what would toast my tacho?

[oommd]

WHat would burn up my tacho in the process of trouble shooting wether my starter solenoid or starter was bad?
After reading the forum I jumped the the starter straight from battery , did this send something strong current to the tacho, It did rev pretty high at the begining.
Im interested because now im looking to replace that and dont want it to happen again.
ALso Im finding that its alittle tricky to find a tacho for a 3 cycl motor.
I have a 1987 70 Johnson outboard.

Im off to pull the burned out one out and get the excact model number to see if I can find a replacement , assuming it is is even for a 3 cycle , doesnt it really matter if I put one in on 4 cycle mode?

thanks

 

[petryshyn]

Re: what would toast my tacho?

First off, how do you know the tach is bad?

Secondly, the tach you seek is an alternator tach not an ignition tach. It is very common amongst marine tachs and doesn't care about 2 or 4 stroke issues.

Teleflex, Faria, OMC and others make tachs for you.

 

[jtexas]

Re: what would toast my tacho?

I really don't see how that could damage the tach.

Before you buy a tach, measure the battery voltage with the engine idling -- should be higher than it is with the engine off. If it's not, replace the rectifier.

If you do in fact need a tachometer, basspro will have it, academy sports, cabelas, iboats, west marine, some walmarts probably have a marine tach on the shelf (although they're marine depts are hit or miss, in my experience).

 

[oommd]

Re: what would toast my tacho?

I meant burned literally, small fire causing soot on the interior of the glass.
I will do some voltage testing to see what is getting key on , motor on etc.

thanks for the info about type and the 3 cycle issue, I saw that the one I took out was set for 4 so wasnt to worried about finding a replacement.

Its a OMC and I pobablt will gey another so the wire colors match.

So its still a mystery as to HOW it happened, dang another issue for a first time boat owner.

Let me know if you can think of other tests.
<> should be higher than it is with the engine off. If it's not, replace the rectifier.<-- Ill keep this in mind and to the testing tomorrow and post my findings

oommd

 

[jtexas]

Re: what would toast my tacho?

I meant burned literally, small fire causing soot on the interior of the glass. ...

Well I'd call that pretty conclusive.........

You should have 12v on the purple wire (IGN post) with the keyswitch in the "run" position, 12v on the blue wire (LT post) when the helm lights are turned on, and AC on the gray wire (SND post) with the engine running.

 

[oommd]

Re: what would toast my tacho?

My measurements on the wires dont add up, does this mean bad rectifier?

<>You should have 12v on the purple wire (IGN post) with the keyswitch in the "run" position <>( check)
<>12v on the blue wire (LT post) when the helm lights are turned on, and AC on the gray wire (SND post) <> ( something wrong there)

The grey drops and the lights are way out, the element in the bulb looks fine after i wiped the soot off , what flamed happened close to the bulb. I will test to see if the element is busted.

So I need to follow this back to what feeds the tach, any pointers.

thanks

 

[petryshyn]

Re: what would toast my tacho?

If you are getting 17 volts at idle, your getting more at high RPM. That's what likely took out the tach.

Do you have a regulator/rectifier, or just a rectifier on your engine?
If you have a regulator/rectifier, you better be troubleshooting it as 17 v is too high.


Cheers...

 

[oommd]

Re: what would toast my tacho?

after some research I believe I have the 3 wire just rectifier, hard to know for sure since the specs I have and many of the stuff I read say rec/reg ( and maybe that is what is supposed to be on it ?) or maybe that is on it?

The seloc online subscribtion tells me that its a part number
Sierra Part #18-5827
Regulator/Rectifiers (EL/TL) for 165.00 , But it doesnt look like what I have and there are four wires in thier pic , this has only 3.
Im thinking they messed up the parts list for my model motor.

Its got one red , one yellow , one yellow/grey stripe.
I followed Joe Reeves test and it tests good on that particular test. I got readings one way and not the other way on all wires to ground and wires to red.

I have a new tachometer comin and do not want to install it till I know that its not going to fry. Im stumped at this point.

The part number on the rectifier is not the one on it ( mystery solved thank to this forum) I believe it currently has parts number 153-3408 which is what the OMC catalog says where on the original 87 70hp johnsons

WHat next o masters of the sea?

 

[petryshyn]

Re: what would toast my tacho?

sounds like you have only a rectifier. Either you'll have to keep some lites on to keep the voltage down, or add in a regulator for the tach. A 7812 would do the trick for about $2. Upgrading to a regulator/rectifier will also work but will be more costly...

Cheers

 

[oommd]

Re: what would toast my tacho?

I like the sound of a 2 dollar fix/test, would you guide me to using it?
My boss is electronics engineer so He could probably help if he understood the problem. I see 3 leads on the chip, would I simply build a little housing and run the 3 wires through it?

Thanks for your time schematic, I can fish with out this right now but Im really enjoying doing this work on my first boat , oh and im cheap!

 

[petryshyn]

Re: what would toast my tacho?

no need to build a box, just mount it on the rear of the tach. It won't need a heat sink for the amount of current the gauge draws. Your boss will select the appropriate "case style" as it comes in many types....

 

[oommd]

Re: what would toast my tacho?

So ill use it on the grey wire to regulate the starting voltage down.
assuming the grey wire sends the pulses to the tachometer.

Yes I get that now that I have looked at the tech info of the chip.
ILL try to get one at radio shack or fry's and see what it does on the new tac and report back my results in a week.

Thanks for both of your replies.

 

[petryshyn]

Re: what would toast my tacho?

NO.....the 7812 is to regulate supply voltage to the tach. (purple wire).

The grey is the signal wire which will remain unchanged....

You also should look into keeping your battery voltage down by adding a load while cruising. 17.17 volts plus will result in battery damage and high water consumption over time....

 

[oommd]

Re: what would toast my tacho?

Im in the process of hooking all this back up and was testing voltage before I committed- got 24v on my purple power wires for a bit?
Then I checked later and its reading 12v again....This looks like tach frying behavior coming from ( ?) Bad battery? Bad alternator? mental condition?

I will put the regulator in place but im guessing it wont protect me from a 24v surge.

 

[oommd]

Re: what would toast my tacho?

UPdate,

I have everything hooked back up without the regulator as I need to solder some leads to it and ask a few more questions.

The 24volt anomaly will come back if it was real but Im not to worried after finding a couple odd things and correcting them.
First I found I was measure voltage coming off one battery and grounding it to another and getting strange readings , That is now fixed by connecting both battery grounds together, Someting I planned on doing after reading about " floating ground" issues.

Had some current running through a wire that should have been switched shut and so on.

Also I have a dead temp guage that was in the chain and not sure what effect it may be having but ill be replacing that and will learn more through that process.
They are cheap enough so ill do that before I get into the forums on "why my temp gauge dont work?"

I think there was some combo of events that sent 24 volts through my tach and since Im not the most through electrician think it was my oversite.

Thank you all for your help.

 

[petryshyn]

Re: what would toast my tacho?

I will put the regulator in place but im guessing it wont protect me from a 24v surge.

I believe the the 7812 will handle up to 40 volts...

 

[oommd]

Re: what would toast my tacho?

oh , ok then it goes in. 2bucks to save my 60 dollar tac is a great deal.

Thanks petryshyn for your guidance.