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Vacuum switch assembly

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  • Vacuum switch assembly

    Hi guys, I wanted to know if anybody knew how a vacuum switch is wired on a 1968 100hp evinrude 100882b. Where do the two wires coming off the switch go? When I started working on this motor I noticed that the switch was disconnected. I cleaned the carbs but it still won't start, it backfires and tries to start but won't. I think that the vacuum switch is one of the issues so I will start there. Any info will be helpful...thanks

  • #2
    Hoo boy, that question made me think awhile. The vacuum switch is in series with the red wire going to the voltage suppressor and pulse pack. It's purpose was to prevent voltage input to the pulse pack until the engine was cranking. And that was because under certain conditions the motor would fire and start running as soon as the key was turned on, without running the starter----sometimes even backwards!

    Having said all that, I believe some replacement pulse packs came with instructions to eliminate the vacuum switch. But I could be wrong on that point. And/or, possibly some motors didn't even have a vacuum switch in the first place. I think they were a mid-year change. But again, I could be wrong again.

    Bottom line is you need 12 Volt input to the pulse pack (red wire) before it can start and run.

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    • #3
      Yeah, after pondering on it some more, I do believe the replacement pulse packs had different circuitry inside and didn't have the phantom starting problem, so didn't need the vacuum switch. But did I mention, I could be wrong?

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      • #4
        Thank you for the input and diagram. One time I was trying to start it the motor did run backwards, I was like what the heck was that! The power pack has been replaced on this motor and it has ran the way it is but has been very hard to start and most of the time I would have to hold the key to the on position with the starter engaged for awhile then release it slowly and it would run. After it started I could stop and start it with ease.From your input it sounds like I am in the right spot I just don't know where to go from here with it.

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        • #5
          Originally posted by Doinit View Post
          most of the time I would have to hold the key to the on position with the starter engaged for awhile then release it slowly and it would run. After it started I could stop and start it with ease.From your input it sounds like I am in the right spot I just don't know where to go from here with it.
          sounds like a bad key switch
          1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - http://forums.iboats.com/forum/owner...988-rogue-2420

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          • #6
            Connect a voltmeter to the red wire going to the pulse pack and observe the voltage any time it should be firing. That is while cranking, when releasing the key, and at run position of the key. It must receive battery power to the pulse pack all through that cycle, or it won't start and continue running. I agree, it sounds like a bad key switch, but don't run out and buy a new one till you have proved it with voltage test.

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            • #7
              I thought it was a key switch so I by passed the switch and I would have to keep the selinoid hot for it to run as well and the starter was still engaged. I am going to see if the red wire has a reading on the volt meter. I was just tying to start it today and it fired through about two revolutions of the flywheel but will not continue running. I am also thinking that it is something basic like a plug but they were replaced recently. Trying not to spend money at guess work.

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              • #8
                How do I hook up a multi meter to the red wire? Will I need both leads from the multi meter to check volts from the red wire? I don't do much of this electrical stuff

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                • #9
                  Set your multimeter to test the lowest suitable DC Volts range. Meters vary, so I can't tell you what that is on yours. Ground your meter's black lead to a suitable place on engine. Connect your meter's red lead to the closest accessible point on the red lead to the pulse pack. You probably will have to probe into a disconnect fitting to do this. Your goal is to touch the meter's red lead as close as possible to the pulse pack. You may even have to pierce the wire's insulation with a needle if there is no other access.

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                  • #10
                    Ok, thank you, I will try that later today

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                    • #11
                      I put the meter on the wires and there is current going thru them. There is no red wire coming out of the pulse pack harness. There is purple,blue,green, and three black. 1 black is ground and 2 go under the flywheel, both of those have 7.5 volts. The purple has 12.5 . But the is no reading on any of them with just the key in the on posisition.

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                      • #12
                        So I have to assume it is a replacement pulse pack with non-original color codes. Also assume purple is what used to be red. (Purple is modern speak for ignition-on feed) They must all be wired correctly, because it does run, even if for a little bit.

                        Your task now is to trace the wires back to see why you lose the 12.5V on the purple/red wire with just the key on. I'd first check it at the key switch. All I or you or anybody else can do is make an educated guess, following the wiring diagram.

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                        • #13
                          Ok I will

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                          • #14
                            Ok , I did not have power when the key was on but now I do have power to the purple wire. Tried to start it and it fired but still did not run, only for a short time and it took a lot of cranking to get it to that point. It ran a little but key had to be held in on position.

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                            • #15
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