http://www.tciauto.com/Products/TechInfo/cooler_fittings.asp
Upper fitting gets cool fluid from the external cooler, and the lower fitting is the outlet to the external cooler.
upper - in
lower - out
In a typical cooling system, the hot side is usually the top line, where heated fluid or air exits the system to be cooled. The return line, which brings the cooled fluid or air back into the system, is generally positioned at the bottom. This configuration allows for efficient heat exchange, as the hot fluid rises and the cooler fluid descends.
your power steering fluid return line, also known as your power steering low pressure line, may have a hair line crack in it and need to be replaced.
The top line is always the return line, The cooler has to fill from the bottom or the fluid would get airated.
disconnect the return line off the reservoir, extend it with a piece of hose into a container to catch fluid. Start car and keep filling the reservoir till the fluid out of the return line comes out clean (2 qts or so). Do it again after a couple of days of driving to get all the contaminents out.
touch it ,the cooler one is usually return
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I had the same question, I looked in my Chiltons manual and it was useless as it normally is. On a 1996 Jeep Cherokee Classic with the 4.0 straight six, the lower line is the return line. It depends on what kind of vehicle you are working on, I assume not all are the same. I tested it the hard way, removed the upper line and turned on the engine.....be aware you will lose fluid if you try this. Once I had my answer I turned off the engine, only took 2 seconds and I lost enough fluid that I will now have to add some.
To flush the system, first locate the power steering pump lines. Usually a high pressure line takes fluid from the pump and is attached to the fitting with a crimped on steel sleeve. You want to detach the low pressure return line, which is usually hose clamped to the return nipple. While adding steering fluid at the reservoir, have an assistant turn the steering wheel back and forth. Get the front wheels clear of the ground with jack stands for this. Continue to drain off the old fluid until clean oil begins to drip from the pump. Then reattach the return line and top off the fluid reservoir. A few turns of the steering wheel, left and right, should be enough to force out air bubbles.
loopback adapter
loopback adapter
A fluid flush is the process of tapping into the return line of a transmission and capturing the old fluid while introducing fresh fluid to the pan. This is the only process that will get 100% of the fluid changed out short of draining the trans, valve body, accumulators and torque convertor and filling the dry transmission from the bottle. A filter change only affects about 25% of the fluid