Dino Fritz wrote:I wish I could find it, but I remember reading an ATSG Bulletin re transmission oil temperatures, where they said that too hot it breaks down, but too cold was also an issue in that water/vapour would remain in the system and cause problems.
This is exactly the same principle as with engine oils. The tests for Dexron type oils are performed at the same temperatures i.e. -40c, +40c and +100c although they don't then get the same SAE ratings as engine oils. Because it is oil, it is thicker when cold, and therefore pumps and flows less easily and also as Dino says, water can remain condensed, and it is thinner when hot.
A typical Dexron type oil will have a viscosity value of around 30 at 40c and one of 6 at 100c. This is much thinner than most engine oils as you can see when you pour it in. It will not have any tested viscosity rating above 100c therefore transmission designers will work within these parameters. This viscosity relates directly to how effectively the oil will do its job.
It is no coincidence that engines are designed to run optimally at about 88c and that engine oils are designed for the same parameters and it is the same for transmissions. The oil temperature should be steady at around 88 - 90c. Any less than that and you will be sapping energy to pump a thicker fluid round the system, any more than that and it loses its ability to perform because it is too thin to be effective.
Since this thread is about oil coolers, in my view if you fit
any separate external cooler it must be thermostatically controlled to prevent overcooling. Routing the oil through the radiator ensures that it will only cool to the ambient temperature of the water coolant which in a steady state should be around 80 - 90c. There is a good reason why the existing coolers are linked to the water cooling system.
