST" position. If using a digital meter, set meter  on scale required to read 0-2000 milli-volts (0-2 volts).
4. Immerse Electrode Tester in the water within 6" (15 cm) of  the aft end of the drive unit. (Figure 24)
5. The following readings indicate that stern drive is adequately protected:

Fresh Water Areas --
  7.5 - 10.5 with Quicksilver VOA Meter
  750-1050 Milli-volts with Digital Meter
Salt, Polluted or Mineral Laden Water Areas --
  8.8 - 10.5 with Quicksilver VOA Meter
  880-1050 Milli-volts with Digital Meter
6. If the reading is not within specified limits, or if reading is within specifications, but there is evidence of corrosion on stern drive, refer to the appropriate troubleshooting chart, following to aid in diagnosis and correcting the problem.


1. Loss of continuity between drive unit components and negative (-) battery terminal.
  Check that continuity devices are not missing or damaged and that connections are clean and tight. If unit is equipped with continuity circuit,  test circuit as explained under Stern Drive Continuity Test.

2. BOATS EQUIPPED WITH SHORE POWER  ONLY: Shore power green safety grounding lead  not isolated from power package ground.
  Disconnect shore power and notice if reading increases. If so, a Quicksilver isolator 76664A1 or  an isolation transformer should be installed.

3. Underwater metal parts on drive unit and/or boat unpainted or paint is in poor condition.
More exposed metal than zinc anodes and/or MerCathode System can protect.  Prime and paint underwater metal parts. This will reduce the load on the zinc anodes and/or MerCathode System.

4. Zinc anodes painted.
Anodes will not provide protection if painted. Remove paint or replace anodes.

5. Zinc anodes improperly grounded or inactive.
Clean anode mounting surface or replace zinc anodes if they have oxidized. (Refer to inactive Sacrificial Zinc Anodes)

6. Zinc anodes consumed (no longer affording protection).
Replace anodes if eroded 50% or more.

7. Drive unit and/or boat bottom painted with anti fouling paint containing copper or mercury.
  Only Tri-Butyl-Tin-Adipate (TBTA) base anti fouling paint should be used. (Refer to Anti Fouling Paint)

* Applicable to units with a MerCathode System only.

8. *MerCathode reference electrode or anode painted.
   Remove paint.

9. *Zinc anode heads used instead of plastic caps  (MerCruiser 120-thru-260 R & MR MerCathode System Only).
  Reinstall plastic caps.

10. *No power to MerCathode controller.
Connect positive (+) lead of volt meter (set on 0-20 volt scale) to "f" terminal on controller and negative (-) volt meter lead to "-" terminal. Meter should indicate battery voltage. Check for blown fuse  on Standard MerCathode System. Clean connection or repair wiring as required.

11. *Poor connection between reference electrode (brown) lead or anode (orange) lead and MerCathode controller.
  Clean and/or tighten connection. Repair wiring.

12. *Faulty MerCathode reference electrode.
Disconnect reference electrode (brown) lead from controller "R" terminal. Connect lead to positive (+) terminal of a 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.
13. *Faulty MerCathode Controller.
With anode and reference electrode leads connected to controller, connect a jumper wire between "R" and "-" terminals on controller. Connect positive (+) lead of volt meter (set on 0-20 scale) to "A" terminal on controller. Connect negative (-) meter lead to negative "-" controller terminal. Reading should be as follows:

(Continued on page 201)

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