Tach question

[tedted]

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 I replaced this system with a newer version of the same.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....
Anyone have any ideas on this? Thanks, Ted

 

[Mule]

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.

 

[meridian]

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.


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[Mule]

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.

 

[Martin J]

Check the new alternator, sounds like a 16 pole HD alt, that would read 400rpm different.

 

[tedted]

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......