Hi,
Engine coolant can cause different things to fail in the transmission. How much of the fluid were you able to remove when you did the service? Keep in mind, the torque converter would have been full of fluid and unless you flushed it, that fluid is back in the transmission.
If you changed all fluid, the next thing we need to check is line pressure. Damage to the torque converter or front pump may have occurred. I'm basing that on the idea that it doesn't slip when you shift manually.
I attached the directions below for checking line pressure. You will need a pressure gauge and scan tool that can communicate with the vehicle. Take a look through the directions and let me know if you feel comfortable performing this.
Additionally, recheck the transmission fluid and see if there appears to be any contamination.
Let me know.
Joe
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Saturday, July 1st, 2023 AT 8:24 PM