Yes, there are many things that could cause this problem. But from experance of owning a 92 240sx ka24de, i have had the same problem. One of many answer, but i have 4 that can help, the rest is SOL find it on google.
1, its really not called a tcu its seperate, its a contol relay ie white and located under the left driveside panel. Look infront of the fuse box you will see a white relay box with the number 28550 40f00. Thats it. Lights back on, what could have caused it is a short in a wire so before you put one in check for it. Example. True statement, my control relay went out because i had a short in my stereo wires, right after i got another control relay.
2, for the speedo could be a number of things but, check your fuses. Make sure the wires behind the cluster is not loose. ( you dont have to take the whole dash apart, take ignition plastics out, plastics under where the hazard lights are, 2 screws on top of the plastics that hold the cluster, twist and bend but becareful not to snap it, 3 screws off the cluster and your set.
3, check your wiring from your speedo to the sensor for speed make sure all connectons are good. If your need more help Email me roksoldier@gmail.com. Trust me i hate the feeling of no lights at night and nothing working especially the speedo rpm and tep, thank god my fuel worked lol.
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