Rectifier already bad?

[MassillonBuckeye]

Replaced a faulty rectifier on 1978 Merc 70HP a couple weeks ago. First couple runs on muffs in driveway, tach was working. Water test later in the week and no tach and running rough(think its fuel at this point). Removed rectifier and testing bad now. Actually with analog meter, its testing bad. On a digital multimeter it's showing some continuity between ground and post a and c, but looks like about 3m resistance on one of the range settings. No continuity on any other range setting. Honestly, I'm not too sure how to read or set this digital meter and I can't find instructions for it. Tenma 72-2050.

Either way, the tach isn't working and I'm fairly confident it's connected properly. It worked when I first connected new rectifier.

What could have caused the rectifier to go bad so soon? Could be overtightening one of the nuts and the post spinning slightly cause this? One of the posts have have spun a fraction of an inch then I stopped. It had worked after that in the driveway though.

 

[Sunfish12]

Re: Rectifier already bad?

Use the analog meter and yes, you defintely could have broke the rectifier. It's not suppose to have any movement.

 

[MassillonBuckeye]

Re: Rectifier already bad?

What else makes the rectifier go bad? I know heat does. Could a bad stator kill the rectifier?

 

[narvlebenoit]

Re: Rectifier already bad?

I would say the post spinning is why its bad.

 

[Sunfish12]

Re: Rectifier already bad?

Reversing battery terminals can make it go bad.

 

[MassillonBuckeye]

Re: Rectifier already bad?

I would say the post spinning is why its bad.

Yeah.. It was just curious why it worked for a bit before conking out. But I agree. When I felt it slip, I felt impending doom! Heh

Reversing battery terminals can make it go bad.

Was very careful not to do that.

Side note: The service manual has you test red lead to ground, black lead A and C terms for Continuity. I have none. And Black lead to Terminal B and Red lead to A and C for continuity which I have none. Failing these tests, the manual says tha diodes are open circuited.
Black VOA lead to Ground, red lead to A and C for NO continuity, which I Have continuity. Red VOA lead to B and Black to A and C for No continuity, which I Have continuity. Failing both these tests indicates that diodes are short-circuited. How can I fail all test which indicate both open circuited and short circuited? Is this possible? Thats what the VOA and testing documentation is telling me.

 

[Sunfish12]

Re: Rectifier already bad?

Because your readings are opposite for every test, are you sure your metering it correctly, (a to c, c to a).

 

[oldman570]

Re: Rectifier already bad?

Sounds as though the test leads are reversed and the recitifer is good. Test the voltage accross the batt. while idling and rev it up while watching the voltage. It should raise up to near 14.5 volts. A post spinning on the recitifer will be bad news as it will breake the wireing in the recitifer. The Tach lead goes to one of the A or C post on the recitifer. If tach still not working check the power and grd. at the tach. and the fuse if so equiped.
Oldman570

 

[MassillonBuckeye]

Re: Rectifier already bad?

Because your readings are opposite for every test, are you sure your metering it correctly, (a to c, c to a).

Yes. Ive also done it more than a dozen times just so I know I wasn't doing it wrong.

The leads are not reversed.

 

[oldman570]

Re: Rectifier already bad?

Now you can replace the rectifier and go from there. Test the stator while replacing the rectifier to make sure it is good. The small rectifiers are very touchy about voltage and can not be run without a battery hooked up. You don't need to feel bad as I have had to replace several. Bad connections anywhere in the charging system wireing or just hooking a charger to the batt. reversed even for a second will sometimes blow the diodes. I've gotton to where I keep some spares handy nowdays.
Oldman570

 

[MassillonBuckeye]

Re: Rectifier already bad?

Now you can replace the rectifier and go from there. Test the stator while replacing the rectifier to make sure it is good. The small rectifiers are very touchy about voltage and can not be run without a battery hooked up. You don't need to feel bad as I have had to replace several. Bad connections anywhere in the charging system wireing or just hooking a charger to the batt. reversed even for a second will sometimes blow the diodes. I've gotton to where I keep some spares handy nowdays.
Oldman570

Gotcha. Ugh! Hmm, well I believe everything is connected properly. Preliminary stator resistance/continuity testing shows good but I'm going to test voltage with DVA adapter this afternoon. Good news is I have a whole ignition system on the way from eBay. Thanks for the input guys. I'll update this thread as necessary.

update: I diagnosed very weak spark and bad coil on #1. Replacing that and the rectifier when my package gets here this week.

 

[MassillonBuckeye]

Re: Rectifier already bad?

Now I'm confused. Purchased the actual Mercury service manual, and the rectifier test method contained shows my rectifier Ok. How can his be?

Heres a video of the test procedure. You can't see them, but the leads are definitely connected to the meter properly.

 

[oldman570]

Re: Rectifier already bad?

The recitifer you tested in the video is good and should work fine. The test procedure in the book is correct. I test all recitifers the way you have and pitch all that test bad, that way I know when I pick one out of the storage bin I know it is good. I would not trust the recitifer that had the post turned and pitch it. The new recitifiers and cheap when you are out on the water and do not want to have any trouble. Nothing worse than driving any distance just to find out that you have to get pulled back, or paddle, to the ramp because of a bad part. It sort of reiuns your whole day. If in doubt do all the test in the manual of the electrial system. As for the tach, it could have been damaged by high voltage from the battery or bad connectios to battery pos. and or grd. Test the back of the tach for 12 volts from the ingition switch and make sure the sender wire is hooked to the recitifer on one or the wires from the stator and not the battery post.
Oldman570

 

[MassillonBuckeye]

Re: Rectifier already bad?

The recitifer you tested in the video is good and should work fine. The test procedure in the book is correct. I test all recitifers the way you have and pitch all that test bad, that way I know when I pick one out of the storage bin I know it is good. I would not trust the recitifer that had the post turned and pitch it. The new recitifiers and cheap when you are out on the water and do not want to have any trouble. Nothing worse than driving any distance just to find out that you have to get pulled back, or paddle, to the ramp because of a bad part. It sort of reiuns your whole day. If in doubt do all the test in the manual of the electrial system. As for the tach, it could have been damaged by high voltage from the battery or bad connectios to battery pos. and or grd. Test the back of the tach for 12 volts from the ingition switch and make sure the sender wire is hooked to the recitifer on one or the wires from the stator and not the battery post.
Oldman570

Ok then my next question is why is all of this incorrect testing material floating around? That first test I linked here can be found on iboats. It was taken from what I think is an aftermarket manual. I bought it as a PDF on eBay. People like myself are searching and using that procedure to test. I was ready to buy a new rectifier. Why are there apparently two different testing methods?

My tach harness on the front of the controls: Do I have to run the ground coming out of that straight to the tach? Right now, the white and black are going to the battery gauge. The tach sender is going straight to tach. Tach ground goes to my ground buss which is right there close which the gauges are also grounded to Shouldn't matter where I ground to as long as its a good ground right? How do I test the tach harness properly?

 

[oldman570]

Re: Rectifier already bad?

The test for the rectifier are by diffrent manufactures of manuals. One by Mercury and the other by Soclac or CDI. As for the tach, I use the black from the controls as grd. for all gauges that I want to work as soon as the motor starts. The purple from the controls should be 12 volts +, and the gray is the tach wire to the rectifier.I use the grd buss for all other accressies, lights, bilge pump, livewell, and items that I want to be able to use without the motor running. Test the tach wire by disconnecting it at the rectifier and tach, test it for any grds, voltage, and opens, broken wire. This can all be done with one end taped up. then remove tape and grd the wire to check for open , broke, wire. The tach wire dose not go to anything in the controls, check the wireing diagram in the Merc manual, and it has other conections in it on diffrent setups. If all else fails run a new wire for the tach from the rectifier to the tach to see if the factory wire is the trouble.
Oldman570

 

[MassillonBuckeye]

Re: Rectifier already bad?

The test for the rectifier are by diffrent manufactures of manuals. One by Mercury and the other by Soclac or CDI. As for the tach, I use the black from the controls as grd. for all gauges that I want to work as soon as the motor starts. The purple from the controls should be 12 volts +, and the gray is the tach wire to the rectifier.I use the grd buss for all other accressies, lights, bilge pump, livewell, and items that I want to be able to use without the motor running. Test the tach wire by disconnecting it at the rectifier and tach, test it for any grds, voltage, and opens, broken wire. This can all be done with one end taped up. then remove tape and grd the wire to check for open , broke, wire. The tach wire dose not go to anything in the controls, check the wireing diagram in the Merc manual, and it has other conections in it on diffrent setups. If all else fails run a new wire for the tach from the rectifier to the tach to see if the factory wire is the trouble.
Oldman570

Why are the tests showing me opposite diagnosis though? First one says BAD, factory manual says GOOD ???

 

[MassillonBuckeye]

Re: Rectifier already bad?

Should I make a new thread? Why does the first testing procedure show my rectifier is bad, and the second procedure shows it good?

 

[sschefer]

Re: Rectifier already bad?

You can hook your tach up to either of the yellow wires from the stator and after setting the pole filter on it, with the engine running, it should give you correct readings. If it doesn't, your tach is faulty. No need to make it difficult.

1. Remove the Battery + wire from the rectifier.
2. Take a simple 12v light bulb type tester and connect one side to ground and the other side to the pole on the rectifier that is marked Bat + or where the manual says the Bat + is supposed to be connected. Start the engine and if the light bulb lights steady then the rectifier is probably just fine.

 

[MassillonBuckeye]

Re: Rectifier already bad?

You can hook your tach up to either of the yellow wires from the stator and after setting the pole filter on it, with the engine running, it should give you correct readings. If it doesn't, your tach is faulty. No need to make it difficult.

1. Remove the Battery + wire from the rectifier.
2. Take a simple 12v light bulb type tester and connect one side to ground and the other side to the pole on the rectifier that is marked Bat + or where the manual says the Bat + is supposed to be connected. Start the engine and if the light bulb lights steady then the rectifier is probably just fine.

There is no documentation on the back of the tach, but there are two little screws. Not sure what they do. Its an Airguide tach. Older one. That worked with the motor for a bit when I first replaced the rectifier. The old rectifier was Definitely bad. I'll try to find more info on it. It seemed a bit "sticky" when it did for there for that time or two. I think that tach is bad.

How difficult is it going to be to find a tach for this older 3cyl Merc motor? I need one that does the 12 pole thing right?

 

[sschefer]

Re: Rectifier already bad?

Here on iBoats they sell Teleflex which are good. Make sure you get the two stroke model.