Want to help your auto run cooler? Check out this ultra-easy transmission cooler install

Did you know you could be hurting your gearbox without realising? If you have an automatic 4WD, and you have taken it on the beach, used it for towing or just generally done some hard work on and off-road, chances are your box has gotten warm at some point.

This is a problem because the most common cause for automatic gearbox failure is overheating. The hotter the ATF gets, the quicker it breaks down and ultimately it won’t lubricate the clutch packs. So for this issue, we are going to install an aftermarket transmission oil cooler on an old BJ74 LandCruiser. If your truck doesn’t come with a factory cooler, or the one it has just isn’t cutting the mustard, then this is a must do mod for your auto 4WD!



A. “When it’s comes to automatic transmissions, an oil cooler is a must. Not only is it imperative that all automatic transmissions have an oil cooler but it is equally important that the oil cooler is good enough for the vehicle’s application.

Unfortunately, not all manufactures put auxiliary oil coolers of any design on their vehicles, and the few that do put a very basic heat exchange systems in are simply not up to the job, particularly in Australia when we put off-road tyres on, bull bars, lift kits, roof racks and engine upgrades onto our 4WDs. Most manufactures of both vehicles and aftermarket accessories don’t give much of a thought towards the transmission temps or the extra load that the transmission now has to carry or deal with. So it’s up to you to remember your transmission and make sure that the very FIRST accessory that you should be considering is a good quality oil cooler that’s perfect for your 4WD and the work load that you intend to use it for.”


Your vehicle’s size and use will determine what size cooler you should run. If you have a big 6-banger GU Patrol or Cruiser, you will generally need a larger core than if you have a smaller four cylinder Zook.

4WD Action’s BJ74 ‘Midi’ Cruiser is a 13B-T four cylinder turbo diesel, so we opted for the smaller core. Space was also very limited on the Midi, which limited the size of the cooler we could run. The Midi gets used as a daily driver and weekender, and doesn’t do any towing, so it doesn’t get worked too hard. If it did, we’d opt for the next size up. We got our universal kit from the local auto shop for around $115. It included the cooler, lines and fittings.


Once you have your kit, it’s time to decide where to mount it. For the best cooling it’s a smart idea to put it in front of the radiator, so it gets enough airflow. Just ensure there is adequate clearance once the grille goes back in. On the Cruiser, we have mounted it under the front grille. 

Generally speaking, oil coolers should be mounted with the fittings facing upwards. This allows air to purge out of the system and prevents the oil running back into the sump when everything is switched off. Also make sure the oil lines are well protected. Usually, it’s best to hide them inside or tucked up behind cross members and chassis rails to prevent them getting snagged off-road. Your best bet here is to read the instructions supplied with your kit carefully, or double check with the manufacturer.


In most cases it’s best to make up mounting brackets to secure the cooler to existing anchor points on the vehicle. You’ll need to make up a bracket for the top and bottom of the cooler, to prevent it vibrating. We’ve marked out and cut down a few bits of flat bar left over from a previous job and secured the cooler using nuts, bolts, washers and spring washers. One thing to note; make sure you only mount the cooler to the vehicle body or the chassis – not both – as the body will move and twist independently of the chassis. And to err on the side of caution, it’s also a good idea to add a dab of thread locking compound to prevent the wiggling loose. Nip up the bolts just enough to hold the cooler in place to start with, so you can tighten it later when everything is installed.


Now you have figured out where to mount the cooler, it’s time to run the lines. Most automatic 4WDs have the oil cooler within the bottom of the radiator. The Midi has a supply and return line to and from the radiator and the box. We sprayed a bit of WD-40 on the fittings the night before to make removal easier. For the best cooling, place the cooler after, or ‘post’ radiator. This way the oil passes through the factory cooler in the radiator, then passes through your external aftermarket cooler. If you’re not sure which oil line is which, undo one of the lines, stick it in a bucket and crank the motor over for one second only. If ATF comes out from the radiator, then that is your return line, if it comes from the line itself, then that is your supply to the radiator.


Remove the factory oil return line, and match up the fittings with replacement ones for the new line. There will be a bit of oil in the line, so use brake hose clamps to prevent spillage and make sure you have a way to catch it. You shouldn’t need to drain the box as there is no oil flowing, but it is a good time to change your gearbox oil if it’s due. Using your new fittings, install the lines and run them to the cooler. Make sure you use thread sealant on all the fittings, and hose clamps as well. When it’s all mounted you should have a line coming out of the radiator, to the cooler, then out of the cooler and back to the gearbox.


Now that you’ve installed the cooler, it’s time to install a temperature gauge. It’s one thing to cool the oil; it’s another thing entirely to make sure it’s staying cool. We chose a dual temperature gauge, which will allow us to see the temp difference before and after the cooler. Most gearboxes have a factory temp sensor, if you have to remove it, make sure you use thread sealant. For a dual temp sensor, we have installed a T-piece in the return line from the radiator. Once you have installed the fittings, run the wiring up to the gauge and wire in the gauge to the accessory circuit, constant power and a good earth following the instructions in the box. We mounted our gauge down by the shifter (as this is where the factory temp transmission warning lights are), cable tied the wiring along the factory harness and finished the whole lot off with split conduit.

[blockquote cite=”” type=”center”]TIP: If you’re installing your temp probein-line, make sure the probe is spaced out from the main oil passage in the t-piece. If you just screw it in the top of the fitting, the probe can block the flow of oil back to the gearbox. To fix this, we added a male to female fitting to the top of the T-piece and then screwed the sensor into that. That way, oil passes by, and the probe still gets a reading but doesn’t affect oil flow[/blockquote]


To minimise air locks in the system, it’s a good idea to fill the oil cooler itself with ATF once it’s fitted but just before you finally connect the hoses. This makes it easier for the box’s oil pump to circulate oil and prevents aeration during initial start up.


Now that everything is mounted, it’s time to check everything and take the vehicle for a test drive. Start the truck up and let it idle for a few minutes to build up oil pressure, then switch it off and check all the lines and the cooler to make sure they aren’t leaking oil. For an accurate level reading, you’ll need to cycle the box through the gears, both forward and reverse, go for a short lap around the block then finally check the level with the vehicle running. Check the oil level of your gearbox and top it up again if needed. Depending on where you mount your gauge sensor, normal operating temperature should be similar to your engine temps, between 80 and 90 degrees.

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