You can first check the fluid with the engine running and wipe the dipstick and then shove it in and pull it out and make certain that you have a defined line of fluid on the dipstick and not just spatter or drops.

In the old TH350/400 trannys you’d start slipping between gears if you were just one quart low. In newer trannys (with a higher fluid capacity) they will slip when they are two to three quarts low.

The torque convertor and the pump will not have enough fluid to create the pressures required to shift and move normally.

After you top it off you might try to ID the leak and decide whether it is worth spending any money on to fix. Most leaks are from age and the front or rear tranny seals and sometimes the pan gasket will leak. If it looks like the tranny pan, gasket, you can hand tighten the bolts/screws that surround the pan and this should stop this particular leak.

If you recently had the tranny serviced they might have under filled your tranny and created these problems.

No bad cars, only bad mechanics. Wax on! Wax off!

Good Luck!

You need a tranny rebuild…