our vehicle’s radiator can do more than just keep the engine cool. There are two other fluids that your radiator may be keeping at the optimum temperature: transmission fluid and engine oil. If you have swapped an automatic transmission into your classic vehicle, you may have added an external cooler instead of using an internal cooler in the radiator. The same goes for an auxiliary oil cooler. While these certainly work to cool the fluids down, there are some major efficiency issues that can occur with external coolers.
he main concern with an external cooler is that there is no regulation of temperature. Wild swings in temperature can cause a whole host of problems that can reduce the life of your engine and transmission, along with a couple of other issues that you may not realize are happening. First, let’s break down the concerns for transmission and oil coolers.Automatic TransmissionsAll automatic transmissions require a cooler for the fluid, as heat kills a transmission faster than anything else. If the transmission temperature exceeds 230 degrees F, the fluid itself is toast as it forms a varnish. Above 250 degrees, the seals begin to harden, the clutches burn, carbon forms inside the transmission, and eventually the transmission fails. Keeping the transmission cool is important for a long life, but did you realize that there is such a thing as too cool? The optimum temperature for an automatic transmission is 175 degrees F, with an operating range of 165-220 degrees. Below 150 (depending on the fluid), the fluid is thicker than it should be, and you may experience harder shifts. Over time, the seals and valves can fail from being overworked by thicker fluid than they are designed for. Modern transmissions use a thinner fluid that is less likely to thicken at lower temps, but any transmission built before the mid-2010s is susceptible to this issue.
An internal cooler inside the radiator tank serves two functions- to cool the fluid while also maintaining an even temperature. Instead of your fluid cooling rapidly and recirculating, the fluid temp is kept within about 20-30 degrees of the engine temperature, ensuring that the fluid is within the optimal operating range. This is most important for stop and go driving. The goal is to reduce the temperature swings inside the transmission to regulate the overall temperature.
