Tachometer

[The chief]

Two seasons ago I had my alternator replaced. I went from the standard 55 AMP Motorola to a 70 AMP Leece Neville. The only problem is my Tach has been off ever since. The mechanic who installed the Alternator had no idea on how to reset the Tach to make it right. Is this an easy fix, can it be reset? I cannot even remember my original idle or top end RPM's. It is important to know the exact numbers as I just replaced my Prop and would like to know if there is a difference. Also, as an engine ages I want to be sure there are no changes going on. I have always relied on the Tach to reassure me the engine is performing properly.

 

[Old Skool Neil]

Here's the tool I used, works great:
http://www.harborfreight.com/digital-photo-sensor-tachometer-66632.html
There should be an adjustment screw on the back of your tachometer. You scan the engine and adjust the tach to match.

Also, Harbor Freight Tools is having a 20% off sale until Tuesday. If you jump on this it'll cost $40 - 20% = $32 + shipping or you can save the shipping if there's a store near you.

 

[The chief]

Sounds good, but two questions. Where do you scan on the engine to get the true RPM readings. Also, I rebuilt my Engine Control Panel around 4 years ago. I seem to recall there where dip switches on the back of the Tachometer. Does that sound right? I do not recall a single point of adjustment. Again, it was a winter project some time ago. My memory could be off.

 

[Old Skool Neil]

The optical tach comes with reflective tape that you put on anything that rotates at crankshaft speed. In my case with the Atomic 4 the flywheel is not accessible but I had 2 other options. The auxiliary drive that drives the alternator and the prop shaft. The trans in the direct drive A4 is 1:1 so it turns at the same speed as the engine. I actually checked them both.

I don't know the intricasies of your tach but I suspect the dip switches are for different inputs and voltages. There should be a small calibration screw on the back somewhere.