Coil problems on a Merc MCM488

[RickW]

After having sat for a couple months the engine won't fire at all. It was running and starting fine before that. Checked for spark at the plugs and didin't see any. As per the manual I have, put main coil wire near a ground to check for spark, but nothing so we replaced the coil thinking it was the problem. Still no go. The distributor, points, condenser, cap, wires, and plugs were replaced at the beginning of the summer because of some problems and the boat had run great since then.<br /><br />After looking in the manual again I see that their is also a "resistance wire" that goes from the coil's + to the electronic choke. I tried checking it's resistance but I get an infinite reading so I assume that the wire is broken or cut. <br /><br />Couple of questions. What is the purpose of the resistance wire, i.e. how does the current flow through it and how does that affect the coil? If I were to replace it with another wire, does anyone know what gauge I should use to get a resistance of 1.8 - 2.0 ohms which is what it's specd at? <br /><br />Let me know if you need any other info.<br /><br />Thanks

 

[12Footer]

Re: Coil problems on a Merc MCM488

 

[RickW]

Re: Coil problems on a Merc MCM488

If you meant to post something 12Footer, could you repost it.

 

[12Footer]

Re: Coil problems on a Merc MCM488

That was odd....Oh yea, I had a phone call,and didn't have time to read-thru it ,so deleted for later...Sorry bout that.<br />Some auto and marine engines have a resistor inine with the points/coil to prevent current surge damage. Without this resistor,the starting current required would be too great for normal operation,and it would toast the coil,points, and sometimes the plug wires and plugs.<br />When they open, the symptom is usually a great starting engine...Starts every time...Then quicky stalls out again.<br />Is this what your did?<br />The resistor wire can be found at the merc dealer. You could repace it with a block-type resistor,as long as it is of the same value, and can disipate at least 25watts.<br />The resistor wire can get hot and open up.But then,it was designed to do this. It may indicate that you had a coil failing,so the fact that you changed-out the coil, was probably not a wasted step.

 

[RickW]

Re: Coil problems on a Merc MCM488

It had been running and starting fine and then all of a sudden, it wouldn't fire at all. I've been told that if the resistance wire is broken that the engine will crank but not fire. Would you agree with that?<br /><br />I ordered the replacement that I needed but it won't be in until next Wedenesday. Do you know if I can go to radio shack and build one myself just to use this weekend? Most of the parts stores around here close at noon today so I don't think I can get a hold of a block tyoe resistor like you mentioned.

 

[12Footer]

Re: Coil problems on a Merc MCM488

Yes,it could keep it from running, but usually (and i stress usually),it starts fine,but dies before you remove your hand from the key. <br />I'm not too sure about "Shaft Shack" these days. They have gotten into cell phones, like they got into the computer market a few years back...At the cost of all else. They might not carry a 1.8OHM @ 25watt block resistor. But you can try.

 

[RickW]

Re: Coil problems on a Merc MCM488

Well, I called a local parts place but he said he had no way of knowing what the specs were on the resistors that he carried. I'd have to call Mercruiser CS to find out. Radio Shack carries what they call "wirewound resistors" rated at 1.8 ohm and 25 w. but they're special order only so I guess I'll just wait for the merc part to come in. <br /><br />Thanks for your help. If I need any more help I'll post here next week.

 

[12Footer]

Re: Coil problems on a Merc MCM488

Ok. Dont you just hate that? Knowing what's wrong, and having to wait for the parts?

 

[petryshyn]

Re: Coil problems on a Merc MCM488

Yo RickW:<br />If you're impatient, buy 2 4ohm resistors and hook them in parallel. It'll give more current capacity and better heat dissipation. Radio shack may stock them as its a more common value. Or, wander around the auto wreckers with an ohm meter and pull one off of the older ignition systems. You also can buy bulk resistance wire from most auto suppliers (if I remember right, its 1ohm per foot) Or from a GM dealer as well.<br /><br />Good luck!!

 

[RickW]

Re: Coil problems on a Merc MCM488

Time to start over. The reason that I thought that the resistor wire was broken and that the wire coming from the solenoid had power in the key on position was because I had the wires backward. Apparently the previous owner did some work on the coil wires and spliced on to the old wires, changing their colors. The purple resistor wire coming from the choke, now has a yellow end at the coil. The purple-white wire leaving the solenoid now is purple at the coil, so I thought it was the end of the resistor wire. Silly me. <br /><br />So now I'm back at the beginning. No spark from the coil. Both connections to the coil's + terminal seem to be working as they should. The points look fine. I tried gapping them higher than spec just to make sure they were opening far enough but still no spark. As a further test, at the coil's - terminal I have 12v when the points are open and 0v when the points are closed. I assume that if the tachometer lead which is also connected to the coil - were grounding out, I wouldn't get any voltage here either. I tried the shift cutout switch and it is grounding the circuit, as it should, when closed but not when open. So everything seems to be working right and I still don't know why I'm not getting any spark.

 

[petryshyn]

Re: Coil problems on a Merc MCM488

The coil has two small terminals and one High voltage tower correct?<br /><br />One small terminal goes to points, the other goes to ign. switch, correct? What is the voltage to these two small terminal when points open ......then when points closed?<br /><br />When measuring, have voltmeter black lead on battery neg. post

 

[12Footer]

Re: Coil problems on a Merc MCM488

Check continuity accross the teather switch if yur boat has one. These go bad quite often,and are often overlooked.

 

[RickW]

Re: Coil problems on a Merc MCM488

shematic, I'm just waiting for my @Home webpage to be set up and then I'll post a link to the wiring diagram from my manual. But this is what I've got.<br /><br />On the coil, as you said + and - small terminals and high voltage tower. On the + side, resistance wire running to the automatic choke and another going to the slave starter solenoid. On the - post, wire going to the points and one that goes to the tachometer. The - terminal shows about 12v with the points open and near 0v when closed. The batteries have been a bit low even after charging so the voltage readings aren't quite where they should be. That being said, we've had low batteries before and it always fired. I'm going to put another battery in tomorrow just to rule out that possibility. I haven't checked the voltage at the + terminal with the points closed. I'll do that tomorrow after I get the new battery in and let you know.<br /><br />12 footer, if you're talking about a killswitch that attaches to a vest kind of thing, no it doesn't have one. Any other suggestions as to how to proceed from here?

 

[petryshyn]

Re: Coil problems on a Merc MCM488

gotta know what you got at the + post with points open and then closed.<br /><br /><br />are you checking for spark right from the coil wire, or from the plug wire?

 

[RickW]

Re: Coil problems on a Merc MCM488

At first I started with the plug wire but per the manual's troubleshooting section, now through the coil wire. Disconnected from the distributor and positioned 1/4 inch from a bare bolt going into the block.

 

[petryshyn]

Re: Coil problems on a Merc MCM488

>Can you remove coil wire and try with a different wire?(anything lying around)<br /><br />>Or atleast remove coil wire and check for continuity with an ohmmeter? (record resistance)<br /><br />>Stupid question......Did you actually see dist. shaft or rotor turning when checking for spark?

 

[RickW]

Re: Coil problems on a Merc MCM488

I think I can find an old wire lying around to swap out but I did check continuity and resistance was, if I remember right, 3500ohm.<br /><br />Yes the distributor shaft is rotating.

 

[petryshyn]

Re: Coil problems on a Merc MCM488

3500 ohms is good.<br />> Are the points arcing?<br />>Measure resistance from + to - on coil(leads removed<br /><br />>Measure resistance from + to high voltage tower terminal<br /><br />got to go.........wife's calling!!!!....<br /><br /><br />later

 

[RickW]

Re: Coil problems on a Merc MCM488

3500 was wrong. Either I misread or forgot. It's actually 800 Ohms.<br /><br />Points arcing - Only when I separate them with a screwdriver. When the engine is being cranked and the shaft is openeing and closing them, as far as I can tell they're not.<br /><br />Resistance from + to - (with leads removed) is 0.<br /><br />Resistance from + to tower is 12,000 Ohms.<br /><br />According to the manual the coil's primary resistance should be 1.3-1.5 Ohms. The secondary resistance should be 9400-11700 Ohms. BTW, in case you forgot, this is a brand new coil. Also, my multimeter only reads in KOhms so I can't tell about the primary resistance.<br /><br />Don't tell me you have a real, nonvirtual life, to return to. How sad.

 

[petryshyn]

Re: Coil problems on a Merc MCM488

yoooooooooo<br />just got back from princess auto!! bought out half the place....<br /><br />hey, 800 ohms on coil wire OK.<br /><br />sounds like coil resistance OK.<br /><br />> Is there voltage to + terminal while cranking?(should be 'some' arcing while cranking)<br />>if there is voltage at + (atleast 5volts with points closed), disconnect condenser and try cranking again. if has poor spark.......then condenser bad.