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2007 Merc 90 hp bottom cyl not firing

D

decibel

New member
Hi all. New poster here from FL.
Just found your forum. Looks like some great info here.

Problem started with a bad bog out of the hole. Pulled plugs and checked fire. No fire on bottom cylinder. New plugs. Swapped coils with Number 2 and symptom did not move. I am open to all suggestions but I suspect rectifier. Is there a way to test it or somehow narrow down better before i order one? Or am I even on the right track? Any help would be much appreciated.

Thanks, Jeff
 
Re: 07 Merc 90 bottom cyl not firing

might be your switch box(?).. think thats the right name anyway. Double check your spark plug wires first, make sure you dont have a hidden break.
 
Re: 07 Merc 90 bottom cyl not firing

Let's start with some basic info first - is this a 2 stroke (classic) 90 or a 4 stroke model (just so we know what parts are where)...
 
Re: 07 Merc 90 bottom cyl not firing

Thanks for the reply guys. Yes it is a 90 2 stroke, 2007. After thinking on it some more. I don't think it is the regulator/rectifier... It wouldn't necessarily be a problem with just one cylinder would it?

I'm leaning more toward the "trigger". If one leg of that is bad wouldn't that cause this problem? Is there a test method for checking it with my ohm meter?

I'm not sure this motor has a "switchbox"... I think it is CMD. Isn't all that done in there?
I really hope it isn't that thing as it would be wayyyy more expensive than the trigger.

I'm gonna try and get by and get a SELOC manual tomorrow, if they have one in stock. If not I could order one from here. Would that type of manual help me with test procedures for this type of thing?
 
Re: 07 Merc 90 bottom cyl not firing

Spark plug wires appear to be fine. I swapped em around and the symptom didn't move with the wires....
 
Re: 07 Merc 90 bottom cyl not firing

One problem that these motors used to have in the small bore version yours is a large bore 3.5" Was that they would get warm and twist or the slide the cylinder sleeve in the bore either breaking a ring or the top of the piston or blocking off a portion of the ports and the engine would act just like a miss.

I replaced three triggers on the first one I found with this problem. What is the compression on that cylinder.

When I rebuild those motors I pin the sleeves before boring them.

just an FYI.
 
Re: 07 Merc 90 bottom cyl not firing

Hmmmmm Hope to goodness that aint the issue. I have no way to check compression here at home but just turning it over by hand with a pull bar at the top of the flywheel, I can feel three distinct compression strokes while rotating and all three feel pretty much the same.

Did some searching here, found some test procedures for the trigger for up to a 2006. Hope it's the same.

I just got back from outside. With the motor running, (on two cylinders) I put a meter on the leads from the trigger. Purple lead and Brown lead read pretty much open. White lead (bottom cylinder) read about 25 or so ohms... pretty much a short.

Sounds like the trigger is bad, am I on the right track???
 
Re: 07 Merc 90 bottom cyl not firing

I will check the test procedure tomorrow in the manual on the older triggers they were test with the engine off and trigger disconnected, I will confirm your finding at the shop tomorrow morning.
 
Re: 07 Merc 90 bottom cyl not firing

Dave, Thank you SOOOO much for the reply!

FYI I checked resistance on all three leads with the motor not running and leads disconnected first, before I crunk the thing up. All three (purple, brown and white) showed open on the meter (from each lead to engine ground). That's why I started it and took readings again. I really didn't know which way i was supposed to do it.... Just workin by feel here.

Heck if I had the dough, and the time... this time of the year the local shops are backed up... I'd just take in to the shop...

BUT, I'm a broke SOB and couldn't afford the shop labor rates. I've always worked on my own junk for just that reason. Just rebuilt the 22re in my 92 Yota. Runs like a sewin machine now. Anyway I feel like a pretty competent DIY kinda guy, just never had any experience with outboards. I hope this isn't out of my league cause my daughter wants to get back out on the water and soon. And this has gotta get fixed cause these times with her are getting shorter everyday that she gets older... soon enough she wont have time for good old dad... LOL
 
Re: 07 Merc 90 bottom cyl not firing

Well, crap. Installed a new trigger today. Still not firing on bottom cylinder.... Uuuugh.

Swapped coils, wires and even plugs again just to make sure...

What should I be looking for???
 
Re: 07 Merc 90 bottom cyl not firing

If you don't mind doing a little jury riggin' you can (sorta) test the switchbox.

If you swap the switchbox wires from a coil that is firing and wire them to your number 3 coil, then just turn the motor over to see if you get spark, that could confirm that the part of the switchbox that feeds cylinder 3 is bad.

Just keep track of where the wires go. And don't let it start - the timing will be out to lunch anyhow, but it could start popping on one cylinder.

The switchbox has 3 separate capacitors, one for each cylinder, a rectifier (interal) and a switch (controlled by the trigger) - plus a few blocking diodes.

Anyhow, it is not "uncommon" for one of the capacitors to fry...
 
Re: 07 Merc 90 bottom cyl not firing

Those are CDM modules on that motor, can you not just plug #3 connector into #2 or vice versa do a spark check and eliminate the module that is bad. Make sure all your grounds are tight and clean on the motor. check any fuses at the harness area as well. Make sure you ground all your spark plug wires when testing the spark do not leave any leads open ideally a spark checker is best.

If you plug #3 connector into #2 cdm/coil and you have spark on #2 then your CDM is bad on #3. Or if you plug #2 connector into #3 coil and you have spark on #3 then you have another issue.

Do not run the motor this way just check spark with the key on and turning over leave the plugs in the motor.
 
Re: 07 Merc 90 bottom cyl not firing

Thanks Dave - totally forgot we were talking about a NEW motor here
 
Re: 07 Merc 90 bottom cyl not firing

Dangit, linky didn't work

http://www.cdielectronics.com/downloads/troubleshooting%20guide/Mercury-Force%203%20Cylinder%20CDM.pdf
 
Re: 07 Merc 90 bottom cyl not firing

No in a CDM ignition each coil has it's own little switchbox built in (to put it simply).

The stator provides power to charge a capacitor at the coil and the trigger (sensor) will tell which coil to fire.

The parts lists I'm seeing may not be current enough, but I only see one part number listed for the CDM's - what's your serial number??
 
Re: 07 Merc 90 bottom cyl not firing

The latest parts list I can find only shows one part number for the CDM's - where are you seeing two different numbers? (now it's just curosity on my part)...
 
Re: 07 Merc 90 bottom cyl not firing

Each of the three cdm's have a sticker on the top of them. Actually, with them mounted, you can only read the top one. Unbolted, you can get to them.

The top two read:
2636 827509-A8

The bottom one:
2907 827509-A7

Why are they different?
 
Re: 07 Merc 90 bottom cyl not firing

Also, I don't know if this is related to my problem or not but the tachometer on this boat doesn't work...

I guess it is just a symptom of my bigger issue of not firing on the bottom cylinder?
 
Re: 07 Merc 90 bottom cyl not firing

Your CDM's are the same, they just came from different manufacture runs on the machine that pukes them out.

The 827509 part of the number is what's important - the first 4 numbers (may) indicate the plant/location they came from and the A8, A7 etc are simply the production run number itself (so your #3 CDM was produced and then the machine was off-line for a bit, or they switch facilities, and did another run of the CDM's and changed the end to A8.

That is normal with Merc and the longer the part stays in production the more potential "runs" there will be (so in 10 years seeing an A28 on the end would be normal).

I suspect it's mostly done that way incase they determine there is a mfg defect in a particular part - easier to either recall or put out service bulletin for a specific part.

To your tach. The tach "signal" gets it's feed from the regulator or rectifier which in and of itself does not contribute to the ignition.

However, a bad regulator/rectifier can cause current to "back up" and toast your stator (which then with both toasted parts would result in the otherwise unrelated issues) OR, the stator could fry and take out the regulator/rectifier and produce the same results.

OR, in addition to your other issue, you could simply have a toasted tach or bad connection in a harness somewhere, or a bad ground.
 
Just saw your post on the 90 hp 2007 merc. I just got my exact same motor outta the shop. After 4 weeks my dealer found the issue.

Middle cylinder had a bad coil pak

The ground wire internal to the ignition switch was burnt inside the switch.


He replaced the dash key switch and middle coil, it’s running perfect today.
 
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