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Author Topic:   Mercury Stator Resistance Readings
jandrewg posted 05-21-2007 09:58 AM ET (US)   Profile for jandrewg   Send Email to jandrewg  
What is the resistance of the stator of a 1995 Mercury 125-HP motor that is stamped 398-9710A31?

[Describes problem with a 19]95 Merc 125-HP motor on a [19]95 Boston Whaler Montauk. [Says that] after running perfectly, [the motor now will not] start [and has] no spark. [After] unplugging the kill switch, still no [spark]. [The 1995 Mercury 125-HP motor] finally started after wiggling the key in [the ignition] switch. Since switch was funky anyway--it was sticky, even after spraying non conductive lube in hole--[the ignition switch was] replaced with new Mercury part. Initially [there was still] no spark, but I wasn't sure I was getting a good ground to plug. After attaching a ground wire to [the plug] there was spark, but not a good, bright blue color, and not the good "snap" I like to hear. The plugs were put back in and [the engine] started and ran perfectly several times. Then [the engine showed the] same no spark [as it had earlier].

Found a suggestion to remove black with yellow stripe wire on switchbox to eliminate problem with low oil or high heat switches. [After removing the black with yellow stripe wire from the switchbox there] still [was] no spark.

I have a manual coming. I would like to get the boat out of the slip it is in and on the trailer ASAP. Would low speed winding give intermittant low spark, sometimes none?

Thanks so much.

jimh posted 05-22-2007 12:36 AM ET (US)     Profile for jimh  Send Email to jimh     
According to the Mercury color code information at

a black wire with a yellow stripe is the ignition stop circuit. It is not associated with the oil reservoir level sensor or with the cylinder head temperature sensor in any way. (If you formed the inference that the black wire with yellow stripe is associated with the oil reservoir level or the cylinder head temperature sensor from reading something published here on CONTINUOUSWAVE I would appreciate it if you would tell me the URI of that article so that I can review it for accuracy.) If you disconnect the black wire with yellow stripe from the switch box you should prevent the safety lanyard switch from being able to shut off the engine. Otherwise, if you pull the lanyard from the safety lanyard switch the spark will usually be suppressed.

I recommend you get an in-line spark tester. This is a very handy device to have when making tests of the spark on your engine. You can test each cylinder for spark without having to remove the spark plug.

I do not know the resistive value of the stator coil you are asking about. Usually you have to have the factory service manual to get that information. If you intend to perform your own service on your motor, the purchase of the factory service manual is a very good investment.

davej14 posted 05-22-2007 07:48 AM ET (US)     Profile for davej14  Send Email to davej14     

What is an "in line spark tester"? It sounds like something I would like to have in my tool box.

jimh posted 05-22-2007 08:52 AM ET (US)     Profile for jimh  Send Email to jimh     
An in-line spark tester is a test device which can be inserted in the high-voltage spark wire. They typically have one connector which mimics a spark plug electrode and another which is a spark plug boot connector. You can remove a spark plug wire from the spark plug and insert the in-line spark tester on that cylinder. The tester has a small spark gap contained inside a transparent glass cylinder. This allows you to visually observe the spark. You can typically find these at stores that sell automotive supplies. I got mine from master mechanic David Zammitt at Lockeman's Boat and Hardware.

Photo: Lisle 20610 In-line ignition spark tester

20610 LISLE In-line Ignition Spark Tester

I made extensive use of this test device when I was diagnosing an intermittent spark condition on my outboard motor. I described this repair in a previously published article.

jandrewg posted 05-22-2007 11:40 AM ET (US)     Profile for jandrewg  Send Email to jandrewg     
Thanks very much for your time and reply. You're right, I really shouldn't be doing much without the imperical knowledge gleaned from a service manual. I have one coming, but it won't be here for another few days, and the boat is in a buddies boat house, apart.

After digging online some more, I found these figures for my 398-9710A31 Mercury stator-

Blue to ground= 3250-3650 Ohms
Red to ground= 75-90 Ohms.

The stator has six wires; 2 identical yellows, blue, blue w/ white stripe, red, and red w/ white stripe.

The white striped wires are apparently the ground side of each of two windings, the high speed and low speed. When resistance is checked between the red and red w/ white striped wires, I get a near spec reading of 90.9 Ohms. When checked between the two blue wires I get no reading at all, an open circuit. I'm guessing that the blue wires are to the low speed winding, but I'm trying to find more information on that. If so, I've found my problem, though often what appears to be a single problem can turn out to be many.

By the way, the above Mercury part number was shown to be NLA on their web site. They had a link to a PDF parts bulletin that said it was superceded to 398-9710A45. A search for that number showed that it too, was superceded. The new and current fix is Mercury part number 389-832075A6. It's a kit that includes the stator and wiring harness adapter to plug into the old design. To use it you must have the newer style voltage regulator that has five wires, not the older one with three wires that attach to the regulator. I lucked out here; I have the new one.

The information regarding the black w/ yellow stripe wire was something I found on another site as it turned out. I apolgise profusely for attributing it to this one. By the time I registered here yesterday, I had read dozens of posts on several sites and got confused about where I had read what. My mistake.

I think of myself as fairly mechanical (my other interest is cars, Mopars in particular, and I have a Dodge Super Bee here in my office that I built a healthy 440 for), but I hadn't seen the inline spark tester you suggested. I'm going to look for one at NAPA Auto Parts today. A great thing to have in my box.

Thanks again. Maybe the above information will help someone.

jimh posted 05-23-2007 01:29 AM ET (US)     Profile for jimh  Send Email to jimh     
The spark tester is very handy with outboard motors. Outboard motors tend to have completely separate high voltage generation for each cylinder. There are typically individual spark coil and individual low-voltage drive signals switched by individual SCR's. Because of this it is fairly common to have a problem with just one cylinder's spark, whereas in other types of motors there is often much more common circuitry among the cylinders. On a two-stroke you can easily have just one cylinder whose spark is weak or intermittent.

I have often heard about the Mercury high-speed/low-speed stator situation, however I do not recall seeing a good explanation of how it works. I have speculated that perhaps these windings are more associated with the battery charging alternator than with the ignition timing or spark generation. If you happen to come across a good explanation of how the two stator coils are used with regard to the spark voltage generation and the ignition timing, I would like to read it.

Generally outboard motors tend to have three sets of coils under the flywheel:

--a high-current coil for the alternator. This coil is rectified and used to charge the battery and provide other electrical current to run the motor. Often this coil is referred to as "the stator."

--an ignition circuit power coil. This coil provides the ignition circuit with its power. It usually has a higher voltage output than the alternator coil. Several hundred volts is a typical value. This coil makes the ignition self-powered. This explains why many outboard motors can run without a battery, or can run even when there are big problems with the battery charging circuit. The ignition is powered by its own coil.

--a timer base coil. This coil usually is on a moveable sub-base so that it can rotate. The position of the coil relative to the flywheel magnets determines the timing of the ignition spark. The timing is varied quite a bit on a two-stroke motor in order to get the best output. A range of 20-degrees or more is common. This coil provides the trigger signal to the SCR's for firing the spark. It is common that there be several sets of coils, one for each cylinder or to pairs of cylinders in six-cylinder engines.

In a permanent magnet alternator the voltage output of a coil is proportional to the engine speed, so as the engine speed increases the output voltage rises. However the coils also reach magnetic saturation, and this limits the output somewhat. For optimum battery charging you want a coil that comes on with a lot of voltage at lower engine speeds. At higher speed you would prefer a different winding ratio because the voltage can get too high to be useful. Perhaps this is what is being done with the Mercury "high-speed" and "low-speed" stators. That is just my guess.

jandrewg posted 05-23-2007 11:03 AM ET (US)     Profile for jandrewg  Send Email to jandrewg     

Again, many thanks for the information and your time. You must spend several hours a day replying to the posts on this site!

Yes, from what I've read, the high and low speed windings Mercury employs are a work around for the problem of magnetic saturation as RPM increases. I have confirmed that the blue / blue w/white stripe wires are the low speed side of the stator, and the red / red w/white stripe wires are the high side. I've read some of your posts from last year requesting a good explanation of exactly how this is accomplished, but my understanding at this point stops about here.

CDI/Rapair makes a replacement stator for my application, and it's interesting to me that, though the Ohm specs are different than the specs for Mercury part (somewhat lower as they are wound with heavier wire), the low speed side has a much greater resistance than the high speed side. You obviously have much more depth regarding electrical components and engineering than I and probably would understand why this would be the case. I'm just starting to get the gist of it, and it's fun.

Back to the stator for a moment, besides the Mercury replacement part number mentioned earlier, and the CDI part referenced above, the other option is a Sierra part number 18-5873. This is what I ended up buying, mostly because I scored one on eBay for $199.99 plus shipping, and the lowest price for the OEM part I found was just over $270. The Sierra part is a kit, as is the OEM piece, that has an adapter harness to retrofit to the older wiring on my motor. Again, if this information helps anyone out there, the same part will work with any early Mercury 2+2 inline four cylinder 100, 115, or 125hp if it has, or is upgraded to, the newer style 5 wire voltage regulator. I am sure this is correct, but please advise anyone to do their own due diligence and homework first.

Finally, thanks also for the link to the Boston Whaler wiring schematic. I've printed it and will use it in the future.

So, once again, I've learned a lot of new things, and have enjoyed the process. The boat dies, and every moment is not a pleasure (I spent several minutes talking to Jesus initially, or at least His name was mentioned a few times), but the information and further education is well worth it. It is much more rewarding than dropping off a broken boat (or car, or whatever), and simply writing a check when it's done.


John Green

jandrewg posted 05-23-2007 11:23 AM ET (US)     Profile for jandrewg  Send Email to jandrewg     
One more thought with regards to the dual winding on a Mercury stator. It would make sense to me also that the main purpose was to give a better, higher amperage charge at lower RPM.

However, when reading posts on a Mercury outboard specific site, one of the first indications of a problem with the lower speed winding is weak spark when cold, but enough that the motor will start. Of course, the motor started so, like me, you'd go off on your merry way and forget about it taking a few more cranks than usual. Once the throttle is down and the RPM increases you've unknowingly switched to the high winding on the stator. Your second clue to a bad low speed side is when the motor is now warm, and you shut it off. The malfunctioning low speed winding is now also warm and has become an open circuit, so the motor won't restart. Sometimes, as in my case, the circuit closes when it cools, allowing the motor to start once again, but giving you an intermittent issue that is by nature hard to diagnose. I got this clue about three times, but was too bull headed to investigate before it finally quit altogether.

So, while I'm sure the low speed winding helps with low RPM charging, it apparently also has the primary role in supplying spark to the motor.

jandrewg posted 05-23-2007 11:47 AM ET (US)     Profile for jandrewg  Send Email to jandrewg     
I just re-read your last post and the reference to the timer base coil. This would have to be what Mercury calls the "trigger". It rotates inside the stator, and is attached to linkage connected to an arm that moves as the throttle is applyed, having the effect of advancing the timing. Total advance is determined by a simple bolt with a lock nut, threaded into the arm, that stops forward movement.

I haven't messed with two stroke timing over the years, though I have had many, mostly motorcyles.

I have had a lot of experience with four cycle engines, the afore mentioned big block in my Super Bee one of them. If a typical two stroke advances the timing 20 degrees or so, it doesn't sound too much different than what I'm used to. My factory MIE 340 Mercruiser big block GM motors in my older Sea Ray have a factory timing spec of 8 degrees BTC (advanced) at idle. Using a dial back timing light, I was curious how conservative Mercury was with total timing for this motor, and found that total timing came in around 2500 RPM (if I remember correctly) and was 30 degrees, a difference of 22 degrees from inital timing.

My warmed over Chrysler 440 is very simular. Since, when using a cam with more than stock overlap and duration specs, more initial timing gives a better vacuum signal to the carb, and therefore a better, crisper idle, I have the initial timing at about 16 degrees. The mechanical advance in the distributor is set to give about 21 degrees of advance, for a total advance of 37 degrees. When that comes in is determined by the springs on the centrifical weights (in the distributor). The lighter the springs, the sooner (lower RPM) the advance comes in.

I may get this information when my manual comes in, but like a four cycle, does a two stroke usually want simular total advance of, say, 35-40 degrees? Does this mean the same would have 15 or more degrees initial?

jandrewg posted 05-25-2007 11:16 AM ET (US)     Profile for jandrewg  Send Email to jandrewg     
A final update on my Mercury 125 stator. I recieved and installed the replacement Sierra part number 18-5873 last night. It was identical to the Mercury piece as far as I could tell, and even included the Mercury instruction manual, with Mercury's copyright info at the bottom.

The motor started instantly, first time. I don't think the starter made a full revolution. I took it out for a 15 minute test run; it pulled well to full throttle, and idled perfectly. Success!

jimh posted 05-26-2007 08:35 AM ET (US)     Profile for jimh  Send Email to jimh     
Thanks for the follow up on your repair. It is always fun to fix something yourself and get it going again.
jandrewg posted 05-26-2007 01:43 PM ET (US)     Profile for jandrewg  Send Email to jandrewg     

Yes, it really is. The motor ran great again last evening. I very much enjoy learning new things, and try to see mechanical problems when they arise as opportunities, though sometimes that perspective is difficult to grasp at the moment.

I'm new here and am watching posts with the excitement of a newbie, but will also try to make a habit of regularly checking in. If you need anything at all, I check my email at least a couple of times a day during the week.

Thank you again for a very helpful site, and for your time in monitoring it.

John Green

jmac3 posted 06-07-2009 02:47 PM ET (US)     Profile for jmac3  Send Email to jmac3     
After reading an outstanding exchange explaining how the stator in Mercury motors work, I have a question.
I have a 1995 Mercury 100 HP 4 Cyl outboard motor. It recent die on me while on a Canadian fishing trip. The problem was no spark. The dealer in Canada said I have a bad stator part # 398-832075A21 or possibly a bad switch box part # 332-5772A7. They could not get the part in time for me to finish my trip so I decided to replace these parts myself. However, after researching the part numbers using my manual on CD. I come up with a different number for the stator part# 398-9710A31.
My motor serial number is 0G272705.
The stator in black in color.

Can anyone confrim which part number is correct?
Thanks in advance,

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