... I was wondering if there was a way to test them without swapping out the tach board on my test fixture.
If you are handy with a voltmeter you can confirm the voltages at the tach board. The sensor chip has 2 components, an Infrared LED that shoots the beam at the flywheel stripes and an Infrared phototransistor that detects the reflections (or lack of reflections). The voltage across the IR LED should be steady, the voltage across the phototransistor will go up and down. This may take 3 hands so to speak but is a way to test the tach board at the board itself. One of the MTH techs ought to have the numbers to look for but if not I will measure a board and post the results.
Separately, if you don't have a meter there is a kludgy way to determine if the two components are working. First you confirm the Infrared LED is putting out a beam by viewing the chip with a digital camera and confirming the purple-ish glow. If the you get that, then you can "stimulate" the phototransistor sensor by aiming a TV remote control at it. Most buttons on a remote send a repeating IR burst when held down. This stimulates the phototransistor as if the black and white stripes are alternating and eventually you'll hear a chuffing sound. It may take several seconds but eventually you'll hear the sound. The engine does not have to be moving or commanded to a non-zero speed; a PS2 steamer will generate the sound when it receives some number (depending on the engine gearing) of reflected pulses. A TV remote with the button held down is simply simulating the multiple reflected IR pulses.
