tacho and rectifier issues

i have recently aquired a 70hsp johnson 1988 small bore,and the tacho doesnt work. after a few looks around i find that the rectifier is totaly fried but the wires were still touching alowing the motor to run fine.
i havnt tested it to see if it is charging but i think it is, as i have had the boat on the water many hours and the battery hasnt gone flat, night fishing included.
just wondering if the recterfier, being fried is why the tacho is not working and would have this stuffed up my tacho , is there a way to test the tacho.

i cant seem to understand why the tacho wouldnt work if the current is still getting though the system even tho its not being rectified if that makes sense.
as far as i can see it is a rectifier only
any help here would be great

Re: tacho and rectifier issues

It is a simple 6 diode rectifier that routes one of the AC leads (yellow/gray) to the gray tach wire.(With reduced Amp output!)
Try to shortly take the gray wire on the connection strip and move to the yellow/gray from stator.
If tach works, then change rectifier.
If the rectifier is already fried, remove it as it may short out and burn your stator.
The engine may be run without the rectifier, just no charge and tach.
Hook a Voltmeter to your battery and start. A minimum of 13,5 V needed for charging, but do not be surprised if you read 15Volts!

Re: tacho and rectifier issues

not sure how you mean, does this pic help to explain to me

Re: tacho and rectifier issues

Pic looks a bit confusing, but this is what it should look like.
2 Yellow wires out from stator, one of them has a yellow gray stripe.
3 wires out from rectifier, 2 yellow and one red.
The yellows from stator to corresponding yellow from rectifier.
The red from rectifier to the red going to battery terminal on starter solenoid.
The gray tach wire, either direct onto the 2 yellow/gray(from stator and rectifier), or on the bridged connection as shown.
If the rectifier is 'out' you run the risk of burning the tach as the full amps from the yellow/gray goes to the tach only and not offloaded to the battery through the rectifier. As a short test, you may test the tach without the rectifier working, but only at low rpm. The higher the rpm, the higher the voltage and amps ....

Re: tacho and rectifier issues

ok thanks for that .
it seems to be hooked up the way you said. bit worried about the tacho tho recon it could be fried. ill do a test on it tomorrow .
not sure on this tacho test bit tho.
in what you say all i would have to do is disconnect the yellow/grey from the board there and try the tacho,as the grey send wire is already there. keeping in mind that the rectifier wires are tempary twisted together at the moment.
does this sound right?


another q i have is
where the loom that the grey wire come from there is another wire about two or three inches long hanging out doing nothing with tape on it, looks like it joined to the connecter board there somewhere, if i put power to it the alarm in the remote goes off, any idea where it might go

Re: tacho and rectifier issues

shantez said:

i have recently aquired a 70hsp johnson 1988 small bore,and the tacho doesnt work. after a few looks around i find that the rectifier is totaly fried but the wires were still touching alowing the motor to run fine.
i havnt tested it to see if it is charging but i think it is, as i have had the boat on the water many hours and the battery hasnt gone flat, night fishing included.
just wondering if the recterfier, being fried is why the tacho is not working and would have this stuffed up my tacho , is there a way to test the tacho.

i cant seem to understand why the tacho wouldnt work if the current is still getting though the system even tho its not being rectified if that makes sense.
as far as i can see it is a rectifier only
any help here would be great

Click to expand...

The rectifier is not necessary for the engine to run; it's there primarily to convert AC voltage to DC voltage to charge the battery, but is necessary for the signal for the tachometer.

It's possible for the rectifier to provide some charge for the battery while having one or more bad diodes, preventing it from providing the signal for the tachometer.

The first thing to do is to test the rectifier; the instructions are in the Engine FAQ forum.

You can test the gray wire signal circuit with an AC voltmeter, placed between the tachometer ground and the tachometer signal posts. You should read >5 volts AC at 1,000 RPM.

Re: tacho and rectifier issues

ezeke said:

but is necessary for the signal for the tachometer.

You can test the gray wire signal circuit with an AC voltmeter, placed between the tachometer ground and the tachometer signal posts. You should read >5 volts AC at 1,000 RPM.

Click to expand...

It takes tach signal straight from the stator lead, not through the rectifier.
The high output chatging system with watercooled regulators are different, but even on those may tach be tested straight from stator.

If the rectifier wires are 'twisted' together and you are afraid it is toast, just leave the stator wires and tach wire where they are, remove the rectifier. That will leave the yellow/gray still connected to the gray.

As for the loose wire, I need a color code. Got a pic?

Re: tacho and rectifier issues

I've been going through some same stuff on my 1976, 70HP. The alarm in the remote is for overheating. The way to test the alarm is to short the temp sender to ground and the alarm should go on. (I found that in another post.) So you may have a short somewhere? I also had a red wire taped up going towards the connection block. It was removed because an amp guage was hooked up. My taped up wire ran to the starter relay, so you could do a continuity check from the taped up lead to the starter relay?

I did replace my rectifier and tach because both were bad. Tach is working, my question is....when I put a meter on the batt, it goes up slowly. 12.55 then 12.56 then 12.57 about 10-20 seconds apart? It is not like when you put a batt charger on a batt and immediately jumps to 13.5v. Is that normal or do I have another issue?

Re: tacho and rectifier issues

Haffiman said:

It takes tach signal straight from the stator lead, not through the rectifier.
The high output chatging system with watercooled regulators are different, but even on those may tach be tested straight from stator.

If the rectifier wires are 'twisted' together and you are afraid it is toast, just leave the stator wires and tach wire where they are, remove the rectifier. That will leave the yellow/gray still connected to the gray.

As for the loose wire, I need a color code. Got a pic?

Click to expand...

Who said the signal goes through the rectifier?

The tach signal won't work without the rectifier - that's a fact.

Re: tacho and rectifier issues

heres a bit better pic of it,
as for the rectifier its fried and i mean theres almost nothing left in the casting.so i know i need one of those.
you can see a wire hanging down with the connecter on the end, brown i would say,altho it looks to be purple where i cleaned it a bit. it looks the same color as the one next to it that goes across to the choke.
when i touch this wire to power on the connecter on the board it sets of the alarm. just not sure where it should go on the board if it even does.

Re: tacho and rectifier issues

I have a feeling that someone may have partly 're-wired' your system.
Purple is the B+ coming out when ign key is ON.
If your alarm goes off, that means either that your alarm horn or sender in oil-tank is defective, or the horn itself.
The one going to the choke primer is purple /white.
Before doing too much more testing and frying more components, I suggest you try to get an OEM manual with diagrams for your engine and 'start all over'.
Most likely your rectifier is fried due to a short somewhere.
For manual, try
e-Bay
Marineengine.com
KenCook
Stay off the aftermarket manuals and on-line manuals.

Re: tacho and rectifier issues

could it be for a amp or volage gauge as they are in the dash but the wireing has been removed from the gauges.
also i did a test on the alarm a few weeks ago and it seemed fine right up untill i removed the rectifier.

Re: tacho and rectifier issues

If an AMP gauge, the red wire from rectifier should go up through the AMP meter, then back to B+ at the solenoid. No need to mess with Ign B+.
If a Volt gauge, that could be pulled out from any 12V source under the dash.

Re: tacho and rectifier issues

ok thanks for that info ill be getting a new rectifier tomorrow and installing it as well as a few new wires as some of them look a bit average and ill let you know how i went with the tacho.