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Joe, Marine Mechanic
Category: Boat
Satisfied Customers: 7251
Experience:  ASE, electronics, Marine mechanical and electronics, pwc, wiring, all around repair tech,
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Mercury 402 won't start after welding. Motor ran doggedly

Customer Question

Mercury 402 won't start after welding.
Motor ran doggedly before repairs as carb float was 1/2 full of gas - now will not fire, even on Ether. Forgot to pull harness before welding.
Ohmic values are ok per SELOC manual and other sites - values as follows:
Stator - 5.9k across red and blue
Stator - 51.2 red to ground
Trigger - 850K between leads
Ignition Coils - .4 Ohms primary/ 1K secondary
DVA readings :
Stator - w/ rectifier disconnected 38V / 308 V
Stator - w/ rectifier connected 31V/ 296 V
Trigger - 1.5V across terminals / .7V to ground each terminal
Ign Coils - input 220V and 207V coil 1 and 2
Ign Coils - output 20V and 17V with DVA, else meter reads .5v each on DC scale without DVA.
Plugs spark outside of engine- after welding plug one had 1 ohm short due to aluminum build up between electrode and ground - this was removed and plug reads open now.
Switch box was replaced since kill lead read 1 Ohm short to cylinder one - with new switch box it read .7 Megaohms. No voltage detected on kill lead from key switch
Appreciate any insight.
Submitted: 1 year ago.
Category: Boat
Expert:  Joe replied 1 year ago.


Thank you for allowing me to assist you, my name is ***** ***** I will be happy to assist you with your concern today.

I am just seeing if you still need assistance with this problem? The reason I ask is because I see that your question has not been answered sooner and I wanted to make sure if you still needed help.

In some cases a question may sit due to several different things including the amount of the deposit/question value, time it was asked, the complexity of the question, number of experts that can help you at a certain time, lack of detailed info and so on. My objective that I wish is to make sure that if you still need help that I can at the very least reach out to you and attempt to help you. I may not have exactly the experience to deal with a certain problem but instead of you not receiving any answer at all I would at least like to reach out and attempt to help you.

At this time please feel free when you are ready to reply back to me and ask to continue working with me, another expert or if you no longer need help please just let me know exactly what you wish to do.



Customer: replied 1 year ago.
No one has answered my question yet
Expert:  Joe replied 1 year ago.

Good morning,

You should not be reading 17 or 20 v on the coils, the pos + side of the coil should read 12v dc and the neg should pulse with a test light during cranking. There is an extremely high chance it took out the switch/cdi box, which you found and replaced it looks like.

Your going to have to go through all of your electronics, what took place can damage more than one thing, in fact it can overload an entire system and fry so many things that it is hard to find them all.

What happens is the stray current makes it's way through the circuits since they are left as completed circuits, too many amps and it starts frying things, you will also need to visually inspect all wires as there's a chance it also melted wires.

Thank you!

- Joe

Please rate my answer when ready. Bonuses are appreciated.

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I'd be happy to answer any other questions you have even if not related.

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Customer: replied 1 year ago.
I realized my output reading on the coils were done incorrectly - with no 'load' - i.e. I was reading off the plug wire with no plug attached- plugs do spark outside of motor while cranking. Engine won't fire on Ether. Readings are from new Switch box.I don't have 12v going into coils; I have (DVA reading) 220v and 207 respectively on green input leads from Switchbox at positve terminal of coil. Is this incorrect?
Expert:  Joe replied 1 year ago.

Is that with your voltmeter set to dc voltage? there should be no way you are seeing 220v, these are 12v systems. The stator will produce ac voltage and the regulator converts into dc and regulates it but nothing near 220v be that ac or dc 220v is way too high and honestly so much so it almost seems to suggest the meter setting.

In your question you stated "will not fire" and thus my understanding no spark as that is what no fire refers to, thank you for the clarification on that. You have spark but, using an in-line spark tester or gap tester, how good is the spark? And does it miss?

You in a nut shell need correct spark and timing, compression, air and fuel to get it to start.... we need to find what is missing.



Customer: replied 1 year ago.
Meter was set to DC Voltage - auto range to 1000V. Reading was taken using DVA adapter. Conversely the Stator readings were 31V and 296V using DVA adapter set to same scale as above. Per troubleshooting guide on the Switchbox ,the Stator outputs are correct (30V + on low voltage lead and 180V+ on High Voltage Lead). To wit, this High Voltage from the Stator seems to be where I'm getting the high voltage input to the ignition coils- Thunderbolt Ignition.I'll get an in-line spark plug tester and report back.
Expert:  Joe replied 1 year ago.

Now I understand what you were explaining, sorry I missed that, somehow I didn't realize you were using an adapter.

Since you have spark I am not worried about testing the voltages at this time on the coil such as pulse - , because you do have spark.

Since you are needing to check strength, color and overall spark, this is the type of tester I am referring to:



Customer: replied 1 year ago.
Used an single -adjustable spark-gap tester - I tested to 30KV at approx 3/4 of an inch gap. Sparks were blue in color and regular in interval.
Expert:  Joe replied 1 year ago.

Good morning,

That seems to indicate good spark then.

Question, when you spray starting fluid into the carb throat does the engine have any signs of life or change?



Customer: replied 1 year ago.
No but I may not be using enough - I used a quick spray at the screen cover. I did get the motor to back-fire twice last night with no Ether, extended cranking and using the choke. I have set the Idle screw to 1 full turn open from fully seated. I was going to try and get some fresh fuel tonight just to be sure I wasn't overlooking the obvious.
Customer: replied 1 year ago.
Tried fresh fuel and no change, other that the backfire occasionally - I even pulled one plug at a time to see if I could isolate the bafire to one or both cylinders. I was able to get both to backfire with the opposite plug pulled. As a last resort I performed a compression test and both cylinder registered 130 psi. I located the TDC mark on the flywheel and will try tweaking the trigger alignment tonight
Expert:  Joe replied 1 year ago.

Did you try more starting fluid again?



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