Transmission cooler, ditch stock cooler?

I've been looking around about doing this mod, but I'm not feeling overly confident with what I've found. Here's what I got, some time back I did some work for a buddy who payed me by getting me some stacked plate transmission coolers by Imperial, one's a 7-1/2"x11" the other a 11-3/8"x11". The rigs a 89' 4.0L with the trusty AW4.
Imperial states the smaller cooler is good on full size vehicles towing up to 2,500 lbs. The larger is for class "A" motorhomes towing up to 10,000 lbs.
So, who has anything to say about ditching to factory in tank system for a totally external system? Can I safely do this and run the samller 7-1/2"x11" cooler, I'd prefer this since it appears as if it will fit a bit easier.
I can (or can I?) assume the best place for it is in the airflow for the main fan? My A/C condenser is gone BTW, OBA is way more important! Of course I will be installing a gauge too, just to keep an eye on those temps.

Thanks
Chris O

The factory plumbs their aux cooler in series with the internal radiator cooler, and that's for towing up to 5,000 pounds. I think you should run yours in series rather than by-pass the radiator entirely.

I ditched my tank cooler and used a big finned cooler that is as tall as the radiator. Anything is better than the tank cooler. Pretty much all the heat from the tranny fluid is either dumped into your coolant or simply returned back to your tranny. Even a small finned cooler allows dissipation of heat into the air where it will actually lower operating temp.

Thank you gentlemen, this is the storta inpout I am needing. I'm still on the rail about this, but Lucas has a good point. I have often wondered wheter that stock tank helped cool the rad when the enigne was running 210. Granted, the tank is in the cool end of the rad.

Thanks!
Chris O.

Just a thought, but one of the reason I believe they put that thing in the radiator in the first place, was to help get it up to operating temp. No other logical reason I can think of.
I come from cold country, I usually install them in series, the external cooler, then the radiator heat exchanger. Get rid of the excess heat first, then circulate it through the radiator, to make sure the oil stays up to around 180 F. Too cold could cause problems also. Not sure about the AW4, but some trannys have a temp. sensor in the pan (bi metal) that switches the valve body routing and affects pump pressures. The heat exchanger in the radiator works in both directions, which equals (hopefully) a fairly constant temp. for the tranny.

tranny coolers

I recall seeing some tranny coolers with a thermostatic valve (bimetal) that didn't open to send fluid to the cooler, until it hit a certain temp. That being said: I'd forget about using the radiator anymore, just run straight to the cooler. Tranny fluid breaks down faster as you start to get it over 180*F, so why send it to a radiator that may be running 200*+?

BLUTO

Just thinking out loud, much of what makes tranny fluid, tranny fluid, is the additives to help keep the viscosity constant. Most of the heat, that messes up the oil, is generated in the pump and the clutches. Gets hot enough in there, to turn steel blue, on occasion. Can´t believe a 180 deg. (even 200) is going to hurt the oil that much. But it may help, all those little balls, in the valve body to bounce merrily around. At like 15 deg or so, fossil engine oil gets thick and the tranny can take a good long time to shift out of first.
Of course, if I lived in the desert, maybe my only concern would be getting rid of the heat. But I did spend a night at China Gardens in Death valley, at like 0 F, next to a frozen waterfall, That I´ll never forget.

Eagle said:

The factory plumbs their aux cooler in series with the internal radiator cooler, and that's for towing up to 5,000 pounds. I think you should run yours in series rather than by-pass the radiator entirely.

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If your going to get a aux cooler for the tranny why would you keep the tranny line going through the rad... the Tranny line runs hotter then the water in the radiator. woudnt that take away some of the radiators effectiveness? becuase you do have hotter material going through the rad.

I got a hayden Aux Cooler model #406, i dont remember of the top of my head but its close to 10.5"x9"x.75". fills the left side of the radiator, from top to bottom. I think its rated for motor homes towing 15K or more. I just replumbed the tranny line to go stright to the cooler and back to the tranny bypassing the radiator. I think it would be a better Idea if you keep the tranny fluid out of your radiator

Most likely, it would make the radiator work some better, to get the tranny oil cooler out of there. But I can´t believe the only reason they have it in there, in the first place, is because thats the way it´s always been done. Heck, engineers spend weeks, trying to figure out a way, to get ride of an inch of wire, in a production run. It´s gotta be more expensive to build a radiator that way, manufacturers are always looking for a way to cut costs.
I´m thinking they probably put it in the radiator, to get predictable pressures, due to a fairly stable temp/viscosity of the oil. Predictable shift points and/or clutch pressures.
I just made myself a viscositymeter, a tin can witha 3/16" hole in it. Heated some ATF to 90 F, put a few ounces of fluid in there, took 38 seconds to empty. Cooled another bottle of ATF to 30 F took a hundred and twenty seconds to empty (no chit).
Until global warming catches up in my area, I´m gonna leave the pre cooler and the primary radiator/tranmission heat exchanger hooked up in series.
At 0 deg F, ATF could be flowing through your great big oil cooler, and coming out, the with about the same viscosity as 90 wieght.
I´ve been working with pumps, gasses and liquids, most of my adult life. Getting rid of excess heat is often desireable, moving thick fluids is a whole nother can of worms. Centrifigal pumps don´t move them well, they don´t flow well and cause excessive pressure in sometimes undesireable places. Check valves work slowly and/or stick.

Some heat is good......too much is bad. As long as your tranny operates in the 180-210 area it will live a long time. The stock tranny cooler helps the fluid to warm up and work at the viscosity that it should, but we tend to push it's limitations with big tires, lots of crawling in hot, low speed conditions, and note that undergearing also works the tranny harder. To offset this, use as big an RV type tranny cooler that you can fit and install a temp gauge in the pan or in-line on the output side before it hits the radiator cooler. This way you know how hot the tranny is getting. Also, shifting your AW4 from OD to 3rd manually on long climbs will build less heat too.

I've run this info past my buddy who owns a 2nd gen Tranny repair shop and he has confirmed this. He also said that the AW4 is a trans that he rarely works on, if kept cool and serviced at proper intervals, it will go 200-300k miles.

So, What size tranny cooler should I get, and how do you bypass the tranny cooler in the radiator?

Terrific said:

So, What size tranny cooler should I get, and how do you bypass the tranny cooler in the radiator?

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how do you bypass it? unplug the supply and return lines from it, and plug them into the external cooler.

It isn't that simple on my xj. I have a factory auxiliary cooler and my lines are either threaded fittings or quick connect fittings at all locations. Do you have photos or a better explanation of how you did your auxiliary cooler and what you did with the factory cooler lines to make them work?

Liquid absorbs heat way faster than air does, that's the reason that liquid to liquid heat exchangers are far more efficient than air to liquid and why the OEM's use them. Aux coolers should be add on's, not replacements.
Running an aux cooler with the factory one by-passed in a cold enviroment coud mean the tranny will never get to the correct operating temp.

My aftermarket trans cooler is run in series with the stock in-the-radiator cooling, I was told that this helps the trans heat up faster in cold weather.

I'm not sure how the in-tank exchanger would help the trans warm up. The T-stat won't open 'till 185* F. The trans will reach that far sooner on its own. Keeping the trans temp stable and consistent makes a bit more sense, however when the ambient temp is quite low the rad might see little flow making for lower temps in the trans cooler return line. I'm curious if the aw4 has or hasn't thermal valves to avoid this.

Can anyone recommend a good oil cooler bypass valve?
What are optimal AW4 operating temps (min, max)?