MCM 3.0 keeps frying ign modules

[Delmagtech]

I have a friends boat coming to me this evening to look at, and I've been doing some research on-line as to what's going on. I thought I would post about it here to see if anyone has been where I'm going.

The boat is an 01 Caravelle 176 with a Mercruiser MCM 3.0 / Alpha.


A month ago, she took this boat to her normal guy for maintenece. He apparently fixed some problem with the nuetral button in the shifter. She took the boat out, and about the time she engaged WOT, it died. Once it died, it wouldn't start again. She brought it back. He figured out that the ign. module was bad. He put one in from another boat, and it fried it almost immediately. He knew it was fried because it wouldn't work when he put back in the original boat.

He sent her to mech #2. Mech #2 replaced module, and fried it himself. Then he said that the gray signal wire going to the tach was shorted, and causing the module failure. Fixed said short in gray wire, installed another module and sent her on her way. She took it on vacation, got if off the trailer, went WOT and it's fried again.

She took the boat to a third mech in the vacation city, he installed a new module, and it was more of the same. It's been on the trailer since.


So now, I'm going to give it a try.

I've read that bad / corroded battery cables can create current spikes that cause big problems for the module, so I'll be looking at that first. Seems along that line of thought cleaning up grounds might be just as important.

The problem seemed to start when the guy fixed something with the nuetral button in the shifter. I can look at that as well, but I've not read a single post where problems with the electrical switch in the shifter (I assume this pulls timing while going in / out of gear) caused damage to the module.

Lastly, I'll check out this gray signal wire going to the tach, but here again, I've read plenty of posts about tach problems, some even claiming a bad tach kept the engine from running, but none yet about permanent damage to the module.

It's a little baffling as the system seems relatively simple. She says with a new module it will cold start and idle all day. But as soon as she opens it up, it's done. Sometimes, after sitting all night, the boat will start again.

I'm wondering if the ignition coil is shorting out when it gets warm or is asked for full load. Im also curious as to whether a problem with the pickup coil could cause issue here. Lastly, I see the cable that connects the module to the coil is pretty pricey, which leads me to think it might more than just a pair of wires. Diodes, resistors or anything else in there prone to this problem?

If you guys have any pointers for me, I'd sure appreciate it. I'm going to start with battery cables and looking at the switch in the shifter.

 

[Bt Doctur]

The most important one is the voltage going to it. What the alt output at idle an above? Watch the meter when doing the test, NOT THE VOLTMETER IN THE BOAT

 

[Delmagtech]

Thanks. I'll use my meter. Hopefully I can get it to run such that I can check the charging voltage. Would you say overcharging is likely to damage it, or undercharging? Safe for me to assume I should see roughly between 13.5 and 14 when charging?

 

[Bt Doctur]

running wild it could go up to 32V , corroded cable connections can cause a spike especially the neg block one.

 

[Delmagtech]

It fired right up for me on the garden hose. It ran great for 10-15 minutes and petered out. I pulled the spark arrestor and grabbed a can of ether. It tried to start a few times, but the ether was hurting rather than helping, so I guess I can still be confidant it's still a spark issue as opposed to fuel.

Battery connections all over look like they could benefit from coming apart and cleaned, but nothing that would bother me if I weren't looking for it.

System was charging just over 14 volts the whole time. I even revved it up pretty high a couple times and the 14 didn't fluctuate.

Afterwards, the ignition coil was cool to the touch and the module was warm. Cap and wires look pretty new, and she says they, and the rotor are recent. I'll pop the cap tomorrow. Being temperature related, I can't see how it's not either the coil or the module, but I hate to ruin another one...

 

[Delmagtech]

Dug a little deeper last night...


Boat ran great on muffs for 10 minutes again (as expected). About 14vdc while running. Electrical switch on shifter linkage does kill engine when I press it up or down.

After ignition failure, I stripped back a little purple wire coating near the ign coil plug. measured 14vdc with key on, and while cranking. When no start, I used an alligator clip and connected straight to battery. No change.

I disconnected wires to the switch on the shifter linkage switch. No change

I jumped wires that we connected to above linkage switch together. No change

I disconnected gray signal wire from tachometer. No change.

Removed cap and rotor. very little corrosion. Scuffed all with emery cloth and reinstalled. No change.

Removed harness that connects module to ign coil. no resistance when measured with dmm from one end to the other. Stripped some wire coating on either side of the resistor or filter or whatever it is in the pink wire. alligator clipped around it. Attempted to start. No change.

I'm fairly convinced it's a bad module, just like the last three guys said. Or it could be that once the coil warms up, it's failing. The bigger challenge in my mind is figuring why they keep getting fried. I have to order parts on line. I'm tempted to grab a module, coil, and harness. Only have found one set of grounds on back of engine could benefit from cleaning.

 

[fishrdan]

I agree with checking for loose/corroded wires. Have you checked the ignition switch and cannon plug yet? Might not hurt checking for AC voltage at the alternator.

I'm tempted to grab a module, coil, and harness.

A complete 3.0L EST ignition kit (distributor, wires, cap, rotor, coil, wiring, etc.) is only $200.....

 

[Delmagtech]

Thanks, fishrdan. I didn't even think about a kit. While it doesn't need all of those parts, it will benefit from them. I had all the separate parts in a basket, so you saved me some money, not to mention a basically free replacement distributor. I didn't look much at the key switch because I still had power after the failure. If the cannon plug is the large round plug where the engine harness connects to the hull harness, I did open that. The pins and sockets were clean as a whistle. I'll take a look at the alternator connections and clean up the engine grounds this week while I'm waiting on the new parts to arrive. It did just occur to me that while checking voltage (when it was cold and running) I was measuring voltage at the purple wire connecting to the coil, but my neg. lead was connected to the battery. If a dirty ground was causing spikes and killing the module, I wouldn't have seen it. I should have grounded to the grey wire to be sure I guess. Anyway, thanks much for the input. The best deal I could find on the kit after considering shipping 235 / 248.50 delivered. Seems like a good value to me.

 

[Delmagtech]

So I got my ign kit yesterday. As much as I wanted to slap it all on there, I decided to keep digging. I discovered the following:


The boat runs MUCH longer than the normal 10 minutes before developing it's no-spark condition with a battery charger connected. I learned that accidently.


Battery cable connections were much worse than I thought after removing. I used a air sander to remove corrosion and added some electrical grease when putting back. The same was true for the ground connections at a few bell bolts.

I systematically tried the new coil, new coil harness, and cap/rotor to see if it would remove the problem once it finally quit. One at a time. None did.

I guess its, in fact, the module.

I don't know whether to remove and unwrap the engine wiring harness to look for more corrosion problems, or just reroute a new power and ground for the ignition. I feel like I have to do one or the other before installing the new parts.

 

[Delmagtech]

I'm studying the wiring schematic and a question.

I understand the purple wire to be 12v to the coil from the ignition.

I had thought the gray wire was the ground for the coil, and that the tach tied into this for signal.

In looking at the schematic, that gray wire goes straight from the coil to the tach and never hits a common ground.

That means that either the coil is actually getting it's ground source from the tach, or it's getting a ground elsewhere. Surely the coil isn't being grounded by it's mounting bolts?

Can someone steer me right on the coil ground?

Thanks in advance...

 

[alldodge]

I'm studying the wiring schematic and a question.

I understand the purple wire to be 12v to the coil from the ignition.

I had thought the gray wire was the ground for the coil, and that the tach tied into this for signal.

In looking at the schematic, that gray wire goes straight from the coil to the tach and never hits a common ground.

That means that either the coil is actually getting it's ground source from the tach, or it's getting a ground elsewhere. Surely the coil isn't being grounded by it's mounting bolts?

Can someone steer me right on the coil ground?

Thanks in advance...

Below is a pic of the basic points distributer setup. The electronic works the same way but instead of the points breaking the connection, it is done electronically. The electronic version does not use a ballast resistor connection. The coil is charged on the primary winding side when a ground is placed on the - side of the coil is grounded. When the ground is removed the EMF changes the 12V to 22000V and is transfer via the secondary winding. The gray wire going to the tach is there for the tech to count pulses and give a reading in RPMs. This is way if your having problems with spark, one of the first things to do is remove the gray wire from the coil. If the problem goes away it's the tach.



http://www.youtube.com/watch?v=W94iksaQwUo

 

[Delmagtech]

Thanks much, Alldodge. After having installed dozens of MSD ignitions over the years, I'm embarrassed to admit I didn't really understand them correctly.

The question came because Im trying to make sure the pertinent connections are healthy so as not to damage any more modules.

 

[Delmagtech]

Should it bother me that I'm reading 14.5 volts at the battery with my meter, at idle, and 13.9 where the purple wire connects to the coil (also while idling). I had already disconnected the gray wire at the tach the other night with no luck. Tonight it ran 8 minutes before it quit. When it did, I cut the gray wire right at the coil. I also unplugged the connector that brings 12v from the interrupt switch. There's nothing left... I've cleaned every connection I can find. It's getting the new parts with crossed fingers.

 

[alldodge]

Should it bother me that I'm reading 14.5 volts at the battery with my meter, at idle, and 13.9 where the purple wire connects to the coil (also while idling). I had already disconnected the gray wire at the tach the other night with no luck. Tonight it ran 8 minutes before it quit. When it did, I cut the gray wire right at the coil. I also unplugged the connector that brings 12v from the interrupt switch. There's nothing left... I've cleaned every connection I can find. It's getting the new parts with crossed fingers.

There is something in the mix we are not seeing and right now I as well do not know what it is. The voltage readings your seeing do not give me any concern. Lets start from scratch, what kind of distributer is in the boat, what is the module part number being used, and can you supply any pics of the setup?

 

[glenn property of pam]

http://www.boatfix.com/merc/Servmanl/13/13E4R2.PDF

I think the wire you are referring to as a resistor is in fact a filter. Turn the voltmeter to a/c then read the voltage at the battery when the engine is running. If you get a a/c reading shut the motor off as that probably means that the alternator has a failed diode. Next disable the alternator (pull wires and tape them up, engine will run off the battery for 10 mins no problem) then restart and test for a/c voltage.

 

[Delmagtech]

You know, I said it couldn't be anything else because the tach circuit was eliminated, the 12v signal from the interrupt switch was interrupted and there was 12v to the coil, and the coil to module harness ohmed out fine, BUT, could it be that the pickup coil fails when it gets warm? The module gets pretty hot. It ran good for four hours when the last guy installed the last module though. I found the positive cables to need cleaning and the grounds were atrocious. Edit. Just saw the response regarding checking for a/c. I just finished installing the new distributor (with new module and p/u coil). I'll put the old one back in and do that test. Pics coming up shortly. I'm working off my phone, so I'll apologize in advance that they'll be big. Lastly, alternator has thin coat of drive oil from the little tank burping out of vent hole. Connections are clean of corrosion.

 

[glenn property of pam]

the alternator makes a/c that is converted to dc by a diode bridge in the back. If the bridge fails then you get a a/c ripple that you can not read with a meter set to dc. The spikes are small and violent so you can end up with a huge voltage spike that a meter set to dc will not register. This may not be the problem but its worth checking. oil shouldn't hurt the alternator. If the old module is toast disconnecting the alternator will not prove anything but you can run the engine with the alternator disconnected and if the pack stays cool then have a shop test the alternator for a diode problem.

 

[Delmagtech]

 

[Delmagtech]

 

[Delmagtech]