Interrupt switch

Author Message   mjennings Visitor Posted on Saturday, May 10, 2008 - 08:05 pm:    I have an 87 Sunbird 3.0 Mercruiser Alpha One that I've finally finished rebuilding. I replaced pistons and head gasket and had it running fine for two outings. The other day I was idleing past the no wake bouys then gave it some throttle and it died. It restarted but this time just going into forward idle caused it to die. I've been told that it needs a new lower shift cable but after troubleshooting it seems the cable works fine. I've also removed the whole lower unit and checked shift mech. and looks ok. My question is what makes the black piece with the two spring wires on it move to engage the interrupt switch? I mean I know the cable is connected to the bracket using the barrel nut, but doesn't the black piece move for just a second to make the engine miss? Is there something in the cable to make that piece move to hit the actual electrical switch because mine doesn't. The cable moves when going into forward gear but the black piece doesn't causing a loud knock and then the prop engages. Any help will be greatly appreciated.   John McGrath Member Username: johnkillcare Post Number: 14 Registered: 04-2008 Posted on Saturday, May 10, 2008 - 10:22 pm:    The interupt switch should only operate when shifting out of gear, not into gear. It is the resistance of pulling it out of gear that operates the switch. If it is activating the switch when shifting into gear it is probably a tight cable, the lower one which goes to the outdrive.   Rod Stewart Senior Member Username: rod_stewart Post Number: 1275 Registered: 08-2006 Posted on Sunday, May 11, 2008 - 01:34 am:    This might help: Shift Interrupt Switch Problems (Not applicable to Bravo drive units) NOTE: The interrupt switch is located on a cast bracket where the shift cable from the throttle quadrant attaches. It is usually mounted at the rear of the engine on the upper exhaust elbow, or a bracket on the valve cover. The front cable is the upper one, and the lower cable runs from the bracket out behind the exhaust down pipe, and to the transom plate assembly. 1. The shift interrupt switch (aka “Kill” switch) is a normally open switch for carb applications. It’s roller (or plunger) normally sits centered in a “V” shaped notch in the actuating arm of the shift cable bracket assembly. The arm tends to stay centered due to a fairly stiff centering spring. 2. Normally, whenever the shift lever is moved from Fwd or Rev gear into neutral, there is a brief moment when the lower shift cable meets resistance. This is caused by the tendency of the rotating prop dog clutches to stay engaged when the prop is turning in water. This resistance overcomes the centering spring, and the switch roller rides out of the V notch and CLOSES the switch. 3. The closing of the switch then shorts out the points (or the Thunderbolt ignition module) and kills the engine’s spark. The engine momentarily dies and this small interruption allows the gear dog clutches to disengage. 4. Once the gears are disengaged, the resistance in the shift cable is relieved, and the centering spring returns the switch roller to the V notch. Spark resumes and the engine picks up to normal idle speed once again. This whole sequence happens in a fraction of a second, and is barely noticed by the operator. 5. Whenever the shifter is in neutral, Fwd, or Rev, the switch roller must be centered in the V notch. If it’s not, the switch will be closed and there will be no spark. The engine will not start or run, though it will still crank over. The position of the switch should be checked first if there is no spark. Also check the switch and wiring with an ohmmeter to make sure it is not shorted out even when the roller is in the V notch. 6. If the interrupt switch is not killing the ignition properly it will be very difficult to shift the drive out of gear, when the boat is in water. (It may still seem to shift fine during a driveway test, with no water resistance.) You can check the switch operation by manually depressing the switch roller with the engine idling. If the switch is working, the engine should die immediately. If it does not die, either the switch is not closing, or the wiring is faulty. 7. If the engine inexplicably dies when shifting into gear, or while accelerating or decelerating, the interrupt switch may be the cause. If the shift cables are out of adjustment, or if the lower shift cable is damaged or stiff (or the drive leg shift mechanism is stiff), this will put extra load on the actuator arm (sufficient to overcome the centering spring) and the switch will close, thus killing the engine. Rod   Bondo Senior Member Username: bondo Post Number: 1009 Registered: 12-2006 Posted on Sunday, May 11, 2008 - 10:43 am:    "It restarted but this time just going into forward idle caused it to die. I've been told that it needs a new lower shift cable but after troubleshooting it seems the cable works fine." Ayuh,...... I Doubt it..... Replace the Lower Shift Cable,+ your Problems will be behind you......   mjennings Visitor Posted on Sunday, May 11, 2008 - 11:26 am:    Alright, I'll try replacing the cable since that seems to be the concensus. How hard is it to replace? I have a Clymer manual that has helped a lot. It doesn't mention removing the bellhousing which is ok with me. Also, I've noticed that the retainer nut is accessible from behind the bellhousing-will that make it easier?   PDunning6 Visitor Posted on Sunday, May 11, 2008 - 06:36 pm:    Hello! Thanks for the info Rod Stewart (good tunes too!). Very helpful. I am experiencing #6 for sure....and possibly a little bit of #7. I am having the shift cables replaced this month (my mechanic owes me...long story) and when he does that I am going to fix the interrupt switch myself. They are very hard to find for my engine (1969 160hp inline 6). I am hoping this will fix all of my problems! If you know where I can pick up a switch...I am all ears! Glad I found this site. It was recommended on our slickcraft group site. Cheers. Patrick 1969 Grew Slickcraft 235

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