rectifier charges but tach doesn't work...

[cdoliver]

First of all. I have read all tach related posts...I'm at the end of my rope on this one. 1979 johnny 115. My tach hasn't worked in years, so I bought a new one (faria). I installed it thinking the old one was just bad. I replaced the rectifier 2 years ago because the battery wasn't charging. Now the battery is charging (sometimes it overcharges) Can a rectifier be bad but still charge and this cause the tach to not work? I tested the rectifier and I get a reading in both directions (not the same reading) so it seems the rectifier is bad...I just thought that a bad rectifier wouldn't allow the battery to charge... Thanks again for the help.

 

[HighTrim]

Re: rectifier charges but tach doesn't work...

It can be bad and still charge, but start with the easiest. It is easiest to check for 12-volt power and proper ground first. A volt-ohmmeter or 12-volt test light are great tools for this procedure. Simply ground the meter or light, turn the key on and touch the positive probe to the purple wire on the back of the tach. You should see 12 volts indicated on the meter, or if you’re using a test light, its bulb should illuminate. Reverse the leads to check for proper ground, or use the ground wire on the tach to ground your meter or test light.

If either the power circuit or ground is the culprit, you can repair the problem and the tach should work fine. In a no-power situation, see if your tach has an inline fuse. These often blow due to constant vibration. After inspecting for power and ground, if it still doesn’t work, it’s time to check the signal-input source, or the tach itself.

A simple way to determine if the tach is bad is to use another tach. This you tried by buying new.

In the event you don’t have access to a new tach, you’ll have to refer to the manual to check the signal source. The signal is a pulse-positive output that’s provided directly from the stator assembly through the rectifier. A peak-reading voltmeter set to “positive” and the lowest volt scale will enable you to read any voltage output (on this circuit) from the stator.

Check the procedure and specifications for your engine, since the inspection point, procedure and wiring are not the same for all outboards. In addition, your ohmmeter can be used to check the continuity of the stator windings, as well as the integrity of the rectifier diodes.

If everything still checks out, you will need to re test your rectifier/regulator as per your manual. What exact test did you do to test it? What were the readings?

These pages may help

http://ww2.tflx.com/pdf/obtach~1.pdf

http://ww2.tflx.com/PDF/99320.pdf

 

[MarkV_Deep6]

Re: rectifier charges but tach doesn't work...

You basically answered your own question. Yes it can be bad and still charge.

I wonder if the internals in the rectifier are the same as many bridged rectifier that sell really cheap. If they are, I would love to be able to replace the guts and re-epoxy everything. Just a thought, at 150-200 a pop, this would be worth it.

 

[cdoliver]

Re: rectifier charges but tach doesn't work...

Thank you for the replies.

I am 100% sure of a good power and ground. 12.4V

The method I used to test the tach was taken directly from this link.

http://forums.iboats.com/showthread.php?t=190638

Here are my exact readings:

I used a Fluke meter set to ohms

Black meter lead to ground:
yellow #1- .7 M ohms-1.1 M ohms
yellow #2- .8 M ohms-1.03 M ohms
red- 1.39 M ohms

Red meter lead on ground:
yellow #1- 1.0- 1.4 M ohms
yellow #2- .8- 1 M ohms
red- .3 M ohms and sometimes nothing...

Black meter lead to red wire:
yellow #1 260k ohms
yellow #2 260k ohms

Red meter lead to red wire:
yellow #1 1.4M ohms
Yellow #2 1.5M ohms


Is this recitifier bad??? Why is the battery charging?
Also, I can connect the signal wire to the sire coming from the stator and get nothing also... I am set to the 12 pole/6p setting on the tach...

 

[HighTrim]

Re: rectifier charges but tach doesn't work...

It seems that your rectifiers diodes are open, the question is why is it blowing? The simple act of mistakingly crossing the battery leads could do it.

When voltage enters the rectifier in the wrong direction it will instantly burn the diodes that make it work. If the heat melts the potting in the rectifier, the wiring can fuse with other wires causing smoke and in bad circumstances fire. That is when it is possible for the 12.65 volts from the battery to flow through the rectifier and destroy components of the ignition system.

Probable causes that may damage the rectifier are as follows.

a. Battery cables loose or reversed as stated;

b: Battery cables disconnected while engine is running;

c: Faulty ignition or battery switch;

d: Voltage applied while testing engine components;

e: Bad or weak battery and/or battery cables.

You mention that your battery is overcharging. Are you speaking in excess of 15 Volts? Run the motor for a good 20 minutes at around 4300 to 4600 RPMS and then measure battery voltage. If so your regulator is likely shot.

In reference to WHY you are still charging, the answer is your outboard has three wires coming from the stator to the regulator/rectifier. One is common, and the others lead to separate banks of charging coils. That way, if one of the coils fails, the other bank should still charge in theory.

Alternators(stators) put out AC current. Graphed-out, it's a sine wave. Simple rectifiers take half of that sine wave and convert it to DC current. Bridge rectifiers take the whole sine wave and convert it to DC. So bridge rectifiers charge at twice the rate of simple rectifiers. If one of the diodes in a bridge rectifier fails however, it still acts as a simple rectifier.

That being said, alternators only alternate if there is a complete circuit. When a rectifier fails, the circuit is broken. That's why Johnson/Evinrude uses two separate banks of charging coils and bridge rectifiers. Those allow back-ups should a connection be broken, hence the reason your tach does not function yet your battery is charging!

 

[MarkV_Deep6]

Re: rectifier charges but tach doesn't work...

I think that's the best explanation of how the rectifier works that I have ever read to this point. Your knowledge is greatly appreciated by many!

I never knew all that redundancy was built into the rectifier. They still burn up so easy, it sure would be nice if there were a way to further protect the diodes.

 

[HighTrim]

Re: rectifier charges but tach doesn't work...

I think that's the best explanation of how the rectifier works that I have ever read to this point. Your knowledge is greatly appreciated by many!

I never knew all that redundancy was built into the rectifier. They still burn up so easy, it sure would be nice if there were a way to further protect the diodes.

Giving me way too much credit, just knowledge accumulated from all the great minds and outboard mechanics on this forum that have taught me. It is to them that we owe our thanks. I am simply an outboard enthusiast that enjoys helping others. There is a big difference!

 

[Yamajoe]

Re: rectifier charges but tach doesn't work...

Ditto, great knowldge and thank you and others very much on this site for passing it own.

 

[ezeke]

Re: rectifier charges but tach doesn't work...

Still, the way to check the unregulated rectifier in relationship to the tachometer has not changed:

http://forums.iboats.com/showthread.php?t=190638

Be sure to disconnect the battery before using the ohmmeter test and when switching the gray wire to the other yellow post on the terminal block. Verify that the tach defaults to zero when the ignition switch is turned to the run position; that lets you know that you have the correct positive and negative wiring to the tachometer from the ignition switch.

 

[cdoliver]

Re: rectifier charges but tach doesn't work...

Thank you, that is very informative.
The charging system has charged above 15 volts (up to 17-18v) before (after the new rectifier 2 yrs ago). I put a new battery on it and the charging seems normal now (never goes above 13.9).
Now... the tach does not work on either yellow wire on the rectifier... bad tach? it's new???

 

[HighTrim]

Re: rectifier charges but tach doesn't work...

From the numbers you provided from Joe's testing technique, they show that your diodes are open, causing the tach failure, not the tach.

You will need to determine though why you have damaged 2 rectifiers.

That is not to say you have other issues as well regarding your tach. Ensure you test it and its wiring from the links provided above and by Ezekes response.

 

[ezeke]

Re: rectifier charges but tach doesn't work...

Temporarily run a separate wire from the terminal block to the tach in place of the gray wire and see if you get a response.

 

[cdoliver]

Re: rectifier charges but tach doesn't work...

A direct wire from the tach to either yellow wire on the rectifier shows no results. There is a high possibility that at least one of the probable causes that High Trim listed has occured in the past. I have resolved all of those problems now. I have a new key switch, new batteries, new battery cables and connectors. It is possible that the cables were put on the wrong terminal for a second in the past as it is very dark where the batteries are... that shouldn't happen again now that I know that it causes damage. I have probably had the battery on charge while trying to start the engine before...guess that's bad.

Now, I will get a new rectifier. I still don't understand if the rectifier is sending a DC signal to the batteries, even if it's on one side of the rectifier, why the tach doesn't work (unless it's a bad tach). We'll see what a new rectifier does... Thanks to you all again.

 

[cdoliver]

Re: rectifier charges but tach doesn't work...

Not to beat a dead horse here... But, I'm looking at a wiring diagram for my engine, and, I just don't understand what the rectifier has to do with the tach working at all (everyone says if the tach doesn't work check the rectifier). The rectifier has two inputs and one output. the tach is taking a signal from BEFORE the rectifier, while it's still an AC current. The red wire is the output which is DC voltage. What does the rectifier have to do with the tach being that it is taking this signal before it gets to the rectifier??? I'm sorry, but I'm frustrated because I just rebuilt this johnny and it runs great, but I can't get the tach to work so I can tell if the rpms are where they should be.

 

[MarkV_Deep6]

Re: rectifier charges but tach doesn't work...

Your rectifier sounds like one of the newer rectifiers that does not have the output mine does. I read a little about that one, but your tach gets signal differently from mine I believe. I have two output wires. Red and Purple along with the two yellow wires.
Hope another person is better able to explain this.

 

[HighTrim]

Re: rectifier charges but tach doesn't work...

They still burn up so easy, it sure would be nice if there were a way to further protect the diodes.

I would like to know why OMC did not fuse them? Yamaha fuses their rectifiers and that often saves them.

Tachs read off the alternating pulses from the stator. The rectifier assembly converts the AC to DC. If the rectifier is damaged, the tach can no longer get the proper alternating pulses that it needs. Any of the leads from the stator charging portion can be used for the signal. The designated tach signal wire is merely attached to one of those at the rectifier as a convenient location. Tachs have rotary switches on the back that set it to different numbers of coils in the charging circuit, times two because there is a positive and negative pulse for each coil for each cycle. Tachs measure pulses.

 

[cdoliver]

Re: rectifier charges but tach doesn't work...

I must have a bad rectifier and and tach... Thats amaizing... I have tried to connect the tach signal wire to the wires coming from the stator with no results.

Heres another question. I've read about the tach resetting to zero with voltage applied. When I apply voltage to the tach, It doesn't do anything. It rests at zero and doesn't move when I give it 12 volts. Is this right?

 

[HighTrim]

Re: rectifier charges but tach doesn't work...

If tach does not 'zero' when power is turned on, the positive (IGN) and/or ground wires are not connected properly. The tach must zero when powered.

When measured with a volt/ohmmeter, the power (IGN) to ground (GND) voltage must be 12-16 volts DC with the ignition on. If tachometer goes to zero and then refuses to track RPM it may indicate the voltage rectifier is not operating properly (even though the rectifier may be partially charging the battery as stated).

 

[Joe Reeves]

Re: rectifier charges but tach doesn't work...

(Small Rectifier Test)
(J. Reeves)

Remove the rectifier wires from the terminal block. Using a ohm meter, connect the black lead of the ohm meter to the rectifier base (ground), then one by one, connect the red lead of the ohm meter to the yellow, yellow/gray, then the red wire (some rectifiers may also have a fourth yellow/blue wire. If so connect to that also). Now, reverse the ohm meter leads and check those same wires again. You should get a reading in one direction, and none at all in the other direction.

Now, connect the black lead of the ohm meter to the red wire. One by one, connect the red lead of the ohm meter to the yellow, yellow/gray, and if present, the yellow/blue wire. Then reverse the leads, checking the wires again. Once more, you should get a reading in one direction and none in the other.

Note that the reading obtained from the red rectifier wire will be lower then what is obtained from the other wires.

Any deviation from the "Reading", "No Reading" as above indicates a faulty rectifier. Note that a rectifier will not tolerate reverse polarity. Simply touching the battery with the cables in the reverse order or hooking up a battery charger backwards will blow the diodes in the rectifier assy immediately.

 

[cdoliver]

Re: rectifier charges but tach doesn't work...

The results are in... And the problem was....BAD TACH... Rectifier was good after all. My brother came over with a good tach and a multimeter that measures rpm's. I get a reading with both. What is the likelyhood of a brand new faria tach being bad??? I'm exchanging it today.

Thank you all for your time and knowledge that helped me diagnose this problem. You are all very helpful.