how do I know when the transmission is full
Don't add trans fluid unless you can check the fluid level. You can check the level from underneath. Make certain the vehicle remains level while checking the fluid. Located just above the top sealing flange of the transmission oil pan you will find a threaded plug. Remove the plug and see if transmission fluid drips out of the hole. If so, the fluid level is correct. If not, remove the red fill cap on top of the transmission, driver side. Add fluid to the transmission until fluid dribbles out of hole. Replace and tighten the plug. If you can't do that, take it to a mechanic to have them check the level, when they get it on the lift, you don't want to run the trans with an overfilled transmission as this will cause problems. But since it is slipping it might also mean it's rebuild time.
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