Mercury Tach Problems - is rectifier bad, or stator??

[CraigC]

Hi, I've got an 87 Merc 150 Black Max outboard that's just developed a tach issue, basically at an idle the tach reads around 2000 RPMs, and it doesn't change much until it's really wound up - best described as erratic tach. I've read about checking voltage on the grey wire and, correct me if wrong please, but it should be like 3.5vac or so at idle? I'm getting around 13 at idle!!! (and closer to 20 at what I guess to be around 2000 RPMs).
Does this sound right?
Is this going to be a bad rectifier, or could this possibly be a bad stator?
The way the wires connect to the rectifier it would almost seem like whatever voltage was on the yellow wire from the stator would be translated directly to the grey tach wire....? Or no?

Also, I don't have the factory manual, I've got another brand covering a lot of different years and sizes, and the wiring diagram they give me for this doesn't show exactly what I've got, specifically it seems to be missing the voltage regulator!??
Does anyone have the correct wiring diagram for this....or can anyone just confirm the wiring is correct?
Looking at the rectifier I've got 3 terminals on it, one marked (+) that has 2 red wires on it, one going to the voltage regulator, a second terminal with the grey tach wire and a yellow wire from stator, and a third terminal with a 2nd yellow wire from stator and a yellow wire from voltage regulator.

Thanks for your time guys.

 

[Barnacle_Bill]

Re: Mercury Tach Problems - is rectifier bad, or stator??

Your wiring appears to be correct. The tach runs off an AC signal from the stator. The rectifier is a convenient tie in place for the gray wire as the stator also feeds it. Thats why when the rectifier goes bad it affects the tach. Test the stator according to your manual.

 

[CraigC]

Re: Mercury Tach Problems - is rectifier bad, or stator??

Thanks for the help, but I was afraid you were going to say that! I feel better with a second opinion though.

I ran the tests on the stator...and it's bad. According to this manual there should be NO continuity between 2 yellow stator wires, and there is! The rest of the stator wires tested good, so I guess that's why it still ran fine, but had the tach go out. So it goes I guess.

This brings me to another question however. I've been looking online for a new stator, and there's a 9 amp, and a 16 amp listed for this serial # but I haven't heard a clear answer for which one I need. What's the test for it? Or how do I tell which one I need?

Thanks again for the help.

 

[Barnacle_Bill]

Re: Mercury Tach Problems - is rectifier bad, or stator??

You are reading something wrong. There should be around 1 ohm resistance with a lead on each wire. No reading (infinity) on each wire to ground.

 

[CraigC]

Re: Mercury Tach Problems - is rectifier bad, or stator??

Oh, sorry, that's my mistake...book says less then 1 ohm between and it tested .6 ohms. It says to test each lead to ground, set to Rx1000 scale, and should have no continuity...THAT'S where I had continuity - it's showing just below 397 on each.
I knew one of the readings was off, just got them mixed up in my head when it came time to type it out. :redface:

That means it's shot, right?

Any ideas on the 9 vs 16 thing?

 

[Barnacle_Bill]

Re: Mercury Tach Problems - is rectifier bad, or stator??

If you had both yellow wires disconnected when you tested them, then yes, it is bad.

 

[CraigC]

Re: Mercury Tach Problems - is rectifier bad, or stator??

Yes, they were disconnected. I've heard that CDI makes higher quality ignition parts then Mercury and they're less $ so I think I'll replace it with one from them. Fairly certain it's a 16 amp stator after more research.

Thank you for all the help - I really appreciate you taking the time to help me out!

 

[j_martin]

Re: Mercury Tach Problems - is rectifier bad, or stator??

Let me help if I can.

The signal to the tach is a boogered up AC on a DC carrier coming off one side of a bridge rectifier. The tach don't really care, because it blocks the DC and flips its switch when the AC goes past a certain threshold each cycle. If you read 13 volts AC on that wire, the tach should read.

The stator may be going, but that amount of leakage to ground isn't going to even heat anything up, and shouldn't interfere with normal electrical operation.

If the rectifier fails, it will usually cause the tach to not read. That is because the signal is generally shorted to ground.

There is a test for the rectifier in the FAQ on this site. If it checks out good, and your battery voltage goes up when the engine is running, the tach is bad. I suspect that's the case.

hope it helps
John

 

[CraigC]

Re: Mercury Tach Problems - is rectifier bad, or stator??

Thanks for the input.
I'm getting a reading on the tach, but at an idle it's reading 2000 RPM which I'm sure, from experience, is incorrect. With increased RPM it does go up, but its surely not ever correct and seems a bit eratic. It never reads below 2000.
My concern is that I didn't think the stator should be putting out 13v on those yellow wires at an idle, I thought it should be lower? I'm wondering if the higher tach reading was caused by this increased voltage, and that it's an indication of a bad stator as well...but I'm not sure??
The stator is testing within specs on all the other tests, except for the above mentioned, and engine seems to run just fine.
I tested rectifier and it checks out ok.

Should I not bother replacing stator then?
Is there a way of testing the tach?

Thanks.

 

[j_martin]

Re: Mercury Tach Problems - is rectifier bad, or stator??

Thanks for the input.
I'm getting a reading on the tach, but at an idle it's reading 2000 RPM which I'm sure, from experience, is incorrect. With increased RPM it does go up, but its surely not ever correct and seems a bit eratic. It never reads below 2000.
My concern is that I didn't think the stator should be putting out 13v on those yellow wires at an idle, I thought it should be lower? I'm wondering if the higher tach reading was caused by this increased voltage, and that it's an indication of a bad stator as well...but I'm not sure??
The stator is testing within specs on all the other tests, except for the above mentioned, and engine seems to run just fine.
I tested rectifier and it checks out ok.

Should I not bother replacing stator then?
Is there a way of testing the tach?

Thanks.

It's perfectly normal. Your electrical system is fine.

Now that we've found out that the tach reads something, that changes the whole trend of the troubleshooting.

1. Noise on the wire can cause an increased tach reading. (Unlikely unless you have some high powered electronics on board, like radar. A loose connection that shakes with engine vibration could do it.

2. There is likely a selector switch on the tach. It's probably set to the right position if the tach ever worked right, but it could be dirty. Sweeping it through all the positions, then putting it where it should be just might clean it up enough to work.

hope it helps
John

 

[CraigC]

Re: Mercury Tach Problems - is rectifier bad, or stator??

Electrical system fine - that's good news, and you just saved me about $150 in un-needed parts, so THANK YOU!

No, don't have any high powered electronics onboard. I've checked, cleaned and tightened most of the connections, but I'm going to go out and double check to make sure I didn't miss any. While I'm at it I'll try sweeping the selector switch on the tach a couple times and then put it back where I find it.
I'll let you know how it goes.
Thanks again for the help!

 

[j_martin]

Re: Mercury Tach Problems - is rectifier bad, or stator??

I just went back and re-read the posts.

If the stator is arcing over to ground, it could make your tachometer go crazy. That arc would be the source of the electrical noise of item 1. If the switch sweep doesn't help, you're back to a bad stator.

hope it helps
John

 

[CraigC]

Re: Mercury Tach Problems - is rectifier bad, or stator??

I swept the tach and while I was at it I cleaned every connection that I hadn't already done on the whole engine/boat, and after that it's working fine now!

I want to say thanks to both of you for helping me with this, it's much appreciated!