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Author Topic:   Mercury Stators
jimh posted 06-03-2006 11:55 PM ET (US)   Profile for jimh   Send Email to jimh  
I have read that Mercury stators are notorious for failing. A colleague has a mid-1990's Mercury. He is installing his third stator. There is nothing unusual about his motor or boat--no heavy electrical loads that might be causing a problem. The stators seem to spontaneously fail in normal service. Is this a common problem with Mercury motors of this era? What is the usual mode of failure? What is the usual cause of failure?

Also, they seem to be quite expensive, over $200. I assume they must be an assembly of all the various coils under the flywheel, so you have to replace everything even if only one set of coils has failed. Is that correct?

Also, I have often heard mentioned a high-speed and low-speed stator. I would very much appreciate a concise explanation of how the dual stator Mercury outboard motors operate. How do they change from one stator winding to another? What controls the change? Are there separate rectifiers? What part of the stator has a high-speed and low-speed winding? Is it just the ignition timing coils?

sosmerc posted 06-04-2006 06:29 PM ET (US)     Profile for sosmerc  Send Email to sosmerc     
The Mercury Stator is indeed what I call a "high mortality" part. It is subject to heat and vibration and can be over-taxed if it attached to faulty components such as a bad voltage regulator or bad switch box. Because the relationship between the stator and the switch box is so critical, I always try to talk the customer into replacing both at the same time, and from the same manufacturer. The 16 amp L4 stator must be used with a combination regulator/rectifier. Also important, conventional flooded starting batteries work best with this system. AGM and GEL batteries have been known to cause premature failure to the regulator and then in turn the stator.

Switch boxes can fail due to a bad key switch, one that leaks DC voltage to the stop circuit when the key is turned off and on.

As a rule, I generally use CDI components. They have superior technical support, and often this is important. One must be especially [careful] to use the correct components for your engine. The Mercury L3 and L4 two-strokes have used a variety of different stators, and you shouldn't assume they can be interchanged.

If I can talk a customer into purchasing both a replacement stator and a switc hbox, I give them a break by charging my cost plus ten percent. So typically it would be about $140 for the stator and $110 for the switch box. If a new regulator-rectifier is needed, that adds about $72. On top of that labor might be $180 for troubleshooting, installation and testing.

I am currently having some [concerns] with the particular CDI replacement voltage regulators (194-5279) used in conjunction with 3- and 4-cylinder Mercury two-strokes. They are not holding the voltage down to 14.5 volts as they should. For some reason, the stock Mercury unit seems to work fine with just about any stator, so I have suspended sale of the 194-5279 until CDI can give me an answer for the problem.

As with everything electrical, it is critical that all connections be clean and tight, and be sure to use the proper components for the application.

CDI claims their stators have some internal regulation on their circuits to protect the engine from voltage spikes. All their stators are now open windings for better cooling. It is also important that the stator be installed in the exact orientation as the original.

jimh posted 06-04-2006 11:01 PM ET (US)     Profile for jimh  Send Email to jimh     
I noticed a replacement part stator for an OMC engine was being supplied without the epoxy potting compound over the windings. Evidently this must be an industry trend to forego the potting compound.

I think all outboard stators are at risk for heat soaking, since they are mounted atop of the engine.

Thanks for the details on the Mercury situation.

jimh posted 06-11-2006 10:00 AM ET (US)     Profile for jimh  Send Email to jimh     
Follow up: My colleague was too hasty in his assessment of the problem with his c.1995 Mercury 115-HP outboard. The problem was not the stator, so he did not need to replace it again. On further investigation he discovered another (non-electrical) defect was causing the problem.
jimh posted 06-11-2006 10:05 AM ET (US)     Profile for jimh  Send Email to jimh     
Regarding the "high-speed" and "low-speed" stator arrangement often mentioned on Mercury motors, my understanding is that there are two coil windings. One coil winding is designed to produce a proper voltage output at lower engine speeds, but will become saturated and unable to produce sufficient output as the engine speed increases. A second winding is designed to produce proper voltage output at higher engine speeds. It cannot produce proper output at lower speeds. By combining these two outputs, the engine can produce more output than from a single winding over the entire range of engine speeds.

The exact electrical manner in which these two are combined or selected is still not entirely clear to me. Please give me some clarification. Are the two coils just connected in parallel and fed to a common rectifier? Or are the two coils connected each to their own rectifier, and the rectified output connected in common?

Chuck Tribolet posted 06-13-2006 03:04 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Or are the low/high speed coils used to fire the spark
plug, and not regulated or rectified?
jimh posted 06-13-2006 08:35 AM ET (US)     Profile for jimh  Send Email to jimh     
Chuck--That is a very good question. We often just call these components "stators" even though they have several separate and distinct coil assemblies. Usually there are three sets of coils:

Battery Charging Coil--A coil with a heavy gauge wire winding designed to produce considerable output voltage and current for generating a battery charging current. This coil is typically applied to a high-current rectifier and converted to pulsating DC, then controlled by a regulator and connected to the 12-volt battery operating the motor.

Spark Voltage Coil--A coil designed to generate the voltage applied to the primary winding of the spark plug coil. This coil may also generate the voltage for powering the electronics modules itself

Timer Base Coil--A coil intended to generate a pulse output typically for triggering a silicon controller rectifier (SCR) to fire the spark plug. It controls the ignition timing.

So when people speak of a Mercury stator having a high-speed and a low-speed winding, which of these three typical windings is being duplicated?

smudge posted 11-10-2006 05:40 PM ET (US)     Profile for smudge  Send Email to smudge     
Hello, I stumbled onto this site after a google search. I am having problems with my Mercury 50 1992. It has 16V output to the battery at idle, which is way too high. This engine does not have a regulator, just a rectifier which I replaced even though it tested ok. Any ideas? Thanks.
jimh posted 11-10-2006 09:02 PM ET (US)     Profile for jimh  Send Email to jimh     
It sounds like your Mercury stator is in good shape.
Mister S posted 11-16-2006 12:57 PM ET (US)     Profile for Mister S    
I went thru similar [repairs] this summer [on a] Mercury 200-HP with two (high-low) rectifier set up. Even though I was charging at [more than] 16-volts, the stator tested bad. Ended up replacing stator, both rectifiers and replaced AGM batteries with conventional 1,000-ampere lead-acid batteries.
jimh posted 11-16-2006 07:59 PM ET (US)     Profile for jimh  Send Email to jimh     
Hello, I stumbled onto this site after a google search.

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We are still looking for a good explanation of what the "high" and "low" stator windings are supposed to be. I have noticed that in some Mercury outboards there are two sets of rectifiers which are driven by two different stators windings. This may be associated with the often cited "high" and "low" stator references.

smudge posted 11-17-2006 05:00 PM ET (US)     Profile for smudge  Send Email to smudge     
The low and high speed coils refer to ignition coils. The low speed coil of my stator on my boat failed several years ago and the boat would not start. I believe they have nothing to do with the charging system.
jimh posted 11-17-2006 06:27 PM ET (US)     Profile for jimh  Send Email to jimh     
I am not at all certain what is meant by "ignition coil". Normally I would associate the phrase "ignition coil" with a device to boost the voltage for spark. Is that what these high-speed and low-speed stators are for?

Most outboards I have seen use individual ignition coils for each cylinder to raise the voltage. Also, I've never heard of using different ignition coils for different speeds.

We need better explanation of what the high-speed and low-speed stators--often mentioned in regard to Mercury motors--are actually doing.

smudge posted 11-20-2006 06:39 PM ET (US)     Profile for smudge  Send Email to smudge     
They aren't ignition coils. I was told the stator uses two separate coils to supply pulses to the ignition coils. If that is the case--and I am only guessing--then it is likely that Mercury do that simply to ensure consistency of the spark energy at different speeds.

I guess what I am trying to say is that at low speed you have much more time for an ignition coil to build up magnetic flux so they use a different pulse coil for higher speeds that will saturate the primary of the ignition coil at high speeds without causing the pulse coils to overheat.

Just a theory...

jimh posted 11-20-2006 08:23 PM ET (US)     Profile for jimh  Send Email to jimh     
I'd like to see a schematic diagram of how the Mercury ignition system is built. That might offer some insight.

I do think there are some inherent problems in a distributor-less ignition system with the ignition timing and spark generation. As the speed of rotation of the flywheel increases, it may be better to have two sets of windings in the ignition system. One winding is optimized for low crankshaft speeds, and a second winding optimized for higher crankshaft speeds.

Perhaps this is also why OMC went to an optical ignition timing system, to get away from the coil-induction system problems.

The stators are always located at the top of the engine block, too. This puts them in a very hot environment. I think that contributes to their shorter life span.

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