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, 1979 Mercury 40 HP 2 Stroke Outboard Motor Problems -

Customer Question
Hi ***** *****....

Hi ***** *****. My name is Steve.

Mechanic's Assistant: Thanks. Can you give me any more details about your issue?

1979 Mercury 40 HP 2 Stroke Outboard Motor Problems - Need Help!Serial Number #5253313This motor would run about 10 minutes and then shut off. Wait about 2 hours and it would start again, and run about 10 minutes and then shut off. It did this several times, then it got to the point that it would not start at all.Here is what I have done thus far.I have replaced all the old parts with NEW ones:Stator: #18-5856Switch Box: #332-4911A5SolenoidWiring Harness on Motor has been replacedRectifier: # ***** Installed this one brand new. When I tried to start the engine, the Rectifier locked OPEN and is now ruined. So I replaced this with the CDI unit noted below.CDI Regulator Rectifier: #194-5279 There is still no fire to the plugs. Once you DISCONNECT the Yellow Wires from the Stator to the Rectifier you have fire; makes no difference whether you turn it over with the key switch, or take the wiring harness loose from the engine and pull it with the rope.Also, if the Yellow Wires ARE CONNECTED no matter how you try to start the engine it WILL NOT start.The ORANGE WIRE from the kill switch has been disconnected the whole time I have been doing these tests.

Submitted: 2 years ago.Category: Boat
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Answered in 46 minutes by:
1/23/2016
Marine Mechanic: Jeff G., Marine Mechanic replied 2 years ago
Jeff G.
Jeff G., Marine Mechanic
Category: Boat
Satisfied Customers: 4,728
Experience: 42 years experience, Certified Master Technician, All Major Outboards, Sterndrives, Hi-Performance
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Can you take a picture of the wiring, particularly going to the rectifier, and the switchbox? If you can't then can you do a detailed diagram of the wiring?

I'll be in and out and will respond as soon as I can.

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Customer reply replied 2 years ago
Hi Jeff,I have 5 pictures for you. Trying to upload here but they won't go. Is there something I am not doing correctly.
When I drag and drop the pics from my desktop the whole page I am on now goes away and no send button.
Thanks,
Steve
Customer reply replied 2 years ago
Here you go. Got it figured out.
IMG_0283.JPGIMG_0283.JPGIMG_0286.JPGIMG_0286.JPGIMG_0279.JPGIMG_0279.JPGIMG_0284.JPGIMG_0284.JPGIMG_0289.JPGIMG_0289.JPG
Marine Mechanic: Jeff G., Marine Mechanic replied 2 years ago

What happens if you disconnect the gray tachometer wire?

Is the picture of before the CDI rectifier?

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Customer reply replied 2 years ago
I took off the gray tachometer wire and it made no difference; it still did not fire.
The CDI that you are looking at came off a running engine and tested good.
Marine Mechanic: Jeff G., Marine Mechanic replied 2 years ago

That's not the correct rectifier for that engine.

Your stator has both the alternator coils, the yellow wires, and the charge coils for the switchbox, which supply the power for the ignition. They are two separate circuits with no common connections. However if the rectifier is not working properly and backfeeding voltage from the rectifier to the stator it can interfere with the charge coils, the red and blue wires from the stator.

Since in your case the ignition doesn't work with the rectifier connected that would be the problem.

You can damage a rectifier by having the external wiring harness disconnected.

Put an ohm meter between one yellow wire from the rectifier and the red wire, everything disconnected. Next reverse the meter leads, repeat on the other yellow wire, what are your results?

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Customer reply replied 2 years ago
each yellow to red is 30 ohms the cdi regulater rectifier will by info from them do from 1966 to 2006, 30-200 hp I had a new rectifier 816770 on the motor still all the same problems and yes I have some four letter words on my toung over this thank you
Customer reply replied 2 years ago
23 January 2016 02:35each yellow to red is 30 ohms the cdi regulater rectifier will by info from them do from 1966 to 2006, 30-200 hp I had a new rectifier 816770 on the motor still all the same problems and yes I have some four letter words on my toung over this thank you
Marine Mechanic: Jeff G., Marine Mechanic replied 2 years ago

From yellow to red is 30 ohms correct, now reverse the leads. So if the red was on the yellow wire, put the red on the red wire. What is the reading?

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Customer reply replied 2 years ago
red to red 0 ohm gray to red 4000 ohms but after testing the red to red I tested yellow wires again to red I got no reading all , wires are disconnected
Marine Mechanic: Jeff G., Marine Mechanic replied 2 years ago

OK Yellow to red 30 ohms and when you reverse the leads so red to yellow 0 ohms correct? This on both yellows? Then the diodes are OK. All the other reading don't mean anything.

Can you do a resistance reading from the red and then blue wires to ground disconnected from the switchbox?

Is the switchbox properly grounded?

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Customer reply replied 2 years ago
Jeff,
1. OK Yellow to red 30 ohms and when you reverse the leads so red to yellow 0 ohms correct? This on both yellows? Then the diodes are OK. All the other reading don't mean anything.1. Answer:
Yellow to Red = 30 ohms; Red to Yellow = 0 ohms.
Same on both Yellows.2. Can you do a resistance reading from the red and then blue wires to ground disconnected from the switchbox?2. Answer:
Red wire to ground from Stator = 60 ohms
Blue wire to ground from Stator = 6000 ohms
Brown and White wire to trigger = 1200 ohmsSwitchbox wires:
Red, Blue, and Orange = No reading at all
White to ground = 1150 ohms
Brown to ground = 1150 ohms3. Is the switchbox properly grounded?3. Answer:
I checked the switchbox with the ohm meter from the switchbox to the motor ground. I got 0 ohms.
Marine Mechanic: Jeff G., Marine Mechanic replied 2 years ago

Your readings are OK for the stator charge coils as well.

Make sure the switchbox is grounded well.

I'll be honest from what you are posting I'm not sure what exactly the problem is. I suspect somehow the rectifier is still bad. The correct rectifier is simply a rectifier, not a regulator and has only three terminals two for the stator and one for the 12V+.

So I would try another rectifier. Other than that without working on it myself, I don't know. I have been working on those engines since new and never ran into this particular problem.

I hate to have you parts change but that may be the only way.

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Customer reply replied 2 years ago
Jeff,
There has been more than one person baffled by this problem. I have been working on this motor for some time now. I am known for being able fix most any kind of motor, a talent that I have been blessed with.
I am 63 years old and have worked on engines for a long time, and have NEVER run into this problem either. It is frustrating; I am thinking about taking it to the lake letting it sink. Chuckle, chuckle.
Monday I will run a special ground wire from the battery to the switchbox and rectifier to make sure both are grounded well.
Jeff if I get it fixed I will tell you exactly what I did to fix it. Will let you know how things go.
Marine Mechanic: Jeff G., Marine Mechanic replied 2 years ago

Just make sure the case of the switchbox, rectifier and the stator are all grounded.

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Customer reply replied 2 years ago
Hi Jeff,
Sorry to take so long to respond. I put on a brand new regular rectifier; took the CDI off and it did not help. Still have the same problem. The motor does not run yet. No more time to work on it right now. But still does not run. Thank you for your help. One day I will get it running.
Marine Mechanic: Jeff G., Marine Mechanic replied 2 years ago

Let me know if I can help further.

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