1985 OMC 65, 100 and 155 HP Models Commercial Service Manual, PN 507450-D, Page 159Get this manual
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Rough Running Engine155 Models rough running engine may be due to interaction of the power packs because of faulty isolation diode in power packsThe diode may be checked by using an ohmmeter set to high ohms scaleCheck each power pack as follows: 1Disconnect the four-wire connectors from power pack to timer base and from the power pack to ignition coils2Connect the ohmmeter leads between the white lead liD" and blackyellow lead "0 Note meter reading3Reverse the ohmmeter leads and note readingAn infinite (very high) reading in both directions indicates the diode is openA low ohms reading in both directions indicates shorted diodeA normal diode will read resistance in one direction and infinite reading in the other direction4If diode is defective, replace power packthe 155 model ignition system, the port side side are two identical ignition systems, and the sameTherefore, the checks will apply to starboard ign ition system