2017-19 GM trucks with Allison transmissions utilize an auxiliary transmission cooler thermal bypass valve to control fluid flow to the auxiliary cooler. The factory valve is restrictive and does not allow sufficient flow to the auxiliary cooler causing higher trans fluid temps especially on trucks with increased line pressure from trans tuning or a built trans.
 
In testing the LDP shop L5P they saw trans temps in excess of 215 degrees after installing one of their built units. This lead to the design of the LDP L5P trans cooler bypass valve. We have seen up to 50 degree temp drops even on stock trucks and have had customers reporting 60 degree temp drops! Their design allows you to install the factory thermostatic valve for cold weather operation to maintain optimal fluid temp year round.  
 
Install time is less than 10 minutes- see attached video. This valve is a must for all 2017-19 diesel( and 14- up gas) GM trucks. See full tech article in August 2020 issue of Diesel World magazine with real life results and install pics!