Tach signal negative volts??

[rolrandy]

94 Johnson 120 v4 replaced rectifer Tach signal at tach reads negatine volts on a 20volt scale. negative volts goes up as the motor rpm goes up.
At back of tach gray AND PURPLE wiring correct double checked that
could tach be bad it has never worked

ANY IDEAS

 

[F_R]

Re: Tach signal negative volts??

Are you sayng you have a negative DC voltage between the gray and ground wires? Hm-m-m-m, would have to think about that one. Actually, it seems like there should be AC on the grey, because it hasn't been rectified yet (ahead of rectifier).

 

[rolrandy]

Re: Tach signal negative volts??

yes negative voltage at the tach betwen the ground and grey wire
and it varies with engine speed
should I be trying to read AC volts

new rectifier either way and the tac still doesn't work

 

[Silvertip]

Re: Tach signal negative volts??

Yes you should be looking at an AC or pulsed signal. If the signal was a negative (or positive) voltage, the tach would only need to be a voltmeter. A tach is instead a pulse counter. It counts 6 pulses per engine rev which is what the calibration dial on the back of the tach should be set to.

 

[rolrandy]

Re: Tach signal negative volts??

Thanks I'll look at ac volts
I think my meter has 2 ranges 250 and 750 an an ac setting
what reading should I be looking for at idle

 

[F_R]

Re: Tach signal negative volts??

It would be approximatly 12 volts AC. But it is not 60 hz AC like in a house. Your meter may not be able to figure it out, I dunno. It would be much faster than 60 hz, even at idle. The hz (frequency) varies with the rpm, that what it makes it work on the tach.

 

[Silvertip]

Re: Tach signal negative volts??

Your meter cannot read RPM from that signal. That's the job of the tach. You would be better off checking battery voltage with the engine running at 1500 RPM. If you read anything more than 13 volts, the charging system is working and the problem is not with the tach sense line. More than likely the tach is dead. Make sure you have 12V on the +12V terminal. Make sure the ground terminal is tight and that there is an actual ground connection. If you have any reading on the sense line and the tach doesn't work, it's probably toast.

 

[F_R]

Re: Tach signal negative volts??

Silvertip says it all. It is senseless to try to figure out the signal from the rectifier. If it is charging the battery as evidenced by rising voltage when running, that part of it is working properly. If that part is working, and the tach doesn't work, it is a bad tach or not hooked up right or something like that.

 

[rolrandy]

Re: Tach signal negative volts??

can I use an automotive tach/dwell meter to read the tach signal
before I buy another tach?

 

[rolrandy]

Re: Tach signal negative volts??

bump

will this work??

 

[ezeke]

Re: Tach signal negative volts??

Go to engine end of the gray wire leading from the main harness and disconnect it from the terminal block. Move the gray wire to the gray and yellow wire from the alternator on the terminal block and connect it. See if the tach works.

​

 

[rolrandy]

Re: Tach signal negative volts??

Ok Tach works if I run with grey wire on the grey/yellow terminal

batt doesn't seem to be charging new rectifeir WHAT NEXT???

 

[ezeke]

Re: Tach signal negative volts??

To me, that is good news because we can figure that the rectifier is the problem, but the tach is good and so is the wiring.

So, what kind of rectifier do you have? Where is it mounted? How many wires?

Here are the ground rules: never work on the rectifier with the battery connected. Never buy a rectifier except from a reputable dealer and only brand new. Never connect anything with voltage to the rectifier to test it.

 

[ezeke]

Re: Tach signal negative volts??

If you have a water cooled regulated rectifier, Check this one from CDI. I don't think you can beat their products. If one in the list of OEM numbers matches yours, I would give it a shot:

http://www.iboats.com/193_4205_Regu...7626582--**********.138359401--view_id.335912

 

[rolrandy]

Re: Tach signal negative volts??

brand new one from OMC water cooled 6-8 wires maybe

 

[rolrandy]

Re: Tach signal negative volts??

went too look actually 5 wires omc p/n 0585192
the one I took out was a CDI no p/n on it 5 wires also

 

[ezeke]

Re: Tach signal negative volts??

Scroll down on the link I gave and you will see that t is a match for the OEM number that you have. At $73.+ and shipping, it is a reasonable price for a good part.

 

[rolrandy]

Re: Tach signal negative volts??

So you think I bought a brand new OMC recifier that is bad???

 

[rolrandy]

Re: Tach signal negative volts??

On the purple wire with the key on I am geting .11 volts turn the key off and I get no volts

does this help diagnosis my problem?

 

[rolrandy]

Re: Tach signal negative volts??

let me also add I just put a new ignition switch in all connections are good and starts and runs ,,