Bruce wrote:

... I'd reverse my "repair" and see what happens. A heat gage going
full scale indicates that somewhere between the instrument the temp.
sender you have a short to ground and/or the sender in shorted to
ground.
...
... this is exactly, leaving out experience, how a professional
instrumentation guy will do it - isolate the instrument and it's
sender from everything else to determine whether it is the instruments
themselves or "everything else", fix either the instrument or
"everything else".

Thanks, Bruce. I think I am getting close to a solution for this issues
thanks to many comments here, on the Volvo forums, and elsewhere. I am
betting that I simply removed the wires from the temp sender when trying to
remove the wires from the tach sender. My manuals and Volvo downloads give
essentially no exact guidance, pictures, or diagrams on which sensor is
which, and on top of that there is really no room in the engine compartment
to look around and see the entire layout. This of course has been a problem
from day one with small sailing boats.

I just sent the following to the Volvo forum. Additional comments are of
course welcome. BTW thanks to you all who are providing serious and
detailed suggestions. I readily admit I am an amateur when it comes to
marine diesels, despite having messed with them 30 or so years. Apparently
some few readers believe it's necessary to remind me of my ignorance in
these matters. Rest assured, friends and others, I am well aware of my
limitations and have been humbled by the sea many times.

Set to Volvo forums:

"I think we're getting warm. Yesterday I tried various combinations,
including just disconnecting the wire to the "w" lug. I had assumed that
this wire, one of 2, belonged to the old tach. When I then tried the engine
with this wire disconnected the tach still worked but the temp gauge was
"dead."

"In other words I suspect that my problem is that instead of disconnecting
the wires I may have mistakenly assumed belonged to the old tach, I actually
disconnected the wires from the temp sender, and then connected one of these
to "w" on the alternator. This fits all the symptoms so far, and could
easily have happened since I am not really sure which sender is temp and
which is revs. My manual does not show the locations clearly. Another clue
is that the pair I disconnected has 2 different sized lugs, fitting the
description of the temp sender wires given to me by Richard Vosbury of
Annapolis. I suspect the larger lug is the actual temp resistor and the
smaller lug is the temp alarm.

Today I will run down to the boatyard and try again, this time using the
correct pair of wires (assuming I can locate them somewhere in the vicinity
of the alternator on the starboard side of the engine). In he meantime, any
more comments on these suppositions? Putting all the comments together so
far is at least giving this confused amateur some ideas on how to proceed.
Thanks, gentlemen."

--
Good luck and good sailing.
s/v Kerry Deare of Barnegat
http://home.comcast.net/~kerrydeare