The 4+3 is a standard transmission(4 speed) with a two speed(direct drive & overdrive) trans bolted to the rear of it. The unit was designed/built by Doug Nash Transmission for the '84-'88 Corvette. It was a decent idea, and did work well to reduce the emissions, and allow the car to pass the Federal EPA fuel mileage, and emissions requirements.
The reader's digest version is, there is a normal 4 speed trans(basically a Borg Warner T-10) in the car, and it shifts just like any other 4 speed. The overdrive section is a separate trans, bolted to the rear of the 4 speed. This O/D unit can either be in direct drive, or in an overdrive mode, depending on engine temp, vehicle speed, and throttle input, and is controlled by the ECM(Engine Control Module). Under
normal driving conditions, the O/D unit would shift into O/D mode shortly after the driver would shift into second gear, and would stay engaged in O/D unless the accelerator pedal was mashed hard, or the driver "canceled" O/D with a switch, mounted either on the console/dash, or on the top of the shifter knob, depending on year model. If you take off with heavy throttle, the O/D will not kick in at all, until the throttle is let up on, or the engine rpms get too high.
The "4+3" name came from the trans having the capability of having overdrive in 2nd-3rd-4th gears, so 4speed + O/D in 2-3-4 = 4+3.
It was more of a nuisance than anything for most folks. The later cars had better ECM control of the O/D, but it's still a pain, unless you keep your foot in it all the time.
The last year for the 4+3 was '88, then they went to the ZF built 6 speed. Your '90 automatic is just that...an automatic, overdrive transmission, known as the 700(or 700R4, if ya wanna get picky). You have nothing to worry about, other than normal, routine maintenance.
Adams' Apple2010-10-05 19:53:10______________
Joel Adams
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