When Cold slow to go into Reverse

[AntMan]

When cold or after sitting overnight, the transmission takes about 10-15 seconds before Reverse engages.

When I select DRIVE it goes in right away as any normal transmission would. Changed trans. oil and filter no change......ANY SUGGESTIONS?

 

[AntMan]

forgot to add, have a trans cooler, mounted low and flat just behind the front passenger side air deflector.