1990 Johnson Evinrude "ES" 9.9 thru 30 Service Manual, P/N 507871, Page 41Get this manual
AnodesTesting and Replacement Erosion or disintegration in salt or brackish water indicates the anodes are performing their functionThe anodes should be inspected andor replaced at intervals or corrosion of the engine will increase
If any anode has been reduced to two-thirds its original size (one-third eroded) it must be replaced!
EJ to low ohms scaleConnect one lead of the ohmmeter to powerhead ground and the other lead to the anodeBe sure surface of the anode is clean to make good contactOhmmeter should indicate very low reading (zero)If not, remove the anode and clean the area where the anode is installedThe mounting screws and the anode itself should also be cleanedInstall and retest