MCM 140 Hot Coil/ Rough When Warmed Up

[Chef Shawn]

Gentlemen that are Smarter Than Me:

Long story short: Got a boat with a MCM 140 (Serial # A457082) for a great price. HE said it ran (it did) but would not shut off with the ignition switch (he pulled the coil wire to shut it down). He said it would run 20-30 minutes in the water and then would run really rough. He also said the coil would get SUPER-HOT.

What I did:
1. Brought the boat home and replace ignition switch with equal quality marine 3-point ignition switch.
- Boat started as usual, still would not turn off with switch. Pulled Coil wire to shut down. Coil super hot.
2. Bought a repair Manual. Studied said manual.
3. Found faulty ground straps: replaced.
4. With the battery connected, key off, 12 volts to + side of coil.
5. Coil wiring= purple, pink, red (from distributor) to + side of coil. If the pink is cut while engine running(key off) motor shuts down. If pink and purple (which are connected) are disconnected, motor will not start. Purple wire cut (pink still connected) motor will run, will not turn off with ignition switch. Coil - side two wires: black from distributor and gray
6. Replaced starter solenoid, now I have 7.5 volts to + side of coil (key off), motor is hard to start, "LOW VOLTAGE" light comes on (dash mounted). Give it enough gas, it will start, will not idle.

This thing is making me mental, please advise. Thanks.

 

[Eric 3.7LX Alpha 1]

Re: MCM 140 Hot Coil/ Rough When Warmed Up

The diode in the alternator is probably toast, the alt. is backfeeding the ignition circuit while the key is off.

You can install a inline diode on the switched feed to the alt, (should be a purple wire)

A diode (not a resistor), could be added externally very inexpensive, connect it so that the switched ignition power to the alternator passes thru it to the alternator. A diode only allows the flow of current in one direction. The diode will prevent current from going back to the ignition switch and this will prevent a "run on" condition when the ignition switch is turned off. Use a Radio Shack #1N4001 diode or an equivalent value

If that works you could keep it like that or rebuild/replace your alternator.

 

[Chef Shawn]

Re: MCM 140 Hot Coil/ Rough When Warmed Up

Thank you for the reply. Forgot to mention, the diode trick is the first thing I tried (I used the same RadioShack part # you suggested) as I had saw that in another thread. However, I installed it on the purple wire near the coil (+) with no effect, so I removed it. Are you suggesting adding a diode inline near the ignition switch?

One thing that is confusing me is that I had 12V to the + side of coil before replacing the starter solenoid and 7V after (both with Key Off, battery connected). Shouldn't I have NO power to coil (+) with the key off? Or can the power make it there through a fried alternator, back-feeding the system?

 

[Eric 3.7LX Alpha 1]

Re: MCM 140 Hot Coil/ Rough When Warmed Up

The diode goes on the (purple) power wire directly upstream of the alternator not the coil or ignition switch. It is a one way device. You want the alternator to recieve power thru the diode and not backfeed it out that purple wire, which will not have voltage on it when the key is off.

The (+) on the coil will have no power on it while the key is off. It is probably getting power from the faulty diode in the alternator.

The doide only works one way, so if it doesn't work flip it around, I don't remember which way to install it, been along time. But Radio Shack would know.

 

[Don S]

Re: MCM 140 Hot Coil/ Rough When Warmed Up

Real easy test, no diodes needed, just unplug the purple wire that is going to the alternator start the engine and see if shuts off.

 

[Chef Shawn]

Re: MCM 140 Hot Coil/ Rough When Warmed Up

A couple of more questions:

I couldn't "unplug" the purple wire, but I had ALOT of it, the previous owners had a long green wire attached (in-line), so while the engine was running I cut said wire , motor ran fine. Shut it off WITH THE KEY (FINALLY!!!!). And it restarted. Checked coil, no power to coil with key off after that wire was cut.

So my question is should I replace the alternator or just do the diode thing? This is my first "family boat", I ran river boats in Alaska when I was young, but those were just car engines stuck into a big *** aluminum frames.. Don't want to disappoint the family on a tubing day....will those $2 diodes really do the job of a $200 alternator (and the headache to remove and install it)?

Will the diode fix hurt the engine in any way? And just to be clear, if I do the diode thing, I should attach the diode to the "green wire that I cut", but I want the diode facing toward the alternator or away?

Thanks again for the help, maybe my family will love me again soon, lol.

 

[Eric 3.7LX Alpha 1]

Re: MCM 140 Hot Coil/ Rough When Warmed Up

It's your call to add the diode or rebuild/replace the alternator. You could throw the diode in there for the rest of the year and rebuild the alt. during the off season.

There is nothing wrong with adding the diode inline, you can add it in right where you already cut the wire. Connect the doide inline with a couple of heat shrink butt connectors, on each end of the cut wire. It is what is broken inside your alternator, (but will look different).

I don't remember which way the diode has to be inline, so if the engine keeps running with the key off, disconnect it and reverse it's orientation.

 

[Chef Shawn]

Re: MCM 140 Hot Coil/ Rough When Warmed Up

Thanks for the help! Time to get the boat wet!!!

 

[Chef Shawn]

Re: MCM 140 Hot Coil/ Rough When Warmed Up

FIXED : For anyone with the same type boat and the ignition switch will not kill the motor, install the diode on the purple wire toward the alternator . Internal diodes were fried and sent direct power to ignition switch and relayed it directly to the coil, which caused coil overheating.

Happy boating.

 

[Eric 3.7LX Alpha 1]

Re: MCM 140 Hot Coil/ Rough When Warmed Up

Glad to here

 

[Chef Shawn]

Re: MCM 140 Hot Coil/ Rough When Warmed Up

Well crap! Test day on the lake, not so good. Probably need to start searching the forums for my newest problems. All went well for the first 10 minutes, then temp gauge went sky-high. Checked engine compartment, smoking, exhaust manifold. Hot hoses both sides of thermostat. Shut motor down for an hour and anchored, planned to limp to dock after motor cooled off.

Started motor in "throttle only" position, idled for a second and went to put it in gear: wouldn't go into forward or reverse??? Felt like something was binding?? (Engine off) Checked engine compartment, everything seemed to be in place. Tried again with a little more force and it slipped into forward and reverse again with no problem, still engine 'off'. After that little 'issue' engine would not restart in either 'throttle only' or regular mode. Got a tow back to dock, scratching my head the whole way.