I'm fairly sure that the 90 system and the 93 system have different tire sensors. Soooo I offer this as just a fwiw. My 90 does the same thing and as I have an extra set of tires & wheels(OEM ones) I changed out each wheel till I found a culprit. In my case it was my right front sensor, no code just the service LTPWS light exactly like you are seeing.
I did some reading over at the NetReg site in the How To section(?) under the maintenance tab. My conclusion is my issue is the battery, they are a fairaday type. It gets stuck, voltage not enough to power the sensor, sensor dropes out momentarily & lights the DIC light but sets no code because the battery starts working again almost instantly, sometimes. Stop car & shut down, restart and no light. Drive slowly for awhile and no light. Drive slowly to a highway and go fast and the light comes on. Drive fast long enough, with varying speeds and the light comes on and goes out sometimes. Drive a highway long enough with the light on and eventually it may go out, and after another while it may come back on.
Scott Fabre says that the battery gets stuck in it's track. This can be stuck like glued in place, or stuck momentarily. Either way, no voltage prodouced and a LTPWS service light. In my case I have three new sensors and had to cop a used right front....that's the one on my car that is the root of the my light(s). I took it apart before I installed same in new tires & wheels. I was doing Scott's test, shake it to see if you can hear the battery moving in it's track. Mine was intermittant in making sound, hence movement, hence voltage for the board to report in to the module. I think that the plastic rails(track) get worn & the battery gets cocked and stuck. JMHO on the diagnosis part. Sorry to be soooo long winded about this.
Tom
Link to page at NetReg site:
http://www.zr1netregistry.com/ZR1_howto.htm