Most 1991 Chevrolets come with a 700R4 transmission. It is helpful to learn how to troubleshoot this automatic overdrive transmission before you take the car to a mechanic to be assessed, so that you know what issues you will need to address with the mechanic. The process uses tools you can find at any local auto shop and can be performed in the comfort of your own driveway with about 30 minutes of your free time.
Look at the level of the transmission oil by pulling the handle on the transmission dipstick out of the transmission filler neck on the back side of the engine. Clean the dipstick off with a clean rag and return it to its original position in the filler neck. Recheck the fluid level, making sure it is between the highest and lowest marks on the dipstick. If the fluid is below or at the low mark, add more fluid using the funnel and the proper manufacturer-approved transmission fluid.2
Smell the fluid and determine if a burnt smell is present. Burnt transmission fluid is a sign of internal transmission damage.3
Turn on the vehicle's engine and place it in gear. Listen for a clunking sound. If you hear this, you may need a professional mechanic to replace the mounts on the transmission. This can happen with older vehicles with more mileage on them.4
Drive the vehicle and listen to the revving of the engine. If you have a high revving sound when you change gears from "Park" to "Drive," as you push down on the gas pedal to accelerate, or if you feel the transmission lag on changing gears, you could have internal transmission problems.5
Check for a dark red fluid leak or a puddle underneath the vehicle to make sure that it is not not leaking transmission oil. Rear main seal transmission leaks are common in older model Chevys.