Tach question

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tedted

Veteran Member
Joined
Feb 27, 2016
Messages
54
Location
USA
Vessel Name
Castoff
Vessel Make
77 Heritage West Indian 36
Hello All,
I've been wondering about my tachometers being out of calibration all season. One is a Stewart Warner on the fly bridge and the other is a Motorola on the lower helm.

The tachs ran off the stator wire from the alternator. The boat long ago was originally equipped with a Delco 60 amp alternator internally regulated. This was replaced with a Powerline 120 amp alternator and smart regulator. I replaced this system with a newer version of the same. The problem ever since I bought the boat is that the tachs read WAY off from actual rpm. They read high by approximately 400 rpm (checked with a photo tach).

There is a small hole in the back of both tachs. Is this for calibrating the rpms with a trim potentiometer? These are older tachs and don't use the DIP switches the newer versions use and I can't find any info on them ( 40 year old technology).
I don't want to stick anything in there without knowing what's gonna happen...lol....:eek:
Anyone have any ideas on this? Thanks, Ted
 
That hole is for a small screwdriver to adjust the needle to correct RPM. Mine are slot screws. My drill was to get a ( Harbor Freight) photo tach..$20+- set the lower one then set the bridge from the lower. Motorola Tach.
 
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I'm afraid to try the adjusting screws on my 30 year old units. I used a harbor freight photo tach and made a cheat sheet showing actual rpm and what's shown on the tach. I didn't bother with the other engine since I sync rpm's by sound.


Sent from my iPhone using Trawler Forum
 
I'm afraid to try the adjusting screws on my 30 year old units. I used a harbor freight photo tach and made a cheat sheet showing actual rpm and what's shown on the tach. I didn't bother with the other engine since I sync rpm's by sound.


Sent from my iPhone using Trawler Forum
Well, be gentle, my boat is 1982 and no problem on the bridge or lower helm. I used a jeweler screwdriver.
 
Check the new alternator, sounds like a 16 pole HD alt, that would read 400rpm different.
 
Tach

That hole is for a small screwdriver to adjust the needle to correct RPM. Mine are slot screws. My drill was to get a ( Harbor Freight) photo tach..$20+- set the lower one then set the bridge from the lower. Motorola Tach.

Thanks folks. That's what I thought. By the way, I got mine on Amazon for 11 bucks......:dance:
 
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