Vehicle: 2018 Chevy Suburban, 4WD, V8-5.3L

Mileage: 156,223 Miles

Problem: The Suburban was brought in because the transmission would run hot after driving four to five hours or more. The customer stated that they had a remanufactured transmission installed about one month ago at another shop (NOTE: second transmission in one and a half years). Earlier in the week, another shop found a diagnostic trouble code (DTC) U0101 - Lost Communication with Transmission Control Module. They cleared that DTC, then the customer decided to take it to another shop.

Case Details: When the next shop got the vehicle, the customer had already driven it for over five hours. The transmission fluid temperature was 230-250 degrees while the engine temperature was at 210 degrees on the interstate (NOTE: the outside temperature was about 30 degrees). The only DTCs stored in the system were:

• U150F - LIN Bus loss communication
• B323B - Lower hands-free liftgate sensor circuit

The technician didn’t know exactly where to start the diagnosis, so he called an ALLDATA Tech-Assist consultant.

They decided to check the cooling system thermal bypass valve after some communication back and forth about which module was storing the U150F DTC, if any modifications were done to the transmission cooler system or the grill shutter operation, and if the cooler was flushed or replaced when the transmission was replaced. 

The technician removed and disassembled the thermal bypass valve. He found the plastic guide to be broken in two pieces, which caused the valve to stick in the bore and always stay in the closed position. 

NOTE: If the valve does not open at 190 degrees, the fluid continues to heat up the further it is driven. Essentially, the issue was causing a restriction in the cooler lines.

Confirmed Repair: The technician replaced the thermal bypass valve, cleared all DTCs and returned the truck to the customer. After driving the truck for over five hours, the customer reported that the transmission was running at least 35 degrees cooler. Problem solved!