89-94 f series with E4OD are equipped with EEC-IV , OBDI to others , memory codes do not have to be cleared for normal operation once the failed or failing component is repaired or replaced EEC-IV only monitors for a failure not performance like OBDII , and most of the PCM,S will clear the memory codes after about 40 warmup cycles if teh concern does not repeat , and when a KOEO test is performed it clears the memory codes automatically , but if the power to the processors keep alive memory is faulty it will erase the codes at every key off cycle but should have a code 15 for it ,if you have an O/D light flashing then it has stored at least one memory code and it can be retrieved if done properly EEC-IV is picky when being tested if the tester isnt operated correctly it wiil erase the codes before they can be retrieved , being you have a 92 model does the speedo ever fluctuate , and see if the cruise control operates and if it shuts off when the light begins to blink if so there could be a problem with the PSOM /programmable speedometer/odometer module , if the cruise shuts off but the speedometer reads normally there is a good chance the PSOM is bad , it relays the speed sensor signal to the processor

 

I know you wrote the question about the e4od tranny a long time ago, but I'm having the exact issue. Did you ever figure out the blinking o/d light?