I've searched, I swear! However, I've not found a definative answer to my question. Which is, how compatible are the various ECU's from the 90-94 models, with each other? A partial answer I've seen is that the ones in the low 50xxx range are for the 90-92 model years, and then the 501xx are for the 93-94 years. Yet, that rule isn't absolute, because i've seen 93's that have in the low 50xxx range. But I'm guessing based on what I've see those that have cited their model number in the thread.
Case study: My ECU has 89661-50142 as the model number. I'm wondering how far up or down in the range I can get away with, and be assured of compatibility.
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I just wanted to add something to this thread in case some one stumbled across it. I'm working on a 90 ls400 with trac that has a non trac ecu in it. I found out the hard way after trouble shooting trac codes. Both tps sensors are adjusted correctly and are in spec, all circuits tested good from the sensors to the ecu, I also pulled the trac actuator wire connector to generate a code which it did, and that code cleared just fine, but the trac (sub-tps) codes where persistent. After four days of careful analyzation, I came to the conclusion that maybe I had a bad ecu. Found out that it was just the wrong ecu. So to recap, non trac ecu will work fine in a model with sub tps as long as you don't mind the annoying trac warning light on all the time. Paul