Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Dumb question - Why AT transmission coolers use coolant instead of it's own ATF?

Just was curious about the engineering and reasoning behind this.

Oil coolers use it’s own fluid (oil)
Power steering uses it’s own fluid
But it seems usually AT uses coolant. Why so?

Some oil coolers use coolant as the heat dissipating method. Pretty common on trucks. Sometimes they’ll spring an internal leak and you get coolant mixing w/the oil. And even on an engine without an oil cooler, the oil is still cooled somewhat by the coolant passages inside the engine. That and the airflow past the crankcase cover.

An automatic transmission has a lot of friction surfaces, clutches and bands, and high pressure pump(s), the torque converter is a heat source, plus just the friction of the gears meshing, and those all combined create a lot of heat, so a water-based method of heat dissipation is required. Water has a lot of heat capacity.

In the summer, it’s a method of cooling. In the winter it’s a method of heating the trans fluid. It’s really more of a means to regulate the fluid temp vs just eliminating excess heat…


I’ve seen oil coolers and ps fluid coolers run through the radiator. I’ve only seen this on pickups, but may be used elsewhere. I suspect the cooling capacity of the radiator and coolant is better than that of the air alone. So many trucks get liquid cooled oil coolers and/or ps coolers. I guess auto transmissions in all applications generate enough heat that they need the “heavy duty” cooling of the radiator and coolant. Maybe?

I’ve seen what twin turbo mentions before also. I just never really understood how a trans could run “too cold”. Of course, I’m in MS. So cold is relative. I’ve bypassed trans coolers in the radiator and used an auxiliary cooler in front of the radiator with no ill effects. Results might be different in the great white north. Actually ran an auxiliary trans cooler and the in-radiator trans cooler in the radiator in my 98 Dodge Ram, after the first rebuild. Didn’t help though. Trans died again anyway.


Transmissions produce a lot of heat from the torque converter, the frictions, the bearings, and gears under load. The oil in the transmission picks of this heat and the oil is transferred to the oil cooler, usually in the bottom tank of the radiator which is the coolest part of the engine cooling system. If you want additional cooling because of high loads like towing or mountainous driving an additional fluid to air cooler can be added after the tank cooler to reduce the temperature of the fluid. The longevity of the transmission is increased by cool fluid.

I once had a problem with a powerglide transmission where the high clutch was slipping. It would drive okay for a few miles; drop into Low; back into High; and then just quit. When I pulled the dipstick out it was blazing hot. It would work fine as long as I kept the selector in Low. The transmission had to be rebuilt._

Not so much anymore. Clutched convertors have reduced that heat a LOT.

@TwinTurbo has the right answer. The trans cooler is IN the radiator to regulate the fluid temp. It warms the trans fluid in the winter. My current ride has an external transmission cooler completely separate from the radiator. The service manual tells me it has a thermostat controlled valve that bypasses the cooler until the transmission reaches 185 F. To check fluid levels after a filter change, you must get it hotter than 185 to check the fluid level.

Interestingly, the dct in our baby mobile needs a cooler. When idling at a stop, it burns less fuel in neutral than in drive, even though both clutches should be opened. Perhaps the shearing of fluid between the clutch plates is enough to sap away a noticeable amount of power

Very confusing question. Oil coolers do not use oil to cool, they use air. What did you mean?

1 Like

Well, they kinda use both. They use oil to cool. The oil uses the air to cool. Unless it’s inside the radiator. Then the oil uses coolant to cool. And the coolant uses air to cool it.

I guess the question could be rephrased. Which is more efficient/offers more cooling capacity - an in radiator oil cooler, or an oil cooler that uses air only? I’d assume the air only cooler. But then I think liquid cooled is better than air cooled in general. So I’m not sure.

Sorry I wasn’t clear.

You’re right but I meant that oil won’t use coolant to transfer the heat to the radiator (or exchcanger since it’s technically not a rad). It’s the oil that flows through the rad, get cooled then goes back to the engine.

While I noticed most AT use coolant instead of it’s own AT fluid.

The ones I’ve seen circulate transmission fluid to an exchanger inside the radiator. What kind are you talking about?

I double checked and I think I was wrong. AT fluid does get pushed to the front with the rad but into a heat exchanger. So this way it would be watercooled technically?

Why not just push it into a AT rad? Not enough place maybe?

You can keep asking the same question, it doesn’t change the answer…

1 Like

Before we go any further, the OP should get the terminology straight. Radiator, cooler, boiler are heat exchangers. Wikipedia had a nice page explaining what a heat exchanger is

At least 2 fluid are involved in heat exchangers. For the engine’s radiator, the 2 fluid are hot coolant and ambient air-- any gas, including air, is widely recognized as a fluid in engineering. In most engine oil coolers, thre 2 fluid are engine oil and ambient air. For an automatic transmission, the 2 fluid involved are transmission oil(notice I’m not using the word fluid) and engine coolant. If you just use transmission oil, what would you suggest the second fluid should be?

Here is an experiment for you. Take two equal sized pieces of meat from your freezer. Put one piece on the counter to thaw. Put the other in a pan of cold water. See which one thaws out first. BTW, don’t eat the one that thawed on the counter or you will get food poisoning.