I have an 84 with the 4+3 and I'm sure the OD doesn't work.  I have several questions.... First what position should the switch on the console be?.... At what speeds/rpms will the OD kick in on each gear?.... If it isn't working now.... do you think a fluid and filter change might work or does it need to be replaced?.... not sure what it looks like... but where do you drain the OD fuild and relace the filter?.... sorry ... first vette not sure what i'm doing with it....lol... haven't been under it completely yet... Any help would be appreciated.

The switch should light up a light in the dash, I think. The '84s had a different system than the later cars.
Check all your fuses first...the basics. If there is no O/D, it will most likely be an electrical issue, whether it be wiring, the switch itself, or the solenoid inside the trans. Check to make sure the connector on the trans is plugged in, too.
The O/D will engage from 2nd gear up, depending on throttle input. If you're just dilly-dallying(tech term), it will engage immediately after you shift into second. If you're on the throttle pretty hard, it will not go into O/D until the engine load is lighter, or you let up on the pedal. There are no specific, set speeds for the O/D to engage, as there are several variables involved, including a cable from the trans to the throttle body, which is a "kick-down/passing gear" cable.
Changing the fluid requires removing the pan on the O/D section of the trans. It doesn't hold very much fluid, but most folks never even think about it, so it never gets changed, which is the #1 reason they fail so often. If you do service the O/D, do NOT use a gasket on the pan....use silicon. A gasket will never seal right, because the pan is stamped from the very thin sheet metal, and it deforms easily, which creates perfect leak spots. Silicon it, and be done. The O/D section uses regular automatic transmission fluid, preferably a Dextron fluid.
As a little bit of trivia, the filter on the O/D section is the same filter as an older, Medium Case Cruis-o-matic FORD trans....

hth

I have an 84 with a functioning OD. The rocker switch should have the right side down to be activated. There should be a panel light lit but it may not come on until the trans warms up (not sure about that). Anyway, the OD on mine starts to function normally after a short warm up. It functions as described by Adams' Apple.
According to my Haynes manual, you can check the fluid level by removing the square head plug on the left side of the OD unit. The picture shows it just above the pan and centered over the pan. Fluid level should be just below the hole. To change fluid perhaps a fluid pump with flex tube could be fed into the hole, pump out as much as possible and refill  through the same hole?

You wouldn't be able to get all of the fluid out like that, nor would you be able to change the filter. Changing/cleaning the filter is an important step for these things. Ya gotta remember that the O/D section is basically a 2 speed, automatic transmission, complete with a fluid pump, and cooler lines. The more old, nasty fluid you get out, the better.
You're correct in how to check/add fluid to them. The square-headed pipe plug must be removed to check, or fill the O/D trans. A rubber hose/extension will absolutely be needed to get the fluid in.
The biggest cause of failure in these things is neglect. Changing the fluid/filter should be done just as any automatic trans should be....at least once a year, or every 12,000 miles, depending on where ya live, and how ya drive. Repairing the O/D section can run into big $$$$, and parts are starting to get scarce. I've got some stuff here, but not much. There's a place in New York that has parts, and that's about it.

I agree that the OD filter and fluid should be totally changed. I've had my 84 for 11 months and put about 3K on it. Who knows when, if ever, that was done. Since I don't have a lift or garage to work on it, I think my local vette guy will get this task and change other fluids as well this late fall or winter.
Total miles on mine is just over 60K.

Ok, I am the non-mechanical guy in this group. What exactly is a 4 + 3 transmission? I have a 90 model couple with automatic, is this something I need to be learning about. None of the lights and information you guys are talking about seems familiar to me, but would like to be sure so I don't screw nothing up. SOMEBODY EDUCATE ME......

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

Kewl, I appreciate that. And extremely good answer I might add, Your the man.

Hey, just to add a post (my first) the manual on my '85 (chilton) says that even if the OD is locked out but the shift button, it will still automatically shift when you exceed 100 mph., will check it out in another week or so, after I go over the car better..  I've had it over that on the way home when I bought it, but had the od on..

The ECM will indeed force a shift into O/D at high speeds like that, to keep the rpms out of the red zone. Full throttle will still keep the O/D canceled to a higher speed, tho.