Hey, welcome to the forum! I had an 87 auto Supra back in the day that did the same shifting yours is doing. My problem was a melted TPS, if code 51 is the AC switch/IDL circuit code then that also points to a TPS failure. If you have downshift problems, check out the kickdown cable adjustment and make sure its in spec. Good luck!
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1989 7MGTE R154 Targa Top White Package w/Blue Interior
Driftmotion 57trim CT26, Upgraded Intercooler, 2.25" Hard Pipes, 3" Turbo Back Exhaust with Test Pipe, HKS VPC w/GM 3 bar MAP sensor, APEXi S-AFC, Walbro 255lph fuel pump, ProSport EVO Series Boost / Fuel Pressure / AFR gauges, Lotek a Pillar Gauge Pod, HKS Type 0 Turbo Timer, Eibach Lowering Springs, Tokico Illumina II TEMS struts, custom powder coated Motegi Racing TrakLite wheels
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