Transmission Oil Cooler. Bypass radiator?

Kq3maga · Aug 1, 2016

Hey everyone,

The other day, i noticed a leak on a tube connecting to the radiator. I saw that it was red fluid, and guessed it was transmission fluid. Being a newbie to automotive repair, I wasn't quite sure why transmission fluid would be going to the radiator. After research, I was able to figure out that there is "cooler" inside of the radiator, which then connects to an external cooler that's in front of the A/C Compressor.

I started thinking about how it seems like it would be a pretty silly design to have an area where it could be possibly easy for engine coolant and transmission fluid to mix together. I then started to wonder whether it would be a good idea, or not, to bypass the radiator all together? I've seen some people say that the oil passes through the radiator to warm the oil up first, which is good for in cold climates. While this may be true, it doesn't seem to make a whole lot of sense since it's going to a component which is intended to cool the oil. Also, it doesn't seem like it would spend too much time in the tiny section of the radiator that long to warm it up or cool it down that much (after all, it's just a short tube, and doesn't pass through the fins of the radiator).

So with all that said, I'm just trying to get some advice on whether it would be a good idea to bypass the radiator or not, and if I should install a second cooler or not?

Also, please be gentle; as I stated earlier: I'm a newbie.

Thanks, all!

bigmark303 · Aug 1, 2016

Nice job on doing some general research before posting.

In your case where you have limited knowledge of the actual engineering behind the cooling system and transmission internal function I would just maintain the factory design to avoid any issues associated with bypassing a properly working system.

Repair the leak is your best bet. Automatic transmissions are very sensitive to heat and bypassing one cooler and not having the proper volume in the external cooler to adequately cool the transmission can be detrimental.

The transmission cooler being part of the engine radiator is a non issue. Lots of vehicles are engineered with that same design and leaks from one to the other through the radiator are very minimal.

adsm08 · Aug 1, 2016

To answer the direct question about if it is a good or bad idea to bypass the trans cooler in the radiator, that idea falls squarely inside the territory of "terrible".

The cooler in the rad is the primary one. It is where most of the heat is shed from the trans fluid. Oil to water coolers are the most efficient, while air to oil coolers are a far second. If you bypass the one in the rad and try to use only the aux cooler you will overheat and destroy your transmission.

Kq3maga · Aug 1, 2016

awesome. thank you for the reply and information, fellas. I really appreciate it.

Tyler46 · Aug 1, 2016

adsm08 said:
To answer the direct question about if it is a good or bad idea to bypass the trans cooler in the radiator, that idea falls squarely inside the territory of "terrible".

The cooler in the rad is the primary one. It is where most of the heat is shed from the trans fluid. Oil to water coolers are the most efficient, while air to oil coolers are a far second. If you bypass the one in the rad and try to use only the aux cooler you will overheat and destroy your transmission.

No.

The small "transmission cooler" inside the radiator is embarrassingly small. I took mine apart. It's very very small. No surface area.

It sucks pecker compared to my 15" cooler I ordered off summit. My trans runs significantly cooler since I bypassed the radiator.

Especially when the radiator runs 10 degrees cooler than the breaking temperature of transmission fluid.

adsm08 · Aug 1, 2016

Yeah, if you add a huge aftermarket cooler you might get away with deleting the one in the radiator. That's not what he was doing. He was talking about using only the small auxiliary unit installed on trucks with the tow package. That one by itself is not enough to keep the trans from overheating.

Also, yes, the one in the radiator is fairly small, especially compared to an air-only cooler, because as I said, WATER TO OIL COOLERS ARE MORE EFFICIENT.

10 degrees might not seem like a lot, but 10 degrees on an engine or trans is often the difference between the designed operating temp and cooking. Most engines run between 195 and 210 degrees normal operating temp. Most of those same engines start taking damage to rings, bearings, and gaskets at 220.

Tyler46 · Aug 1, 2016

I think you may have missed this part.

Kq3maga said:
So with all that said, I'm just trying to get some advice on whether it would be a good idea to bypass the radiator or not, and if I should install a second cooler or not?

And you also have to remember that the 2.3 and 3.0 guys don't even come factory with an air cooler.

I was running that water-cooled-only setup under geared on 35's and I LOVE to smash the gas at every stop. Took 20,000 miles of that, 5 grand shifts and heavy off road abuse to finish off the first trans I had in it.

Bypass radiator - yes.
Install second "additional" (or bigger) air cooler - yes

I would upgrade it before doing anything though.

Sent from my iPhone using Tapatalk

AllanD · Aug 2, 2016

Actually the air-to-oil cooler mounted up front is first in the flow path his is to shed as much heat as possible...

The cooler in the side tank of the radiator is almost a "re-heater"
what many fail to understand is that cooling the fluid too much can
be just as bad if not worse than not cooling it at all, because transmissions
like everything else has an ideal operating temperature that temperature is right around 160-180F...

which is the temperature of coolant on the cold side of the radiator...

I have mechanical temp gauges on both sides of my radiator.
they are there as a backup to prove/disprove gauge cluster indications...

Tyler46 · Aug 2, 2016

The top fitting on the transmission is the return, if you follow the lines it flows from the bottom fitting on the passenger side of the trans, into the top of the radiator, and the bottom line goes into the air cooler. Then returns to the trans.

The solution? A $40 cooler from summit, skip the radiator.

Done

Sent from my iPhone using Tapatalk

Tyler46 · Aug 2, 2016

Sent from my iPhone using Tapatalk

RonD · Aug 2, 2016

Kq3maga said:
Hey everyone,

The other day, i noticed a leak on a tube connecting to the radiator. I saw that it was red fluid, and guessed it was transmission fluid. Being a newbie to automotive repair, I wasn't quite sure why transmission fluid would be going to the radiator. After research, I was able to figure out that there is "cooler" inside of the radiator, which then connects to an external cooler that's in front of the A/C Compressor.

I started thinking about how it seems like it would be a pretty silly design to have an area where it could be possibly easy for engine coolant and transmission fluid to mix together. I then started to wonder whether it would be a good idea, or not, to bypass the radiator all together? I've seen some people say that the oil passes through the radiator to warm the oil up first, which is good for in cold climates. While this may be true, it doesn't seem to make a whole lot of sense since it's going to a component which is intended to cool the oil. Also, it doesn't seem like it would spend too much time in the tiny section of the radiator that long to warm it up or cool it down that much (after all, it's just a short tube, and doesn't pass through the fins of the radiator).

So with all that said, I'm just trying to get some advice on whether it would be a good idea to bypass the radiator or not, and if I should install a second cooler or not?

Also, please be gentle; as I stated earlier: I'm a newbie.

Thanks, all!

Welcome to TRS

Fix the leak

As to why the radiator is used for a transmission fluid cooler, it is because not all vehicles are used in warm climates, some are used in very cold climates.
So the radiator also acts as a trans fluid warmer in those conditions.
All machines with moving parts have a best temp range of operation, automatic transmissions are no different than an engine in that respect.
A liquid cooling/warming system transfers heat better than air cooled, so the radiator transmission cooler is better(by size) than an air cooler

Failure of the trans fluid/coolant divide in a radiator is very very rare, it can happen, but it is no reason to not use the radiator transmission cooler.
It would be the same as not buying a vehicle at all, ever, because in might break down one day, lol, and that happens ALOT more than coolant and ATF getting mixed from a bad trans cooler.

A second air trans fluid cooler should always be used, heat is more the enemy of automatics than cool, and a transmission under load will generate excess heat even in cold weather.

If you are in the Southern part of North America then you are probably fine without radiator trans cooler and only using a larger air cooler.
But these are AIR cooled, so air flow is required, if you sit in traffic alot, especially on hot days then.................
You could install a fan, but using the radiator trans cooler just makes more sense since it already has a fan installed

Transmission Oil Cooler. Bypass radiator?

Kq3maga

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bigmark303

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adsm08

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Kq3maga

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Tyler46

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adsm08

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AllanD

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