Question about CDM or ignition coil on outboard engine?

CAnderson

CAnderson

Well-known member
Here is what I have: 1996 Mercury SeaPro, 40hp, 2-stroke, 2cyl, tiller handle.

Symptoms, rough idle, will gain RPM until about 1/4 - 1/2 throttle and then, only run at a little above idle from 1/2 to WOT. The engine will not move the boat above trolling speed, which normally runs 27mph at normal hunting load. The issue seemed intermittent and would usually raise it's head on cold days. I don't believe that is the case anymore, but don't want to deal with the issue even if it is only intermittent. A big wind blows into the bay and the engine decides not to run right, that could be a real bad day.

- I replaced the fuel line expecting it to be a fuel delivery issue and had suspected that there was a leak somewhere in the line as the fuel bulb would never get firm. Now, new fuel line, bulb stays firm and issue is still present.
- Next was to replace the inline fuel filter and water/fuel separator canister. Both were due to be changed anyway. I inspected both. Neither showed signs on contamination or sediment to suggest the old fuel line was breaking down. Inspection of the old fuel line (only 2 years old) showed no signs of breaking down either. With both filters changed, no improvement in problem.
- Next was to inspect the fuel pump assembly to determine if there were any problems present inside. No problems were seen, new gaskets and such were installed as normal maintenance items.
- Last inspection on the fuel side of things was to remove the pick-up tube assembly from the tank. No issues, no sediment in the tank, no clogging of the screen, etc.
- Next was to move to the ignition side of things. I started the engine and she idled a little roughly. I would describe the sound as being like a lopey cam in a muscle car setup for a little street strip racing. (Mind you, this 40 hp, 2 cyl engine is no modified race engine). I pulled the plug wire from the #1 cylinder. Engine continued to run without any noticeable change in idle sound.
- After replacing the wire on cylinder #1, I pulled the plug wire from #2 cylinder. The engine began to run noticeably rougher and began to sputter. When I pulled the spark plug wire from the #2 cylinder, I also got a little shock. This suggested two things, there is likely something wrong with CDM on cylinder #1, and i need to replace the plug wires on both cylinders and consider it a normal maintenance interval.
- I repeated the test above, but this time, disconnected the wire harness for each CDM, one at a time. Again, when the CDM was disconnected from cylinder #1 there was no noticeable difference in the idle. When I disconnected the CDM wiring harness form cylinder #2, the engine sputtered worse than before and stopped running.

At this point, I believe the CDM on cylinder #1 is bad and should be replaced.

1. Can anyone confirm that my test accurately leads me to that conclusion or am I missing anything else in the puzzle?

2. Should the CDM units be replaced in pairs? Essentially, should I suck up the cost and replace both, perhaps holding onto the one from cylinder #2 as a back up. They are $113 each. Not a hateful cost and certainly MUCH cheaper than buying a new engine.

Thanks for your thoughts, time, opinions, experience and suggestions.

Chad A
 
Clark, thanks for the offer. I will shoot you a PM with my address.

I can check plugs, but they are brand new. I just replaced them last week as a maintenance item that i change each year.

I can swap the CDMs from cylinder 1 to cylinder 2. I assume, what I should see is the same as the first test, just this time the engine should die when I disconnect cylinder 1.

But then I guess, if the engine continues to run with the CDM from cylinder 2 connected to cylinder 1, then that might suggest the problem is elsewhere and before the CDM itself.

Is that logic correct?
 
Clark, thanks for the offer. I will shoot you a PM with my address.

I can check plugs, but they are brand new. I just replaced them last week as a maintenance item that i change each year.

I can swap the CDMs from cylinder 1 to cylinder 2. I assume, what I should see is the same as the first test, just this time the engine should die when I disconnect cylinder 1.

But then I guess, if the engine continues to run with the CDM from cylinder 2 connected to cylinder 1, then that might suggest the problem is elsewhere and before the CDM itself.

Is that logic correct? Yes,that is my thinking,it will verify one way or another right away.
Click to expand...
 
Thanks Steve. I pulled out my factory service manual (FSM) and there is a pretty complete testing procedure listed in the manual to test the CDM. I will have to get my meter from work tomorrow.

I will swap the CDMs first to complete that line of logic and then I will go through the testing procedures outlined in the FSM in hopes the results confirm the prior findings. The FSM shows some similar symptoms shared between the CDM and TPM so I will definitely need to narrow down which is the problem.


Ouch, just looked up the price on the TPM! Let's just continue to hope that it is the CDM. I can replace both of the CDMs twice for the cost of one TPM.

Chad
 
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Chad,
My '84 Merc 50hp had similar symptoms a few years ago. I found some online troubleshooting procedures and determined it was the trigger module. Is that the same as the "TPM"? The Merc dealer wanted two-hundred-something dollars for it. I found a brand new one on eBay for under $100. I installed it per instructions and the motor has run like a million bucks ever since. Sounds like you have a pretty good grip on the troubleshooting. I just thought I'd mention looking at alternative sources for the part; it saved me more than a few dollars.
Gordon
 
Thanks for the suggestion Gordon. The TPM is the timing protection module. In my case, the factory module costs around $400 but CDI appears to make one for around $273.

I did just notice that engines utilizing a TPM require an inductive type spark plug. I know for a fact that I do not have inductive type spark plugs in my engine currently. That may be part of the issue. I am going to have to do sme reading on what the symptoms would be for using non-inductive plugs when required to use inductive plugs. The new plugs will be about $16-20 for both vs $8. That would be the cheapest and simplest solution.


Hope that just made sense.

Chad
 
Swapped in the upgraded inductive resistor spark plugs. No noticeable change in the way the engine ran. I then went through the series of diagnostic tests for each CDM. Both tested well within specifications and thus don't appear to be contributing to the problem.


Next up is running through the testing procedures for the TPM and Stator.


Chad
 
from everything you have said I would check you r stater just did the same thing on a 40 Johnson now it is running good
 
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