Did a diagnostic check of the ECU on one of our '89 3.0 equipped Monteros today. Got codes 22, 23, & 24. Crank Angle Sensor, Top Dead Center Sensor, and Vehicle speed sensor respectively. FSM gives trouble shooting instructions for the VSS, but just says to use the Multi-Use tester to check the other two (naturally I don't have one of those). Don't recall seeing a location for the TDC sensor, but I assume it is in the distributor with the CAS.
Further items of interest are that the check engine light is not on - but does come on at initial start - so the bulb is good. Also it has an intermitent no start condition where it occasionally refuses to start after a short stop, but will fire right up after it sits for a time (1/2 hour or so). Of course it has never done it when I'm around so I have not been able to determine if there is no fuel or no spark. It seems to run ok, a little low on power and hesitates a bit off idle, but I figured that was due to high miles.
So where do I go from here? Replace the distributor? I have spares. Or does someone know of a way to check the CAS and TDC sensor without the special tester? Would the engine even run if these were bad? The only experience I've had with one of these was a GM and that was a no-start condition.

yes in distributor

since you know the present codes, erase them.
see if they come back right away, or only after the no-start

my guess is faulty connections.

I have not seen any complaints about these componants here, but that might be because the are not a servicable item.

unless there is dust or water in the dizzy, they last a long time....as far as I know.

Exactly what I did today. Pulled battery cable to clean the posts and so cleared the codes. Drove around a bit afterward and then rechecked for codes. Nothing yet, so will have to wait for a no-start and then recheck.
Good thing I checked online, looks like the 89 distributer is different than the 90 to 94s - same with the ECU. If I do change it out, who knows what new problems I may have created installing the later one.