will this work?

R

RB

Guest
My boat tach just quit. I have a 115hp Johnson '77. An OEM replacement
goes for about $130.

There are automotive units commonly available for down around $30, and I'd
love to be able to use one of these el cheapos if it will work.

Most auto tachs have a switch on them for selecting 4/6/8 cyl use. I think
putting the switch on the 8cyl setting (4 cycle, of course) would cause the
scale to read right for my 4 cyl 2 cycle boat engine.

Both tachs seem to be triggered by a 12v pulse from the electrical system.

If I'm right on the above, then it seems I can get away with using the
automotive tach. Thoughts/comments...
 
If your boat engine is a 4 stroke type using a standard ignition
configuration, an automotive electronic tach should work. Take care that the
automotive type may not be as water and weather resistant as the boat type.
It also may not be as heavy duty for taking the waves and bumps that you
will get in a boat, therefore the readings may not be as stable. There is a
very good reason for the big price difference.

If you check out the tachs used for competition vehicles, you will see tachs
going in to the many hundreds of dollars. These will not be water resistant,
but will be very accurate and stable with hard use. There is good reason for
the price difference.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
=========================================


"RB" <rbig@bellsouth.nospam.net> wrote in message
news:qfD7c.64552$xL3.63238@bignews1.bellsouth.net...
My boat tach just quit. I have a 115hp Johnson '77. An OEM replacement
goes for about $130.

There are automotive units commonly available for down around $30, and I'd
love to be able to use one of these el cheapos if it will work.

Most auto tachs have a switch on them for selecting 4/6/8 cyl use. I think
putting the switch on the 8cyl setting (4 cycle, of course) would cause the
scale to read right for my 4 cyl 2 cycle boat engine.

Both tachs seem to be triggered by a 12v pulse from the electrical system.

If I'm right on the above, then it seems I can get away with using the
automotive tach. Thoughts/comments...
 
"RB" bravely wrote to "All" (22 Mar 04 09:08:45)
--- on the heady topic of "will this work?"

RB> From: "RB" <rbig@bellsouth.nospam.net>

RB> My boat tach just quit. I have a 115hp Johnson '77. An OEM
RB> replacement goes for about $130.

RB> There are automotive units commonly available for down around $30, and
RB> I'd love to be able to use one of these el cheapos if it will work.

RB> Most auto tachs have a switch on them for selecting 4/6/8 cyl use. I
RB> think putting the switch on the 8cyl setting (4 cycle, of course) would
RB> cause the scale to read right for my 4 cyl 2 cycle boat engine.

RB> Both tachs seem to be triggered by a 12v pulse from the electrical
RB> system.
RB> If I'm right on the above, then it seems I can get away with using the
RB> automotive tach. Thoughts/comments...

Last I heard seems boat engines don't care if they use car tachs. As
long as the pulses match then the rpm will read right. Your math seems
right to me since a 2 stroke has twice as many pulses. Since there are
twice as many pulses you can always divide by two with a flipflop and
have the switch set on 4 cyl if it matters a lot to you.

.... Now touch these wires to your tongue!
 
On Mon, 22 Mar 2004 09:08:45 -0600, "RB" <rbig@bellsouth.nospam.net>
wrote:

My boat tach just quit. I have a 115hp Johnson '77. An OEM replacement
goes for about $130.

There are automotive units commonly available for down around $30, and I'd
love to be able to use one of these el cheapos if it will work.

Most auto tachs have a switch on them for selecting 4/6/8 cyl use. I think
putting the switch on the 8cyl setting (4 cycle, of course) would cause the
scale to read right for my 4 cyl 2 cycle boat engine.

Both tachs seem to be triggered by a 12v pulse from the electrical system.

If I'm right on the above, then it seems I can get away with using the
automotive tach. Thoughts/comments...
Click to expand...
Ahhhhh. the '77 Johnson... now that brings back memories - not all so
nice. Mine did the same thing just before the ignition modual left me
for dead 20 miles offshore in the thunderstorm (not an experiance I'd
recommend). I was told that this was common for the tach to check out
as the sealed module developes cold colder joins inside that cause the
failures. About 10 years ago when I had the beast, it was about $180
for a new module. BTW, the only thing that saved me was a hamer -
banging on the module was how I got home.

Oh, that's right... you had a question - silly me. Yep, you can use an
automotive tach like you said. It will work.

Gary W.
--
 
Have you opened the tach for attempted repair? 2 tachs on the flybridge of our
CrisCraft packed it in. I carefully unrolled the faceplate cover and found the
"ground" from the tach movement baseplate mechanical connection had corroded
and lost it's conductivity. I added a very small wire from the meter movement
to the ground on the circuit board, carefully hammered the roll back over the
housing and the tachs have been working error free for over 3 years.
 
Thanks much for the good replies and info.

I did try the taste test with those wires that were mentioned. They tasted
pretty coppery, and then they bit me. Bad wires! Naughty naughty!
 
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