75 with no spark then spark

[svarn8]

I am chasing a gremlin. I have changed out stator switch box trigger rectifier and flywheel. it is a 1994 75hp mariner. It will crank and run perfect all day then it will not fire for 3 hours to 3 days then it will crank right up and run perfect . I have 3 of these 75 hp motors so the parts were easily accessible. I bought a dva and have a friend that works on mercury's every day. He is yamaha certified. but we can not figure out why it will run 1 minute and then not, yes I have unplugged black and yellow wire, kill wire. and swapped out wiring harness. any ideas??

[svarn8]

carbs are clean and has all new fuel lines, water seperater, and fuel tank .

[svarn8]

out of all of the electrical parts which one would be the most likely to teeter totter between working and not working I am leaning towards ordering a new switch box .

[Whaaaaat]

Went thru a very similar situation recently. Turned out to be the engine harness. Must have gotten wet or something inside the cannon plug. There where wires talking to each other that shouldn't have been.

[svarn8]

I disconnected the plug at the motor and tried starting it with a trigger start button and it still would not run. I switched out harness from cc back to motor also unplugged the black and yellow kill wire. I will see if I can switch out engine harness on motor. thank you for the idea.

[Whaaaaat]

Don't need to swap out the whole thing, just disconnect wires and ohm them to the other wires. With everything disconnected all wires should read open

[svarn8]

since it is running today. we rechecked the stator with the dva and are getting readings from 397 to 465 then a spike up to 700 specs are 180-400 so it now appears to be over charging 2 weeks ago it was 280-300. It has been an exciting work in progress every time i check something it changes.

[svarn8]

I have since retested the stator and numbers are back at 294-297 I believe he had his meter set to ac instead of dc. so I rechecked it and it is in range. I have the red stator with the shunt resistor. also tested at the blue wire and get 294-297 . I am baffled why it is running perfect today and would not crank 2 days ago. Is there any other wire in the harness other than the black wire with yellow tracer that could cause no spark.

[svarn8]

I disconnected the harness plug from the motor and tested it and all ports were open. I wiggled wires and all seemed fine. I also disconnected all wires on motor and cleaned them and checked for corrosion and breaks. all wires are clean and reconnected.

[hsbob]

try spraying a cool spray when it quits work. this could rule out on phase of the box's defect.

it does sound like a connection or wire being corusion [sp] issue. i had it on the wires connectors to a control box [on a25hp]. i finely sand all connectors and problem solved.

[perfmarine1]

Have you tried running with voltage reg unhooked?

[svarn8]

I have not tried it with all vr wires disconnected.. I did try unplugging the 2 yellow wires that go to vr when it was not firing, but it did not help. when it looses spark it is just out of no where and then all of a sudden it will crank again. I am planning on replacing the positive and negative cables tomorrow. thats about the only thing I have not swapped out at some point. the last 4 days it has fired every time I touch the key.

[svarn8]

today I switched out the ground cable to the battery and it turns over fine but no spark today. can the switch box be this sporadic?

[Whaaaaat]

Try with boat harness disconnected. Still sounds like a wiring issue.

[svarn8]

I have tried that would not crank. Went back out this evening after a light shower which dropped the temp about 15 degrees. It fired right up. I ran it 3 different times and fires right up.. I am wondering if heat is causing switch box to act up. I am going to try the cold spray as mentioned by hsbob . I was going to do it today, but could not find my can of compressed air. I think I finished it off working on the bike. Thanks for all of the suggestions. I will figure this thing out. All of the suggestions really help the thought process, thank you again.

[svarn8]

park it in hot sun it won't run, park it in shade and it will, cold spray on switch box did not cause it to crank. I am getting 225 volts going into switch box via[ blue wire and ground] I am getting a reading of 5 on green wires and ground on coils. I deem switch box is bad, but for no reason let it cool off this evening and I bet it cranks up..