Factory tow package did not have a brake controller.

The 'cooler' in the rad is the opposite. Its there to help the trans warm up in colder climates. 200* coolant does not cool a trans that should run between 160-180*.

Wouldn't the cool side of the rad (where the tranny cooler is located) be less than 200F, thus cooling both the tranny fluid and engine?

In theroy we would like to think so. However the cool side in our rad is only about 180*. Still a bit hot for an AW4. I highly doubt we see over a 40* drop in coolant temp from inlet to outlet.

I dont have any hard data so I digress as I may be full of .

Probably closer to a 20 degree drop. Maybe we'll get some posts from owners that have tranny temp gauges. Thought about installing a tranny temp gauge on our XJ, just never have. Have the big cooler and have never towed more than 3 motorcycles on an open trailer so just called it good.

I guess it would be hard to say, but depending on the flow rate of the trans fluid in the cooler lines the "small" factory tow package cooler may be more than adequate. I was confused about the cooler in radiator deal, but like mentioned before it helps maintain the ATF temp and keep it from getting too hot, but also helps to bring it up to operating temp faster. To my point though, if the fluid isn't flowing really fast, that small cooler will take a lot of heat out of the system, especially when the fluid gets a lot hotter than the air (increased thermal transfer rate). The other side of the question though is that too big of a cooler will keep the trans fluid from getting up to operating temperature and staying there if you're not pulling a big load. Like motor oil, it works when it's cold, but the transmission is designed to have the fluid at a certain operating temperature (like the engine and its oil) for clearances, seals etc. It's not always about getting things as cold as they can, but keeping temperatures in the designated "sweet spot".

my 90 came with factory tow package and came with the class3 hitch installed from factory with the secondary trans cooler my wire connector is the 7 pin

I've never ever heard of over-cooling killing a properly maintained auto tranny.....over-heating will kill one every time. What's the definition of over-heating? - don't know what that magic number is and don't want to find out either.

That's a good point too. On the other hand, why go through the expense of putting a more complex combined cooler/radiator on the vehicle that isn't going to cool to less than the temperature of the engine? If the goal was to keep the transmission as cool as possible, why not have only a seperate cooler that will cool more efficiently and not heat the fluid if the vehicle is just sitting and running? Also, why tell people to only check the fluid level when it gets up to "operating temperature" (my point being there is an operating temp and not because of obvious thermal expansion)? Just something to chew on I guess. I'm an engineer so I think about things in a different way sometimes.

I'd be willing to bet that factory XJ auxiliary tranny cooler and radiator (both are dinky) were designed by a bean counter, not an engineer.

So there is an XJ in the JY that has the factory tow package on it. I was able to grab the factory pig tail harness from it for the grand total of *Adam Sandler voice* FRREEEEEEE. And no, I didn't steal it, the guy at the counter said I could have it. I can also get the cooler for pretty cheap, just have to get them to process the Jeep and drain the fluids (came in from a wreck) so I can pull it off. I could get a bigger one, but I think it will work, and then I can use the factory lines (provided I can get them off in one piece) with out having to invent plumbing and mounting.

Anyone run acorss a PDF wiring diagram that shows the factory trailer harness? I see the Jeep is wired up to operate electronic brakes, but I don't know how it moduates them with out the control unit that is normally mounted under the dash. Is it just a one size fits all kind of thing? I'd like to see the wiring diagram to understand how they're operating it.

I've been towing the boats I have owned for the past 9 years, from about 2000 pounds weight to about 4500 pounds. I've found that for up to the recommended max trailer weight for our XJs, surge brakes should be fine. I don't think you need to worry about electric brakes. But if the trailer you want to pull just has electric brakes (?), then I guess you need to rig up something for the controller/lighting. MAKE SURE your XJs brakes are up to snuff.

I don't have any plans to pull a trailer with electric brakes any time soon. The only substantially heavy trailer I might pull would be the boat my dad has, but that has surge brakes on it. I'm just trying to figure out how the factory electric brakes were intended to be used. I've never pulled a trailer equipped with electric brakes before, but from talking with people who do (and riding with them) I know the controller is needed to compensate for empty/loaded weight and differences in road conditions from pavement to gravel etc. Maybe this would be a good topic for a new thread, if there's not one already...I'll have to go look. I just want to know what I'm doing if I need to pull a trailer with electric brakes so I don't kill myself or anyone else because I didn't understand my setup.

Think about it, how "used" and "crudded up on the inside" could a jy tranny cooler be? How thoroughly can u clean a used cooler? It can't be dis-assembled to verify how clean/dirty it and the jy fluid lines are. Do u want the possibility of someone else's tranny trash going into your tranny?

Universal trailer wiring (4 wire) is green = right, yellow = left, brown = running and white is ground. Those 4 are standard even in a 5 pin, 6 pin or 7 pin connector. Brake controllers should have a wiring diagram in their wiring instructions, I would think.

I'm not too worried about crud in the cooler. All the tranny fluid gets circulated through it so if it's that bad I doubt they would have been able to drive the Jeep enough to flip it and have it slide down the road on it's roof. A good flush with solvent and we'll be good to go.

I already have the pig tail wired up and I know the color codes. I've wired a number of trailers/cars for a 4-pin. Brake controller instructions are NOT going to tell me how the Jeep is wired from the factory to operate trailer brakes with out the controller.