On Thu, 08 Apr 2010 22:34:24 +0100, goofball_star_dot_etal
wrote:

On 08/04/2010 08:39, Bruce wrote:
On Wed, 07 Apr 2010 14:59:02 +0100, goofball_star_dot_etal
wrote:

On 07/04/2010 14:14, Bruce wrote:
On Tue, 06 Apr 2010 23:49:41 +0100, goofball_star_dot_etal
wrote:

On 06/04/2010 21:33, goofball_star_dot_etal wrote:
On 06/04/2010 21:04, Armond Perretta wrote:
My 2002 (1993) has 3700 hours and runs well. The deluxe panel also was
fine
with all gauges except the tach performing properly. I have replaced the
original tach with the latest (both wires removed from the old sender and
one of these to "w" on the alternator, the hour meter wire removed and
set
aside, etc.). I just started up the engine for the upcoming season and
the
new tach is fine. However now I have a new issue with the temp gauge.
Here
are the symptoms.

Prior to starting, when I turn the key one click, all gauges including
the
temp move a bit, indicating that the wiring is fine to all instruments.
Nothing happens with the tach of course. When I then start up, all gauges
including the new tach perform as designed, except that the old temp
gauge
(which ran perfectly last year) pegs to the left once the engine starts,
giving what looks like reverse polarity connection symptoms. I did not
change any wiring to this gauge when fitting the new tach.

Next, when I shut down after running the engine under load and letting it
come to temp, all the gauges including the temp show what appear to be
correct readings, but only when the engine is NOT actually running.
That is,
it seems the temp sender is OK and sending correct info, the oil sender
same, the voltage meter same, the tack off of course, etc. I think this
means the temp sender is fine, but that running the engine is somehow
sending the incorrect voltage to the meter. Only when I start up and the
engine is actually running (and the tach showing the revs) does the temp
gauge peg to the left. Is it possible I just picked the wrong one of
the 2
old tach wires for the new tach to connect to "w"? No other wiring
changed
from the time the temp gauge was OK. Ideas welcomed.


Check for voltage drops between various earth points when the alternator
is charging.

In particular, check the integrity of the alternator earth path (since
it seems that the error is associated with charging current), the engine
earth path (since, probably, the temperature sensor thermistor uses the
engine as earth), and the instrument earth (since that is where you see
the problem). In an ideal world one would have a single point earth but
this is not always practical.

I'm a bit confused. Assuming a normal inboard engine installation, the
alternator earth is normally either through the case to the engine
block or through a separate earth connection to the same place. The
engine is normally grounded/earthed by a direct connection back to the
negative side of the battery.

It almost has to be connected this way as the starter, which IS
earthed through its case to the engine block will never develop its
normal power without an adequate earth/ground path back to the power
source; the battery.

Cheers,

Bruce
(bruceinbangkokatgmaildotcom)

Fortunately the starter is not running, so does effectively does not
exist during normal operations. I bet some meters kick when the starter
is operated on many boats..
There comes a point when it is quicker to get a meter out and fix the
problem rather than to speculate what should be, or might be. It stinks
of a 'common mode' problem to me, and I would check the earth conections
first. I look forward to hearing the result.

I was commenting on the remark about multiple ground circuits, as
there aren't any. The Temp and pressure senders are grounded through
their case to the engine block. The alternator is grounded through its
case to the engine block.The starter is grounded through its case to
the engine block. If an electrical ignition engine the ignition is
grounded to the engine block. The engine block is grounded through a
big cable to the battery Neg. post. No multiple paths.

Some common paths there, and the possibility of a connection not being
as it should be. If there is a potential difference between the sender
earth (ground) and the instrument earth, it might cause the observed
problem if there are two legs of a bridge circuit which go to different
'earths'. There is no such thing as a single point (earth). Whether an
engine block, terminal block, star washer, or other connection can be
treated as electrically equivalent to one is what matters. It depends on
the circumstances, such as the frequency and the currents involved etc.
Mixing sensitive instruments with high current devices such as starter
motors by sharing a common path (wire) is not a good idea for the poor
instrument, but it well may be ok for two intruments to be connected
through a shared supply wire. For RF interference issues, a low
resistance is no guide to whether a connection is low impedance or can
be regarded as a single point.


As far as the instrument problem is concerned the logical plan would
be to undo the last thing done, i.e., disconnect the Tach and see what
happens?

Can't do any harm.

Hardly rocket science but if only one thing changes - the bloke
connects a different tachometer - and all the instruments fail then
????.

Then there is a fault. We knew that. Tachometers don't wreck temperature
gauges all on their own. He may have inadvertently disturbed something
or connected wires at random but I was trying to suggest something that
he may not already have thought of, rather than the most likely cause
(finger trouble..)


Of course is one or more "sending units" are not properly grounded
then that instrument will not read correctly but that is not a
multiple ground path, which I was replying to. It is a poor or
nonexistent connection to the common ground.

with most 3rd party tachometers it is very possible to connect them
improperly as many times the marking is ambiguous (unless one actually
reads the instructions :-) I once came across a temp gage set with the
gage marked "+", "I" and one extra pigtail with no marking. The sender
had two terminals, one marked "W" and the other unmarked. It turned
out that the gage body was grounded through its mount and the pigtail
was the light "I" was connected to the unmarked terminal on the sender
and "W" was a warning circuit.

Cheers,

Bruce
(bruceinbangkokatgmaildotcom)