The PCM sensor uses 5v from the PCM, it sends a voltage back to the PCM that varies depending on it's temperature and the PCM interprets that returned voltage as a temperature reading.
OBD1 is not nearly as sophisticated as OBD2. Example:
Coolant temp reads 50F, ambient reads 70F, intake air temp reads 125F. All are "valid" temperatures as far as the range of over all expectations go. After 5 minutes of running the coolant temp doesn't change. OBD1 thinks "it's in a valid range", OBD2 says "wait a minute", OBD2 knows that value should have gone up and based on load, run time, ambient temps and intake air temps has a pretty good estimation of where it should be and if it's not there will throw a code. OBD1 will not as it doesn't do that type of calculations.
Now if the coolant temp sensor returns an invalid signal, such as not having 5v going to the sensor, OBD2 will throw a code. OBD1 might. This is really where being able to view the live data from that sensor comes in handy,
One thing I found when putting together the parts for my EFI swap was a source that listed the data and values from the various sensors I am using. Microsquirt (my PCM) will let me look at that data live so I can see a flaky sensor that returns an "in range" value that is not correct. Such as telling me my coolant temperature is 50F, which is "in range" for that sensor, when it's actually past the thermostat opening temperature.
Don't forget a grounded signal line from the sensor to the PCM will also confuse it. Unfortunately, I only have GM sensor data, but if we knew what the data was for your sensors we could back probe the voltage to the PCM and see if what's going there is rational or not.