With the advent of the 6-speed transmission, the number of transmissions equipped with cooler bypass systems has dramatically increased. Sure, we had the systems on earlier applications like the 5R55s and even back into some 4-speed families, but they were nowhere near the volume we see today. Most 6,7,8,9, and 10-speed applications on the market today have some form of fluid temperature-controlled cooler bypass valve system on the transmission somewhere, whether the valve is located in the cooler line, radiator, or internally within the transmission (most likely the valve body).
You may be wondering why we have this valve on these units. The answer is to help meet the EPA CAFE standard. This is the same reason we have all the new LV and ULV-type fluids. Those fluids are pretty fickle when it comes to operating parameters, so it is beneficial to get the transmission to full operating temperature as soon as possible, which can have a positive impact on the vehicle’s fuel economy and transmission operation.
All valves typically have the same design features: a wax element contained within a valve housing with a plunger operated by the element (Figure 1). As the wax contracts due to the fluid temperature, force is placed on the plunger, causing it to retract into the channel, controlling the cooler flow and opening a passage to allow the fluid to bypass the cooler. This action effectively “loops” the cooler lines, preventing the cooler from affecting transmission fluid temperature. The reverse happens when the fluid temperature increases as the valve opens, and the cooler flow increases, reducing the fluid temperature.
It seems pretty simple, so what could go wrong? In theory, this system should be bulletproof, but in practical application, we see tons of issues with valve operation.
Not all valves are created equal. Different transmissions have different valve temperature settings, leading to issues as severe as transmission failure. “How?” you ask. Let’s take a look at the Ford and GM 10 speeds. Look at the factory fluid fill instructions. GM has you checking the fluid at a reality normal fluid temperature (Around 150° F) while Ford has you checking the fluid at a very high temperature (206°-215° F). What is the difference? It is the valve opening setting for the cooler bypass valve. I’ve seen numerous shops that had rebuilt the Ford 10 speeds only to have the customer return with a slipping concern or dropping out of gear issue, only to find out when they checked the fluid level it was low. The reason is the cooler was dry after your rebuild and cooler flush, and as soon as the transmission reaches full operating temperature, the valve directs flow to the cooler, effectively lowering the fluid level in the pan, which, of course, causes slipping and gear dropout concerns.
Some Asian manufacturers recommend that you run the transmission without the return line connected to the transmission to eliminate air from the cooler and to make sure that the bypass is closed before checking the fluid level. Yes, that is a little extreme, but the use of a temp gun to make sure the valve is supplying flow to the cooler is something you may want to consider when checking your fluid level on a late-model application.
As was stated earlier, not all valve opening points are the same. As an example, the GM valves open at:
- Early 6L applications 194° F (90° C)
- Late 6L applications 158° F (70°C)
- 8L 194° F (90° C)
- 10L 143° F (61° C)
The valves have a tendency to fail. Debris, as well as wax element failure, can cause the valve to stick in the bypass position, leading to no or little cooler flow. Of course, the clutches and the rest of the transmission get a little upset when the fluid temperature gets that high, leading to DTCs, default actions being taken, and possible transmission failure. Some of you may bypass the valve, so you no longer have to worry about the valve sticking. While in theory, that should work, but in practical application, it does not always do the trick. I have been involved with several GM 8L applications that refused to relearn the shift adapts after they were cleared, only to find out that the temperature was well below the minimum adaptive learn temperature threshold, so be aware. As I have told you through the years, some manufacturers require their dealer technicians to replace the valve if the unit was rebuilt or if a reman was installed. That means they know a problem exists and are unwilling to take a chance on the original valve.
Some shops are not aware of how the valve works, so they do not understand that when you flush the cooler, the valve must be open, allowing flow to the cooler. If the valve is not open, you are simply flushing everything but the cooler. Some cooler flushing machine manufacturers have tools to help with this issue, while some technicians have developed their own tools to address the problem. Some shops use a heat gun on the external design valve to force it to open during the flushing and/or fluid-filling process.
As you can see, even simple systems can create havoc for your shop if you are not aware of its operation. Until next time, remember, “Quality means doing it right when no one is looking.” the ultimate in transmission testing equipment Transmission
