Did they swap anything else over from the 87 Supra? Such as the fuel injector resistor packs on the firewall? I'm assuming that since the 87 harness is still connected that they kept the 87 fuel injectors and fuel rail...
If I was in your situation, I'd get the fuel injectors and wire harness from a 89+ turbo supra. That way the factory gauges will operate properly and it should just be plug and play. If a wire harness is hard to find, you can get a 87 ECU, 87-88 Gauge cluster + dash harness, the resistor packs for the fuel injectors, and maybe some other odds and ends to get it running right.
I'm sure it's possible to rewire the connectors on the wiring harness to make it connect to the ECU properly, but you will still need to address the fuel injectors. pre89 7MGTE's use low impedance injectors, whereas 89+ used high impedance injectors (I think that's right, might be the other way around though; trying to recall from memory).
Anyway, help this gets you pointed in the right direction.
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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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