electrical diagnosis...proper sequence

[fishndvm]

If this sucker was a dog I could deal with it. But it isn't...which is why I come here for a diagnostic plan! I've got a 1986, 25hp, pull start Merc 25XD, ser # A193883. I acquired it from a buddy of mine who is meticulous at keeping his equipment in tip top shape. He gave me the engine some time ago, and I have replaced the impeller and broke down and rebuilt the carb. Today I checked for spark and...nothing. My buddy swears that when he laid this girl to rest about ten years ago, she was purring like a kitten. I believe him. What is the proper sequence to follow to try and diagnose the problem? Other than making sure that the lanyard is attached to the stop (safety) switch, I'm not sure where I go from here and in what logical order. Any help would be greatly appreciated. Yes, I guess I could get it to a Merc dealer, but then I would learn absolutely nothing. Not my style. Thanks a million. By the way, it seems this model runs on spark plugs that do not have the usual electrode at the end, but rather a little nipple. I'm not sure what these types of plugs are called. And yes, I have a factory manual. Sometimes this manual is worth it's weight in horse manure.

 

[DHPMARINE]

Re: electrical diagnosis...proper sequence

You have surface gap spark plugs.Is there a part of the manual you don't understand? Can we help you with it ?<br /><br />DHP

 

[Silvertip]

Re: electrical diagnosis...proper sequence

If the manual you have is a factory manual you have the best documentation available. If its a generic (all models in one book) style it may be lacking in detail. That said, Just because the kill switch lanyard is connected doesn't mean the switch it operates is in the OPEN position. You need an ohm meter or test light to verify it. Lanyard on = open. Lanyard off = closed = no start. How did you test for spark?

 

[fishndvm]

Re: electrical diagnosis...proper sequence

Thank you both for your input. I have a real manual. For whatever reason, there is no flow chart in the ignition circuit section for troubleshooting as there is in the battery/starter section. The wiring diagram for the manual start model shows a stop switch, but my engine actually has two it appears. One is the lanyard switch, and the other is at the end of the tiller handle. I'm not sure why it doesn't show them both.<br /> The manual states that I can test the stator with an Ohm meter, and do a cursory test on the coils with the same. However, it also states that the coils could test OK with the Ohm meter, and still be shot. This then would require the use of a "Quicksilver MultiMeter/DVA Tester" or a volt meter capable of measuring 400 volts DC and using a "Quicksilver Direct Voltage Adapter". Then it says to follow the instructions in the meter's instruction booklet to perform the tests! I'm assuming that these tests are performed on a running engine. Well how does it run if there's no spark? <br /> If I did it correctly, I disconnected the kill swithes and cranked her over numerous times. Still nothing. Where do I go from here? Is the test sequence on the various components in the order as they appear in the manual? And once again, it isn't running. Are the tests possible without it running? Yes I'm new at this, so I may be missing something very basic here. Can anyone tell me what it is?<br /> Thanks just for reading this.

 

[Laddies]

Re: electrical diagnosis...proper sequence

OHM test the stator, if you have no spark with the blk/yellow wire disconnected at the switch box, if the stator tests good you probably have a bad box as a bad coil would still have spark on the other cyl.

 

[Silvertip]

Re: electrical diagnosis...proper sequence

You tested the kill switch and proved it wasn't the problem. You need to also test the tiller kill switch to prove its not the problem. Also check the wiring to and from both of these switches. If they are pinched anywhere and make a ground connection, that represents another "kill switch" If these checks are ok, you need to get deeper into the ignition system. I seriously doubt its a coil problem as both would not likely go bad at the same time. That pretty much leaves the box or the stator. Ohms tests are electrical resistance measurements and are not done on a running engine. Voltage tests can be done at cranking speeds.