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Cooling the Trans

A very cheap do-it-yourself auto trans cooler.

by Julian Edgar

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At a glance...

  • Why you need a trans cooler
  • Building your own
  • Cheap and pretty easy!
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Heat kills transmissions - as a rule of thumb, every 6 degrees C above 100 degrees C temp cuts the life of the transmission fluid in half. And it's not just the trans fluid that suffers, either. Any auto trans rebuilder will tell you that by far the majority of auto trans failures can be related to overheating. So if you have a high performance car equipped with an auto trans, one of the first upgrades on your list should be to fit a transmission oil cooler. In a car modified for extra power, more heat will be being dissipated in the transmission, and in cars with high-stall torque converters (either factory or aftermarket) the amount of heat being generated can be enormous. In hot climates? - well, it all just gets worse.

So a transmission oil cooler is A Good Thing - albeit expensive, especially in large sizes. But what if we told you that you can get excellent copper/aluminium heat exchangers for nearly nothing? And that they make ideal auto trans fluid coolers, are available in a wide variety of sizes and shapes, and can be fitted with the greatest of ease?

You'd like that? Well, read on!

Transmission Fluid Cooling

All cars already have auto transmission oil cooling. Usually, aluminium fluid lines run forward to the radiator where they are connected to the bottom or side tank by short lengths of rubber hose. Inside the rad is a heat exchanger - it's instrumental in both bringing the trans fluid up to temp and then holding it there.

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However, if the trans fluid rises excessively in temp with all the heat being dumped into it, the temp reduction effect of engine coolant at (say) 80 degrees C is much less than that which is available is you use the outside air at (say) 25 degrees C. (This assumes that the efficiency of both the water/air and air/air coolers is the same, which it may not be. It starts getting complicated when you go into it...) The standard radiator heat exchanger is also quite small - typically a plate-style design perhaps 150 x 30 x 30mm with commensurately limited heat exchange abilities.

In short, the standard heat exchanger is insufficient for performance applications.

Sourcing a Heat Exchanger

So where do you get this cheap and very effective trans cooler from? Salvaged from an air-conditioner, that's where!

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All domestic, industrial and car refrigerative air-conditioning systems use fluid/air heat exchangers, both at the condenser and evaporator ends. The most common have an S-shaped copper tube winding through many aluminium fins. Two, three and four row designs are common. These heat exchangers are available in a huge variety of shapes - from wide and long but very thin (eg 2-row), to short and fat (eg 4-row).

Household split-system air-conditioners (where the evaporator mounts inside in a slim enclosure and then compressor and condenser are outside) use thin evaporators, while conventional through-the-wall air-conditioners use fatter cores. Car evaporators are short and fat, while car condensers and large and thin.

In short, there are literally millions of variations on the theme, but all are based around fluid/air heat exchange.

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The type that you want depends on where you're going to mount it. Positioned in front of the radiator you'll want a thin design, so that air can still travel through it to reach the radiator (and air conditioning condenser). However, if you're going to mount the cooler in a guard (fender), a more 'block' shaped design will be fine, especially if it is sealed to a forward-facing duct.

There are three primary sources for these heat exchangers - air-conditioner repairers, scrap metal merchants and municipal tips. The last is the least consistent - there might be five air-conditioners available.... or none. Years ago when I lived in a country town, I sourced a lot of heat exchangers from a local air-conditioner repairer - he was happy to give them to me when he saw I was making use of them. (I actually built a turbo car intercooler from them, in addition to making a monster auto trans cooler.)

But if you can't find a helpful repairer, a scrap metal merchant is likely to have a pile of car radiators and air-conditioner condensers and evaporators. The rate at my local scrap dealer is AUD$3.30 per kilogram - and the cooler used in this story has a mass of just 1.5kg... yep, it cost me under five bucks! Sometimes the scrap guys will just give them to you, rather than bother going through the sale paperwork...

While You Are There...
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Take the time when you're at a scrap metal merchant to check out the other non-ferrous and ferrous bins. Stainless steel sheet (excellent for making heat shields), and aluminium sheet, angle and square tube can often be obtained at the same time. The latter is perfect for making brackets and a myriad of other uses. (In fact the top mounting plate for the auto trans cooler in this story was made from scrap aluminium.) Look for short offcuts, especially those cut 'square' as this will reduce your workload when you need to make something from them. When I picked up the heat exchanger used in this story I also got about 10kg of aluminium square tube and angle offcuts - enough to last me for years and years.

Before you go looking for an aircon evaporator or condenser, have a good idea of the max dimensions that you want the core to have. It's amazing how things look bigger (or smaller) when viewed in a scrap yard compared to when you later offer them up to the front of the car.

When you've found a core that is suitable, there are a few more things to look out for. Firstly, check for damage. You're buying scrap so you can't really complain when there is some minor damage, but make sure that you select only cores that don't have damage to the pipework. Fins can usually be straightened if they're not too flattened (just use a pair of screwdrivers and long-nose pliers - and a lot of patience) but dented or broken pipes are a lot more work to fix. In short, to repair damaged tubes you'll need access to oxy-acetylene brazing gear. (And that's not impossible if you know a plumber!)

Also inspect the diameter of the pipes, especially where you will be connecting the hoses. Typical trans coolers use 5/16th inch (8mm) diameter connections, and you'll find that many air con cores are either the same or pretty close to this. But some are not: they use much larger diameter pipes. Others have inlet and outlet tubes that vary in diameter. In both cases, unless you have access to brazing equipment, making changes in this area will cost you.

If the core is thick and uses very fine-pitched fins (ie they are close together) you will need a high pressure differential if air is to be forced through it. That could be provided with a fan or aerodynamically - but either way, just sticking a thick, fine-pitched core out the front of the car won't be very effective.

Some cores come framed in sheet metal, which makes mounting them neater and simpler. Many scrap metal merchants will have literally dozens (some will have hundreds!) of cores to pick from, so you can usually be selective until you find a good match for your requirements. (Incidentally, at my last scrap metal merchant visit there was a reasonable-sized factory intercooler available for scrap price. It had a little tube damage - easily repairable with an epoxy compound.)

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The heat exchanger I used was located after a number of visits to the scrap metal merchant. Their stock changes and so it makes sense to drop in on a regular basis until you find what you want. In the two-week period when I had an eye out for a suitable core I also located:

  • A 740 x 250 x 40mm two-row core from a split system (it was a little big for my application but would be ideal in a really high-powered car). It's shown above. It cost $2 from a dump shop, complete with the fan and all the rest of the wall mounted unit.
  • Click for larger image
    I also found 390 x 130 x 80mm 4-row core from an old under-dash car air-conditioner - another $2 dump shop purchase. This one would be ideal for a guard (fender) location, especially if a shroud and fan were added to it. Note the much tighter fin pitch of this design.

Installation Choices

There are three ways in which a trans cooler can be plumbed into place. (All approaches are easy, because the tubing already extends to the front of the car - all you need to do is to replace the factory pipe-to-radiator hoses with longer items to reach your new cooler.)

The trans fluid cooler can be located:

  1. In series with the standard radiator cooler, being plumbed-in before it
  2. In series with the standard radiator cooler, being plumbed-in after it
  3. To completely replace the standard radiator cooler

But why would you do (1) or (2)? You don't want the radiator heating the fluid when you're trying to cool it, do you? The key point to remember is that you want the fluid to be warm. Some auto transmissions monitor fluid temp and change their behaviour accordingly, so that the shifts are of the same firmness irrespective of trans temp. But others rely on the radiator heat exchanger to bring the temp up and until then, the shifts are a bit harsher or softer. If you bypass the radiator heat exchanger completely and fit a monster trans cooler, in cold weather the fluid is likely to be kept too cold.

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(Note that some more modern transmissions use thermostatic control of the transmission fluid cooler. That is, no fluid is sent to the cooler until the temp is too high. In these designs it isn't possible to overcool the trans fluid.)

But even in a non-thermostatically controlled trans, the potential for overcooling depends on the climate in which you live. If it never drops below 10 degrees C and is often 35 degrees C, you're probably not going to have to worry about too-cold trans fluid. (I mentioned earlier a country town in which I lived. It was near the Outback in South Australia, and the car to which I fitted a huge trans cooler - a Holden VL Commodore Turbo - had a trans temp gauge. The transmission never even looked like staying too cold, even with the radiator heat exchanger bypassed.)

And if you want the radiator heat exchanger still working, should you plumb the cooler before or after the radiator heat exchanger? Most aftermarket transmission oil cooler manufacturers recommend a position in front of the rad heat exchanger, presumably so that a minimum temp will still be attained by the fluid. However, some others suggest the opposite - mounting the new cooler after the rad heat exchanger!

Perhaps some rules of thumb for hard-working performance cars are:

  • Hot climate - bypass the radiator with the new cooler
  • Moderate climate - fit the new cooler after the radiator heat exchanger
  • Cold climate - fit the new cooler before the radiator exchanger

In the case of the car to which I was fitting the cooler - a 1988 Maxima V6 Turbo - I had noticed that the trans didn't shift as sweetly until it was up to temp, so I decided to fit the ancillary cooler plumbed in front of the radiator heat exchanger.

Installing the Cooler

The first step is to cut the heat exchanger's copper entrance and exit tubes to length. File these smooth so that a hose will easily slip over them and then take the core down to the local petrol station and thoroughly blow them out with the air hose. You can also check for leaks while you have the compressed air source.

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Mounting the core is up to you. You can do what most aftermarket trans cooler manufacturers recommend and separate the fins enough so that you can place a fastener (eg a bolt) through the core and attach it to the radiator or another mounting point, or you can make a sheet metal frame that goes around the edge of the core, giving convenient mounting points. The core that I selected came pre-packaged with stainless steel trim around the edges and these were easy to drill and pop-rivet brackets to. The top bracket I made continuous across the width of the core, which had the double advantage of giving good strength and also concealing the top copper tubes. (This is the panel with the mounting lugs that you can see here.)

After the fins have been straightened and the brackets attached, use a can of black spray-paint to give the core a good coat.

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After mounting the cooler at the front of the car, you'll need to plumb it in place. The easiest way of working out which is the pressure feed from the trans is to pull off both hoses that go to the radiator heat exchanger (arrowed - this is the view from under the car) and have an assistant start (and then quickly stop!) the engine. The hose that trans fluid gushes from is the pressure side. If you intend plumbing-in the new cooler in front of the rad heat exchanger, connect this hose to one side of the new cooler and the other side of the cooler to the radiator. If you want the cooler placed after the radiator heat exchanger, then plumb it into the other hose.

When you start the car make sure that there are no leaks and remember to top-up the trans to take into account any oil lost and the additional volume in the new cooler.

The Results

The core that I ended up using was a 2-row copper/aluminium design with widely-spaced fins. So, how well did it work?

A digital probe was used to measure the temp of the inlet pipe to the cooler. (Because in this installation the trans cooler is plumbed-in prior to the radiator, this was the pipe coming straight from the trans.) The measurements showed that when the car had been working hard and was then stopped, the temp rose very fast. In this situation the probe showed over 80 degrees C once the car had been stationary for a few minutes. However, when moving again (and with air being pushed through the trans fluid cooler) the system temp soon dropped to about 40 degrees C. (Note that these measurements were in an ambient of 20 degrees C - higher ambients will mean higher trans temps.)

Conclusion

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When the size of the trans cooler has little to do with how much you'll pay for it, fitting a huge copper/alloy cooler becomes a cheap and simple proposition. If you've got an auto trans car, you've got no excuse!

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