A simple transmission question

[firemediceric]

I am wanting to flush my transmission on my 1981 300 SD. Last time I dropped the pan and changed the filter. I don't believe there is any way the filter could need changing again, so I would like to just flush the old fluid out and replace it with new fluid. My thought, as I do on my Ford truck, is to remove the return line, attach some tubing to direct the fluid into a bucket, and then run the old fluid out while replacing the wasted fluid with new.

My question is which line is the return line and where is the best place to disconnect it?

I see one line runs on the passenger side of the transmission and one runs on the driver's side. Both look to connect to the radiator with a section of rubber hose. Direction as to which line I should disconnect to run out the fluid and where I should disconnect it at would be most appreciated.

Thank you,

[mbdoc]

Since you can drain 99% of the fluid, why not just flush out the hoses & cooler?

[mach0415]

If I am not mistaken, the return is not a suction line. The sump is suction ie pan and filter. The pressurization of the cooler line simply pushes fluid through the cooler to be returned to the trans. So technically it is not flushing, per se, but you accomplishing the same task by removing all old fluid. Any non-evasive trans flusher, such as BG, uses a bladder to utilize trans pressure to force fluid in via the cooler lines, while reclaiming old fluid. The fluid pressurized line goes to the empty side of the bladder, while the return side is connected to a capacity-charged with new fluid on the other side of the bladder. Pressure on the old pushes on the bladder to force new in, so you never overpressurize and blow seals. When the sight glass shows clean red fluid and the pressure equalizes on the gauge, it is done. So, simply putting one of the cooler lines in a bucket will not do anything but drain the trans. Some BG flushers use a low pressure pump for low line pressure transmissions, like Ford 5R44E/5R55E trans found in Explorer/Sport Trac and Ranger.

[vstech]

how many miles do you have on the filter? why are you wanting to change the fluid? the easiest thing to do is get under the car, align the TC drain with the window and drain it, drain the pan and open the two flex lines at the hard lines, and blow some air through it to empty the cooler. done all fluid out. then fill with 5 quarts and let the car idle for a bit, then put it through all the gears then put in the remainder of fluid to fill level. most sit at 6.5qts.

[firemediceric]

Originally Posted by M.B.DOC

Since you can drain 99% of the fluid, why not just flush out the hoses & cooler?

That's exactly what I want to do. On my truck I follow the procedure outlined at http://www.thedieselstop.com/forums/f47/changing-atf-7-3l-powerstroke-engine-4r100-automatic-transmission-198641/ which calls for the return line to be disconnected, the truck to be started and the internal trans pump pushes the fluid through the lines and cooler and out the disconnected line. Once the new fluid that the transmission is getting refilled with starts emptying out of the disconnected line, then the lines, cooler, pan, and torque convertor have all fresh & clean fluid. Quick, easy and thorough.

I'm thinking it would be best to disconnect the rubber hose from the hard line going to the cooler, but I'm unsure of which line is the return; passenger side or driver's side.

[vstech]

Originally Posted by firemediceric

That's exactly what I want to do. On my truck I follow the procedure outlined at http://www.thedieselstop.com/forums/f47/changing-atf-7-3l-powerstroke-engine-4r100-automatic-transmission-198641/ which calls for the return line to be disconnected, the truck to be started and the internal trans pump pushes the fluid through the lines and cooler and out the disconnected line. Once the new fluid that the transmission is getting refilled with starts emptying out of the disconnected line, then the lines, cooler, pan, and torque convertor have all fresh & clean fluid. Quick, easy and thorough.

I'm thinking it would be best to disconnect the rubber hose from the hard line going to the cooler, but I'm unsure of which line is the return; passenger side or driver's side.

on your TRUCK this is the only way to clean out the fluid.
however, your MB has drain bolts in the Torque Converter, the main pan. so all you need to do is line up the drain plug with the window in the coverplate, remove the two plugs and drain the fluid. then open up the two flex lines, and blow air through one of them. the fluid will blow out the other one. re attach everthing, and refill.
done.
no damaging the pump from running open.