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View Full Version : Help for old tachs.



penbroke
05-12-2003, 09:26 PM
If your old tach reads wrong (usualy too high) and looks like this:

http://www.donzi.net/photos/penbroke_tach_1.jpg

The label may be different but the inside most likely looks like this:

http://www.donzi.net/photos/penbroke_tach_2.jpg

The dark blueish round object with the screwdriver slot in the upper right is a potentiometer (pot) which allows calibration of the tach.

It can be accessed with a small screwdriver thru the hole indicated by the arrow. Just peel of the sticker covering the hole.

http://www.donzi.net/photos/penbroke_tach_3.jpg

Hook up a (known) accurate tune-up tach and adjust the pot 'till the boat tach reads the same as the tune-up tach. Get the boat wet and compare the two at several engine speeds. They probably won't be in complete agreement but I was able to get mine to read within about 100 rpm across the whole range. The top end of the range is probably more important than the bottom. Before the final setting, rotate the pot several times back and forth to rub the contacts clean.

Be sure to replace the sticker over the access hole to keep the bugs out.

Keep a close eye on the tach and make it earn your trust before you "rev it on the red line". Actually if you are pushing your luck rpm wise you should be using a better quality tach anyway.


TRY THIS AT YOUR OWN RISK!


No guarantee but it worked for me!

Frank
Trying to keep the old stuff working.

DOND
05-12-2003, 09:54 PM
Frank, Great Info! I have the same tach but it is labeled Holman Moody.Mine buries itself all the way to the right at any rpm. Do you have any suggestions on what could be wrong?
Thanks, Don

MOP
05-12-2003, 10:37 PM
Pebroke Excellent post! I tell you one I ran into, I was checking a bumpy tach. I found that over alot of years the terminals got corroded and a good cleaning is worth a try. Try to hold the tachs terminal studs still, many times they will spin and break the wire inside. If its the type with no nut under the terminal take two nuts and lock them together to hold it.

penbroke
05-13-2003, 08:19 PM
DOND: Check the wiring. I posted some wiring diagrams a while ago that likely apply to your boat. wiring diagram thread (http://www.donzi.net/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=1;t=007091#000000)

This tach is strange (to me) because the power to the points goes thru the tach and is also tied to the voltage regulator. If you have installed one of those solid state modules to replace the points or a 1 wire alternator with built-in regulator the tach may not work.

I'm not sure but if everything is original and the wiring checks out I would guess your tach is defective.

MOP: Thanks and thanks for the tip! I always enjoy your tips and tricks of the trade.

Frank