1963-1973 Mercruiser all Engines and Drives Service Manual Books 1 and 2, Page 262Get this manual
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::J 4 Rever-e lest leadand note re adi ng on meterIf both readin gs are zero, or both readi ngs are ver;high diode is defe tiv good di ode will give one 10 readi und one high readin"6 Repeat teps -thr