The truth about fitting an aftermarket auto transmission oil cooler

Is it a good idea for you to fit an aftermarket automatic transmission oil cooler to your fine four-wheel drive chariot? Should it be an air-type or water-type cooler? Let’s find out…

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The four-wheel drive fraternity is full of do-it-yourself ideas that often include modifying your perfectly functional ute or wagon with some kind of aftermarket accessory or ‘enhancement’.

When it comes to off-roading, adventuring, towing and touring, particularly when it includes dirt roads, dodgy tracks and plenty of low-range driving, there’s no shortage of gear you will hear every brand and respective bogan fanboy ‘advising’ you why it’s a good idea.

But in this report, we’re going to talk about why fitting an automatic transmission oil cooler might be a bad idea (at best) or (at worst) a recipe for expensive, self-inflected disaster.

It all started with an email that landed in the AutoExpert inbox, hot off the keyboard from a guy named Victor:

A couple of years ago I hired a 2.3 ton caravan and towed it with my five-cylinder BT-50 to Dingo Piss Creek. When I went north of Longreach with higher ambient temperature, the radiator temp would redline if I went over 80km/h and the transmission would also kick back a gear. So I drove at 75km/h; all good.

Chatting on a two-way indicated this could be fixed by installing an air-cooled transmission oil cooler. Yes, I did this and the problem went away. Why the hell would anybody cool a transmission with hot radiator water in Australia?

I’m now driving a 2021 Hilux dual-cab because I was unfortunately T-boned by P-plater. The Hilux transmission is also cooled by hot radiator water. Doesn't sound right, does it?

I've just boned-up for a camper trailer with 1.9 tonnes of aggregate trailer mass and no thunderbox fitted. The Hilux has been fully fitted out with all the off-road fruit.

I was hoping you could advise if I should fit an air transmission oil cooler.

Maybe the transmission oil needs to be at a certain hot temperature for efficiency. Is it possible to overcool and therefore damage your transmission by fitting an aftermarket cooler?

Cheers from god's own country.

- Big Vic

So Big Vic, presumably, fitted a bull bar, a winch and dirty big driving lights - and all that other crap that you stick on the front end and on top - so that you can be part of club bogan. And the BT-50 overheats when you go over 80 towing a caravan. Let’s be clear - an ordinary BT-50 can tow a caravan. Agreed? Off the showroom floor, that BT-50 was the ugly Ford Ranger and it was more than capable of pulling three tonnes of aluminium thunderbox.

So what happened along the way? I'd be willing to suggest that the over-temp problem was caused by adding too much weight, failing to maintain the vehicle correctly, or a combination of the two.

When the engine coolant and the heat-transfer system is not working, this is exactly the kind of result you would expect from poor maintenance or something additional added to the system that shouldn’t be there, such as obstructions to air flow; things like the radiator being full of grass seeds or poor air flow conditions to the vehicle’s frontal air dam.

But as for bolting on a new gadget to alleviate a problem which shouldn’t exist in the vehicle’s natural state - proceed with caution. (Continued below…)

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BACK TO BASICS: UNDERSTAND THE PROBLEM BEFORE SEARCHING FOR A SOLUTION

The fix for your engine coolant over-temp problem is not to fit a cooler for the transmission oil, it’s to fix the fundamental problem - whatever it is - with the radiator and the cooling system for the engine, I'd suggest.

This band-aid fix of fitting an oil cooler might help, but I'd also suggest you want to be careful about cooling the transmission down too much, because like all other oil, it’s very carefully chosen to suit a vehicle and manufactured accordingly.

For example, engine oil at room temperature is kind of a problem; it doesn't do its job very well, and that's why it needs to be chemically modified so it can be thin at low temperatures and maintain reasonable viscosity, but keep that thin film toughness at operating temperature.

This is a chemical engineering challenge. I'd suggest that for transmissions, the optimal temperature for an automatic transmission fluid is probably in that region of about 90 to 95 degrees Celsius, and what you'll notice with most automatics is that when you get in the car, it's cold, you drive off, and the shift quality is a bit rough, initially. You might have noticed this.

Then the transmission oil gets up to temperature and the shifts get better. This is because the oil is at its optimal temperature, which means it delivers its optimal operating properties. That make sense, right?

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Modifying for here...

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Could make it worse here...

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And leave you struggling here.

But there is a risk that you can cure the problem ‘north of Longreach’ where the ambient temperatures are 40-something in summer, but that could become too effective when you're touring in the snowy mountains down south, during the snow season. The cooling system for the transmission could be much greater than you need in varying conditions, so it goes from optimum temps up north, to delivering oil back in that is much lower than 95-Celcius because the ambient temperature outside is something like -4 or something. That’s not good.

So yes, Big Vic, you've got to be careful. This doesn’t just mean ‘proceed with caution anyway’, it includes not modifying something if there are serious consequences. Notice how snorkel manufacturers don’t declare their products are fitting with full immunity from voiding your factory warranty? Why do you suppose that is? Bullbar manufacturers love to claim they can fit an airbag-certified bullbar, and boy do they sell more products based on that premise. But again, notice how ute or 4WD brands won’t tell you it’s cool warranty-wise to fit bullbars? Occam’s razor, folks >>.

When you modify a car, particularly when you modify a vehicle with far-reaching potential negative effects meaning expensive, like a transmission, it’s not going to be a cheap fix if you kill one prematurely. And don’t even bothering with pleading your case using Australian Consumer Law.

Watch the full report to fully understand in greater detail why fitting an aftermarket ATF cooler to your ute or 4WD is a potential financial disaster waiting to happen…

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