Mercury 1995 115 runs well until below 1000 RPM

[richw46]

My 1995 Mercury 115 started having problems last fall. After a long run to the marina when we got into no wake and I slowed to 1200 RPM it was putting along fine. I dropped to 1000 RPM and it immediately died and refused to start. (Died like the kill switch was pulled or key was shut off.) I kept cranking and it eventually fired but almost ran the battery down. I parked the boat and the next morning it started normally and I put it back on the trailer
Winter came and the boat was put on jack stands in my garage
This week I did the normal spring routine including changing the spark plugs. Yesterday I took it to the lake and it started on the first crank. I ran up to 45 mph with no problem, slowed, U-Turn, went at half throttle to my first spot. As I approached the RPMs came down to 1200, idled in gear ok, dropped to 1000 and the engine died again and wouldn't start even with prolonged cranking. I fished for maybe 20-30 minutes, cranked the engine and it fires up. I go to the next hole and again, it dies at 1000 RPM. This happened one more time and then it refused to start any more. I went to the dock on the trolling motor. At the dock I cranked and got a blue-smoke sneeze and that was all.
I'm pretty sure this isn't a fuel problem because it's an immediate stop. The first time it happened I choked it and it just flooded the engine. After that I just turn the key and either it will start within a few seconds or not at all. It seems like it will start after it has been sitting a while, like something needs to cool off. First crank in the morning is always successful. I don't think it's the kill switch in the shifter because switching it on-off doesn't make a difference, but it might be something else in that circuit. In my service manual it shows a black wire that runs from the switch to the motor in one direction, to the ignition switch in the other. There is also a yellow/black wire from the switch to the ignition.
I have a fishing trip coming up in 3 weeks and of course now it acts up. I'd appreciate any help I can get.

 

[houndman]

I recently had the exact same problem with my 1996 Merc 115. It left me stranded a couple times and I had to use the trolling motor to get back to the dock. Turns out it was the voltage regulator.

I downloaded the troubleshooting guide from the CDI website. The second thing to try was unplugging the two yellow stator wires going to the voltage regulator. That fixed my problem. I've made two trips to the lake with the yellow wires unplugged and it ran perfectly. I haven't actually replaced the voltage regulator yet. I wanted to make sure that was my problem before I purchase a $200 regulator.

 

[richw46]

I wouldn't think that the yellow wires would be the problem. I thought those were for charging the battery. Does it always run okay until you turn it off? Does it start okay when it's cold but not warm/hot? Those were my symptoms and it turned out to be the stator. I'm in the process of making a YouTube video on how to replace the stator and switch box. Here's the section of the manual dealing with the charging system. It's difficult to scan because the manual is thick and difficult to get perfectly flat on the scanner bed.

 

[houndman]

It would start up fine while cold but once the engine was warm it would run fine on plane but as soon as it came off plane and the rpms dropped below about 2k it would suddenly shutoff as if it was switched off. The hotter the engine got the worse the symptoms would be until it finally wouldn't start unless it sat long enough to cool off.

I checked the resistance and voltage of the stator and the trigger while the engine was warm and both checked fine but there was no spark at any of the plugs. I was thinking it had to be the switch box but I didn't want to replace it until I was certain. That's when I downloaded the troubleshooting guide from CDI. As soon as I unplugged the two yellow stator wires it instantly started throwing lightning bolts at the plugs. I'm uncertain as to why the voltage regulator would cause no spark. My theory is that the voltage regular is emitting high frequency interference which is mucking around with the switch box.

I've since done internet searches regarding this and many folks have had similar issues with the voltage regulator causing no spark but I've yet to read a cogent theory as to why this happens. I've ordered the Factory Service Manual so that I can try to figure out why the voltage regulator circuit would cause this symptom.

 

[houndman]

This is the third item on the CDI troubleshooting chart:

Disconnect the Yellow wires from the Stator to the Voltage Regulator and retest. If the engine now has spark, replace the Voltage Regulator.

 

[richw46]

Well the stator creates the voltage for the spark and to charge the battery. It sounds like the stator to me. CDI switch box is about $35, stator is $249 and Mercury says to replace both if either is bad. If the motor is firing okay when it runs I'd say the trigger is fine and it doesn't damage the stator or switch box if it goes out.

The readings I get on the stator don't match anything in my service manual, but it does for the trigger. When I got the new stator I tried matching those readings and it still didn't match. When I took readings between the red & red/white and the blue & blue/white I found a difference between the OEM stator and the new CDI stator. I had no readings on the blue & blue/white, which is the low speed coil. I was able to get mine started a few times when cold but it eventually went totally dead, no reading between blue & blue/white. On the new CDI stator the reds were 29.7 ohms and the blues were 564.9 ohms.

 

[richw46]

There's another thread I started, regarding the switch box and rev limiter

Does the switch box impact the rev limiter?

I just installed a new CDI brand stator and switch box on my 1995 Mercury 115. The engine now starts and runs very well. I'm happy with that. However, it now seems to be hitting the rev limiter at 5,000 RPM. With the old switch box I never experienced that. I'm pretty sure it's the rev limiter...

forums.iboats.com

It also has references to this same problem I've been having. After I replaced the stator and switch box I could run up to 5000 RPM and the motor would stall to below that and then start climbing again. The rev limiter is supposed to peak at 5250-5500. I've disconnected it and will try on Wednesday for another run to see if that works.

 

[houndman]

Well the stator creates the voltage for the spark and to charge the battery. It sounds like the stator to me.

I hear you, however, I've taken it to the lake twice now all day long and it's run perfectly with the yellow stator wires disconnected. I'll find out for sure when I replace the voltage regulator. I certainly hope it's the voltage regulator and not the stator since the VR is much easier to replace.

I'll report back my findings.

 

[richw46]

The only thing I could think of would be that the charging coils in the stator are shorted, so this could impact the stator's performance in creating electricity to power the CDMs or ignition coils. Disconnecting the yellow wires from the voltage regulator would eliminate that issue but now you won't be charging your battery.

I would run tests on the voltage regulator (disconnected) and see if the specs are good. If they are then my guess (I emphasize guess) would be the stator.

I hate electronic problems. They are a real pain to trace and figure out. I had an issue several years ago where I could run below 35 and everything was fine. Go above that and every once in a while it felt like you ran over a log (bang) and the alarm horn would chirp. It was just a fraction of a second. Something that happened for just a fraction of a second when over 35 is very difficult to diagnose. I was fortunate enough to find a short post on a forum where someone had said he was told that was the rev limiter. Mine is mounted on top of the engine heat sensor. I guess the rev limiter was grounding out through the heat sensor and chirped the alarm. I took off the rev limiter and the sealing plastic on the back had a big crack in it. I replaced it in 2020 and no more problem, with that, anyway, unless my current 5000 RPM problem is related to that too.

 

[jimmbo]

That's one of those 2+2 motors, below about 1800, it is only running on 2 of the 4 cylinders, so if one of those quits the engine will die.
Check all the wires going to the stator for Continuity, as the stator is moved, as the wires do flex, and perhaps is opening, also look for signs of shorting to ground.

 

[richw46]

Thanks, Jimmbo, I replaced my stator and switch box and the motor starts and runs normally now, up to 5000 RPM and then it shuts down until the RPMs drop and it suddenly starts up again to 5000 and drops, etc. It shold be able to reach 5250. But that's another post I opened:

Does the switch box impact the rev limiter?

I just installed a new CDI brand stator and switch box on my 1995 Mercury 115. The engine now starts and runs very well. I'm happy with that. However, it now seems to be hitting the rev limiter at 5,000 RPM. With the old switch box I never experienced that. I'm pretty sure it's the rev limiter...

forums.iboats.com

Houndman says his problem is similar to the one I posted here but he's able to get his motor running by disconnecting the 2 yellow wires from the voltage regulator, but the battery can't charge now.

He's going to replace the VR and see if that gets everything working again

 

[richw46]

Went to the lake today and had success in running at WOT with the rev limiter disconnected. The motor ran up to 5250 with no stalling or other problems. I ran about 2 miles down the lake at WOT, stopped, fished for several hours, then made the return run; no problems.

So it's not a fuel problem and disconnecting the rev limiter shows that the stator isn't the problem. That leaves the switch box and the rev limiter. However, in April before the stator went totally out, I was able to make the same run with the same results with OEM parts. I suspect the switch box and the vendor has offered to send me a different model of it that is compatible with my motor.

While I'm waiting for the new switch box (he said maybe 10-14 days) I'll put back the old switch box (OEM) and see if that makes it falter at 5000 RPMs. If it does, then most likely it's the rev limiter.

BTW, I was watching the tach and a couple of times when the prop cavitated because I trimmed the motor too high or made too tight of a turn it went to 5400 RPMs, very briefly before I throttled back. The rev limiter can prevent that, I think. I never had the motor falter like that before, so I don't know if the RL was ever working or not.

 

[houndman]

Turns out my problem was caused by a bad bullet connector on one of the stator wires behind the oil tank. While installing the new voltage regulator, I needed to remove the oil tank to get to the voltage regulator connections behind it but the tank was "stuck" to something. The bad bullet connector had melted into the plastic oil tank. About an inch of wire on each side of the bullet connector had melted insulation exposing bare wire. I removed the bullet connector and bad section of wire and direct-soldered the wires together. I took it to the lake today and it ran fine.

Before the problem started I noticed a lot of interference on my fish finder display and the tachometer had become "flaky". Neither symptom were apparent today. She's fixed!!!

I assume that the bad bullet connector connection was arcing and causing a lot of high frequency interference which was mucking with the switch box.

Unfortunately, I now have a brand new, unused CDI voltage regulator that I don't need and can't send back.

 

[richw46]

I have plenty of good but can't use spare parts laying around. I think of it as something I can plug in to find out if that's the problem on the next issue.

If the voltage regulator is going bad you can possibly see the voltage on your fish finder. The last 2 I've had included an on-screen option for battery voltage. That's when I saw mine was going bad the first time it happened. I was seeing 20 volts. That's when my wires got fried. The Ranger shop replaced it but didn't replace the bullet connections and a few years later I found some charred wires going to/from the regulator because they were corroded from the first time.

I'm not a fan of soldering wires together because if they get really hot, like hot enough to melt plastic, they can also melt the solder. Hot wires can also weaken a soldered connection over time and eventually break. I use good butt splices and cover the connection with self sealing heat shrink; thick stuff. Get a good connector crimping tool. I got this one years ago and it's been very handy. https://www.amazon.com/gp/product/B0006M6Y5M

One more tip: I have learned that if I'm going to buy a part I can't return, I'll remove the suspected bad part and look it over. That gives me a chance to eyeball the part for cracks, indications of overheating, unusual wear or other things. If I can find a YouTube video on how to test an electrical part I'll do that too and when I get the new part I compare the readings of the old and new part. If they're the same then, if possible, I might return it.

I'm glad you're back on the water. Think of the money you have saved, doing it yourself and, as a bonus, you are now smarter than before about your outboard. [Silver lining cloud here]

 

[richw46]

I put the OEM switch box back on the motor and connected the rev limiter wire again. The motor behaved just like the new CDI switch box, so it must be the 3 year old Mercury limiter that's the problem. With the wire disconnected the motor runs normal, so I may just leave it off unless I can find a deal on a new limiter.

Original problem was caused by the stator and replacing it put me back on the water. The switch box was replaced too but it turns out the OEM part wasn't damaged.