Since installing my alternator-driven tach, I have been needing to calibrate it more accurately. I just checked Ebay and found that they have photo tachs for under $20. including shipping. I figured, even though I will not use it much, any shop would charge me at least that much to turn theirs on. Just passing the info on for anyone that might need one.
I bought mine from the same place. Works just fine.
For those that install alternator driven tachs - here's a little tip
For MOST Detroit Diesel Delco gear driven 50DN alternators
The R" Terminal output is 12 volt and (2.94 (53/18) gears & 6 pulses per revolution)
which equals ---> 17.66 pulses/rev)
so- if your tach came with a 12 pulse / revolution setting
To re-calibrate for 17.66 pulses/rev - first start the engine & record the default
12 p/rev RPM reading at a steady idle- then turn the adjustment device
until the tach registers approx 2/3 or (12 / 17.66) of the original (12 p/rev) RPM
reading
The tach came calibrated according to the info I gave the vender, but I am not sure of it. It is adjustable while in operation, so I will use the photo tach to clock my no-load max and set the dash tach to it. I figure that will get me as close as I can get. It makes sense that the lower speeds should then automatically be right on, but what's life without surprises.