1985 OMC 65, 100 and 155 HP Models Commercial Service Manual, PN 507450-D, Page 119Get this manual
If warning horn does not sound, ground the tan vacuum switch lead (from the engine wiring harness) to the powerhead with the key switch in "On" positionHorn shou Id sou ndIf the warning horn does not sound: Check voltage at the tan lead to vacuum switch with circuit testerIf voltage is not indicated: Check fuel warning system wiring circuitIf voltage is adequate but the horn did not sound when tan lead was groundedCheck for poor connections or defective hornIf warning horn sounds: Vacuum switch may be defectiveTurn key switch to "OFF" positionTo check sw itch, connect ohmmeter to terminal post and switch ground leadPump vacuum tester to value listed in chartOhmmeter should show continuity and drop to infinity when mercury drops below maximum toleranceIf not, replace switch