It should come on when there's a problem, I had an '89 GT and recall that the light most likely came on when you first turn the ignition on as a circuit/light check. If you have concerns, code it out, that will reset the computer as well as tell you anything that may have fallen out of the parameters. Besides the injection, I think it may also get triggered by the low
oil level indicator too. The water and air temps are part of the injection sensors so I didn't mention those as separate entities.
By the way, '89 was the first year of universal application of Mass Air and the last year prior to air bags, one of the reasons it's my favorite year....air bags...eccchh.
Blykins,You shouldn't need a scanner on that system, you should be able to use a test light on the underhood plug and count the flashes. Did you use the sensor on the speedo drive? It may have to do with the loss of the overdrive.