Need Help Starting Mercruiser 140 3.0 - Electrical Problem

[mj1622v]

I have a 1984 Mercrusier 3.0L with a conventional ignition system. When I try to start the motor it just cranks and cranks. After examining the engine, I determined that I am not getting spark to my plugs. Voltage of the ignition coil reads about 4.5 volts when the wires are connected to the coil, and about 12 volts when the wires are disconnected from the coil. I removed my distributor cap and the points are in the closed position. When I pry open the points, my coil again reads 12 volts on my multimeter. Maybe the points aren't firing or adjusted properly? Maybe needs whole new distributor? I'm somewhat new to conventional ignition systems, so any input is appreciated, thanks in advacne everyone!!

 

[HT32BSX115]

Re: Need Help Starting Mercruiser 140 3.0 - Electrical Problem

Howdy,

Well, the INTERNET is your friend....

You should probably have a look at the following...

http://auto.howstuffworks.com/ignition-system4.htm

http://autorepair.about.com/cs/generalinfo/a/aa052502a.htm


We can probably talk specifics when you finish your homework!


Cheers,


Rick

 

[jennis9]

Re: Need Help Starting Mercruiser 140 3.0 - Electrical Problem

First - welcome to iboats.

we have the same merc - and just had a similar problem.

I am not mechanically inclined, but have a reminder for you. If you ever replace the ignition wires on that motor - make sure they give you wires for the conventional ignition and not the electric ignition (it's easy to do). we burned a few plugs and had a heck of a time diagnosing our other issue - and it seemed like it was electrical. On top of a mechanic that was a little less talented than he said...

We replaced everything right, points, plugs, cap, rotor - but had no idea we had the wrong wires - then the carb went at the same time and we had a mess on our hands.

So be patient and wait for the advice here, or you may have too many things going on for the guys to narrow it down to.

Good luck.

 

[mj1622v]

Re: Need Help Starting Mercruiser 140 3.0 - Electrical Problem

I know how the basic components are supposed to work, but does anyone have any idea why the coil is drawing power?

 

[HT32BSX115]

Re: Need Help Starting Mercruiser 140 3.0 - Electrical Problem

If the points are closed, it will draw current anytime the "key" is on....Also, it will draw current if the condenser is shorted.

If the coil is the wrong resistance (or the ballast resistor is missing) it will draw a lot of current....... enough to get hot.

Are the points opening when the distributor turns?...and when the points open does the coil stop drawing current?

I'll also add that there should ALWAYS be 12v on the + side of the coil. The points ground the other (-) side. (that is, if the condenser is NOT shorted)

Take your multi meter and measure the current (AMPS) supplied to the coil.

Select the 10A scale (and reposition the leads) and connect them in series with the 12v to the coil. you should measure somewhere around a few to several amps. (when the points are closed)......

when the points are open you should measure zero amps.

If you measure current with the points open, your condenser is shorted or very "leaky".....or you have a short in the points themselves.

 

[mj1622v]

Re: Need Help Starting Mercruiser 140 3.0 - Electrical Problem

Ok, here?s what I've got. The points are not opening as the distributor turns. When the points are pried open the coil reads a full 12 volts, but only 4.5 volts when the points are closed.

When measuring the amps supplied to the coil my meter read about 3 amps when the points were in the closed position. I separated the points and put a thick piece of cardboard in between them to hold them open, and still got a reading of 3 amps with them open.

Shorted condenser? Short in the points? Both? Really appreciate the help!

 

[Don S]

Re: Need Help Starting Mercruiser 140 3.0 - Electrical Problem

Disconnect the gray wire on the coil. It goes to the tachometer, and if it's shorted, it will cause the problem.

 

[HT32BSX115]

Re: Need Help Starting Mercruiser 140 3.0 - Electrical Problem

Only you can see why your points are NOT opening. If the cam is worn out so completely that it's round, you'll have to replace the distributor (or rebuild it.....or switch it to a Pertronix system)

Usually the points just need adjusting. You should REALLY just replace the points and condenser. They need to open when the dist turns.

If you have 4.5v on the + lead, that might be ok when running. That means you either have a ballast resistor, or a VERY poor connection (if there's no resistor), or a burned resistor that has FAR more resistance than it should.

See if you can find the resistor, and (reposition your leads on your multi-meter) and read the voltage across the ballast resistor. It should equal the the supply voltage (12v) MINUS the voltage you measured across the coil. (It's just a voltage dropping resistor)


They usually set up the system so that it has 12v on the coil during cranking (for maximum spark while the starter is sucking battery voltage down) and a much lower voltage when the engine starts running so that you don't overheat the coil. Those are usually 6v coils.

You need to start with new points and condenser though and maybe check the ballast resistor (if you can find it)

 

[mj1622v]

Re: Need Help Starting Mercruiser 140 3.0 - Electrical Problem

I adjusted the points this morning, and they now are opening fine as the distributor turns. I also removed the wire that runs from the condenser/points to the coil. With this wire off, my coil's voltage shot right back up to 12 volts.

ordered a tune up kit for the engine for 11 bucks that comes with a new rotor, points, and condenser. I'm hoping that will take care of things. Haven't checked or looked for the resistor just yet, I guess that's next on the list if this fails.

 

[Trent]

Re: Need Help Starting Mercruiser 140 3.0 - Electrical Problem

As was said above.... Disconnect the gray wire on the + side of the coil then try to start. A shorted tach will cause this problem.

 

[mj1622v]

Re: Need Help Starting Mercruiser 140 3.0 - Electrical Problem

Thanks for the tip guys, I removed the gray wire and checked it out, but it seems to be ok. Only drains voltage when the points and condensor are hooked up to the coil.

 

[Trent]

Re: Need Help Starting Mercruiser 140 3.0 - Electrical Problem

Did you try to start the engine with the gray wire removed? If so run a jumper wire from the + side of the battery to the + side of the coil and see if she starts and come back here. (with gray wire removed)

You need 9.5 volts for running so you dont burn up the points.. This is done thru the resistor wire. While starting you will have battery voltage..around 12.5v or a little more.

 

[mj1622v]

Re: Need Help Starting Mercruiser 140 3.0 - Electrical Problem

I jumpered from the coil to the battery and attempted to start the other day. Both times, with the wire attached, and with it removed, the engine fired up. I see no loss of voltage on the coil when the gray wire is attached either, but lose about 8 volts when the points and condensor are attached.