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Keeping Your Transmission Cool

► Auxiliary Transmission Oil Coolers

► Transmission Heat Temperature Failure Chart

► How to Install an Auxiliary Transmission Oil Cooler

► Cost for Transmission Oil Cooler

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Save Your Transmission by Reducing Heat


The factory transmission cooling system uses the engine’s radiator to cool the transmission fluid.  Thus, when your engine runs hot your transmission also runs hot. The transmission can also overheat in stop and go traffic, when traveling on mountain roads, when towing another vehicle or trailer and on long trips.

Numerous other conditions, including low fluid level can also cause your transmission to overheat. More often than not, when a transmission overheats, it can go undetected until the trouble symptoms begin to show up.  By then, the damage is done – a replacement is normally needed.

The chart below provides a graphic illustration of just how devastating excessive heat is to the life expectancy of an automatic transmission.  The general rule of thumb is that for each 20 to 25 degree increase in temperature the transmission’s life expectancy is cut in half.  Keeping your transmission cool is the key to longevity.

Transmission and Transaxle Heat Temperature Failure Chart


Overheating is a Transmission Killer

According to The Automatic Transmission Rebuilders Association (ATRA), an International trade association for the professional automatic transmission repair industry, approximately 90% of all automatic transmission failures are due to fluid (ATF) breakdown caused by excessive heat.  As ATF heats up it becomes oxidized and loses its ability to lubricate and cool the transmission.  Poor lubrication causes friction, which produces more heat.  This is why it is extremely important to routinely check both the ATF level and condition in your vehicle.

Spending 5 to 10 minutes once a month checking your vehicle’s transmission fluid level and condition could save you thousands of dollars in repair bills.

175 Degrees:  100,000+ miles  (Normal is typically 179 to 195 degrees)
200 Degrees:    90,000 miles
225 Degrees:    55,000 miles  (Pressure Drops)
250 Degrees:    25,000 miles  (Valves Stick)
275 Degrees:    17,000 miles  (Varnish Forms)
300 Degrees:      4,000 miles  (Seals & Clutches Burn)

The problem of transmission overheating can be resolved by installing an auxiliary transmission oil cooler.  As a DIY project, you’ll pay between $30 and $60 for the oil cooler and spend 1 1/2 to 2 hours on installation.

You can have a local transmission shop install a transmission oil cooler in your vehicle.  Expect to pay between $150 and $250 for the job.  Either way, installing an auxiliary transmission oil cooler is one of the best investments you can make in protecting your transmission from overheating and certain failure.

Keep your transmission from overheating and you eliminate the #1 cause of premature transmission failure.

As repeated throughout this Website, excessive heat is a transmission killer.  Keep your transmission from overheating and your chances for transmission failure are drastically reduced.

The best way to insure your transmission does not overheat is to install an auxiliary (secondary) transmission oil cooler.  The cost is minimal and the installation is easy. A person with minimal automotive skills can install a transmission oil cooler in a couple of hours or less.

Transmission Oil Cooler Installation (Step-by-Step Guide)

Note: Installation instructions may be included in the transmission oil cooler kit you purchase. If so, follow those instructions first and use this information only if needed.


Screwdriver for hose clamps
Hand Drill and a selection of drill bits
Razor knife to cut rubber cooler hose
Hacksaw or small pipe cutter to cut cooler lines
A drain pan or small container to catch fluid

Before you begin:

1. Cut hoses so they are a couple of inches longer than your rough measurements.

2. The cooler must mount no closer than 1/2” in front of the radiator or A/C condenser.  The secondary transmission cooler must be securely mounted so that it does not come in contact with any moving parts.

Mounting the Cooler

1. The transmission cooler kit you purchased will include the mountingtransmission oil cooler hardware needed to mount the cooler.  The cooler should be mounted ½ to 1 inch in front of the radiator or A/C condenser for vehicles with air conditioning.

2. Find the oil cooler lines.  These are metal tubes that connect to the transmission on one end and the radiator at the other end.

Position the auxiliary transmission oil cooler so its inlet and outlet tubes face the existing transmission oil cooler lines.

4. With the mounting hardware provided with your oil cooler kit, mount the cooler 1/2 to 1 inch in front of the radiator or A/C condenser.

Note: The cooler may be mounted in a different location, but doing so may reduce the cooler’s effectiveness.


Connecting the Cooler

1. Connect the cooler using the illustration below as a guide.  On all automatic transmissions, the transmission oil flows from the transmission through the original equipment cooler, (which is located inside the radiator), and then back to the transmission.

When a auxiliary transmission oil cooler is installed, the heated transmission oil flows from the transmission to the original equipment oil cooler inside the radiator – then to the auxiliary oil cooler and then back to the transmission.  See illustration below.

2. Place a catch pan under the radiator.  Using an open-end wrench, disconnect one of the oil cooler lines where it enters the radiator.  Now, ensure that the vehicle cannot start during cranking.  Have a helper crank the engine for just a few seconds.  The transmission oil will flow either from the radiator (where you removed the cooler line) or from the disconnected cooler line.  If the oil flows from the radiator, this is the outlet.  If the oil flows from the disconnected cooler line, it means the oil flows out of the radiator at the other cooler line, making it the outlet.

Now that you know which is the outlet, install the hose connector adapter that came with the auxiliary oil cooler kit to the outlet neck on the radiator.

3. Attach the rubber hose to either of the outlets on your new auxiliary cooler.  Slip a hose clamp over the connection point.  Run the other end of the hose to the adapter you placed on the radiator in the previous step.  Mark the needed hose length and cut hose accordingly.  Slip the hose over the adapter fitting and secure with a hose clamp.  Tighten hose clamps to 15 to 20-inch pounds of torque.

Repeat these same hose fitting instructions for the second hose – being certain to torque the clamps at 15 to 20-inch pounds.



Check the Installation

1. Check the auxiliary cooler to be sure it is mounted securely and that there is some space between the cooler and the radiator or A/C condenser.

2. Check the hose clamps for proper tightness

3. Check to be sure the rubber hoses are not touching any other parts and that there are no sharp bends or kinks.

4. Start the engine and allow it to warm up to normal operating temperature.  Check for leaks as the engine is warming.

5. Check transmission fluid level and add fluid if necessary.  The installation of the auxiliary cooler should require some additional fluid to be added.

IMPORTANT NOTE: During the first week after installing a transmission oil cooler and then periodically from then on, check connections for leaks and check hose clamps to be certain they remain tight.  Also, don’t forget to check the transmission fluid level and condition periodically.

Congratulations, with a auxiliary transmission oil cooler installed, you can drive your vehicle with more confidence and peace of mind knowing that a cooler running transmission is a longer lasting transmission.

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