Possible stator troubles on a Yamaha. - The Hull Truth

TheBottomLine · 06-25-2009, 10:59 AM

Let me just start of by saying that this has to be one of the most bad donkey sites on the net for boating. I have been able to solve numerous issues with just searching. Much thanks to all who contribute. Now with the issue at hand. I have been having weird issues with surging on my port engine for a while now and finally last week it kicked the bucket. I have absolutely no spark and have worked it down to either being the stator or the CDI unit. At the moment I have taken out the stator, but have no idea on how to test it. If anyone could help me out in this arena I would greatly appreciate it.

1998 Yamaha OX66/EFI 250hp

I know this is my first post, so please be gentle.

A1 Outboard · 06-26-2009, 04:57 PM

The stator is normally checked on the engine for voltage output. Your stator has 2 charge coils which power the ignition system. It would be unusual for both coils to fail. You can test the charge coils of the engine with a ohmeter. Both coils should have a little over 300 ohms resistance and not be shorted to the metal core. The one thing that will cause no spark is the stop circuit. A short to ground anywhere in the circuit will cause no spark at all. The CDI unit has 2 separate circuits for ignition so once again it would be unusual to have total failure. I think unless you have the specialized test equipment that a dealer has you should reassemble the engine and take it to a qualified tech for troubleshooting and repair. You could replace every component in the ignition system and still not find the problem. Yamaha ignition components are very expensive and not returnable if they dont fix the problem.

TheBottomLine · 06-26-2009, 08:49 PM

Thanks for the response A1. What connectors and contacts should I measure? I have two plugs, a round one with three green wires and a square one with four wires. The round one measures .9 ohms all the way around and the square measures 134.1 ohms (Brown to Red) and 91.3 ohms (Blue to Black w/red stripe). Thanks for your help.

WEDOCQ · 06-26-2009, 10:23 PM

You can diagnose by using a process of elimination. If there is no spark to just one bank, then it's probably one of the CDI's. If there is no spark to BOTH banks, it could be the trigger OR the stator. My stator failed in the same manner as yours, gradually worse performance until absolutely no spark at all. Ebasicpower.com sells aftermarket stators for hundreds less than OEM.

A1 Outboard · 06-29-2009, 09:37 AM

Sorry to disagree, but each CDI box has its own power supply which is called a charge coil. These are two seperate coils which are not connected in any way. The trigger on this engine consists of six seperate coil which are common only through the ground. If the ground is lost the coils will ground through another coil which will cause ignition to happen at the wrong time. The fuel injection system also uses the signal from the trigger to fire the injectors. The four pin plug is the connections from the charge coils. The three pin is the connections to the stator for battery charging. The fact that you got differing ohm reading from the spec. is possible from the use of a different meter than what was used when the spec. was developed. The only reliable method of determining the if the charge coil is good or basd is a loaded output test. This is done with a jumper harness while cranking the engine. The test is done using a peak reading or true rms meter. The ohm spec I gave you previously may be incorrect because I am at location where I cannot check my manuals for the correct spec. Ignition troubleshooting always starts with a spark checker or gap board on the sparkplug wires. Then the engine is cranked with the sparkplugs in place. The results you see on the spark tester will lead you to the next step. When you have no spark at all you disconnect the stop circuit at the engine and crank again. If spark returns you have found the area of the problem. Still no spark you do a loaded output test on the charge coils. If the loaded test shows no output you test the charge coils unloaded or disconnected. If you have output voltage now you check for wires shorted to ground or a CDI problem. This test procedure goes on further depending what the results of the test you have just performed are. Skipping a test or or doing the tests in the wrong order will have you replacing components that are not defective. These tests cannot be done without the correct jumper harness and a peak reading voltmeter. You should NEVER pierce a wire to obtain a loaded reading.

TheBottomLine · 06-29-2009, 02:33 PM

Many thanks to WEDOCQ and A1 for your responses.

A1 let me go over some the troubleshooting steps that were done. The original mechanic did in fact test with a spark/gap board and found no spark. Then we went ahead and swapped the key ignitions and found no spark. He then went ahead and placed a remote start harness on the engine directly and had the same results. At that point it became an 80/20 decision and the 80 was leaning towards the stator. But now based on your explanation it seems that since the charge coils are independent I should have a spark on at least one bank. And the fact that I have none at all would lead me to believe that I have some sort of grounding issue. Let me know if I’m way off base?

A1 Outboard · 06-30-2009, 08:26 AM

Your tech has eliminated the boat harness and the key switch. It is still possible to have a short to ground in the engine harness. Another way to check for shorts is at the connector on the CDI. Make certain you disconnect the boat harness from the engine harness whe you do these tests. You can also do the cranking out put test and ohmeter tests for the charge coils and the timer base at this connector. When you are doing these tests be certain you do not damage the pins in the connector. When you reinstall the stator be sure to use Loctite on the bolts.

TheBottomLine · 04-22-2010, 10:00 AM

Well its been a couple of months since I replaced the stator and all was perfect until this weekend. I am now starting to see the same symptoms that I had before the engine died last year. The port engine will maintain 4200rpm until I accelerate past that. Once I try to push it past 4200rpm it will just start to drop until I pull back. If I pull back to anything under that it will be fine. I can then resume to 4200rpm with no problems. And to add more to my frustrations I have mechanics telling me its a possible fuel pump problem, even after the above explanation. That makes absolutely not sense to me. I am leaning toward a bad grounding problem. Any help/troubleshooting processes would be greatly appreciated.

A1 Outboard · 04-23-2010, 06:51 AM

The problem may be in the fuel system. The easiest way to test is with a fuel pressure gauge to check for the pressure dropping off at the RPM where the problem shows up. A good tech will also use a vacuum gauge to check for a restriction on the inlet side of the fuel system. The problem could well be as simple as a dirty filter.

TheBottomLine · 04-23-2010, 07:17 AM

What I dont get is that before, with the same exact symtoms, I did not do anything except change the stator and it ran like a champ. I will move forward with diagnosing the fuel pumps and let you know. Thanks for the input.