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Draining the GM TH400 Transmission:

OK, it looked empty the first time, so here it is again. Oh BTW, the TH400 is a 3speed tranny, the GM 700R4 is the TH400 4speed.

Subject: Draining a GM TH400 Transmission

To drain the torque convertor, one must first remove the domed inspection cover on the lower half of the bellhousing. This should be retained with 2x 1/4-20 bolts at the midpoint level of the bell housing. After removing the inspection cover a clear view of the engine flywheel and joined torque convertor should be available. The Torque convertor is the donut thing in there. Now comes the hard part. While someone is under the vehicle, a competent repair helper, should carefully 'tick' the engine over with the starter while the technician under the vehicle attempts to locate the drain plug. It is in the 'inside' of the donut about 2" from the center shaft. It will look like a 1/4" square-topped bolt and be no longer than 1/4". The entire plug is less than 1/2" when removed. After locating the plug, bring a drain pan nearby, because there is no clean way to do this. IT IS IMPERATIVE TO LET THE TRANSMISSION COOL OFF PREVIOUS TO THIS PROCEDURE, SO AS NOT TO BURN THE TECHNICIAN! Remove plug and let drain as much fluid as possible. Do not try to turn the engine over while this is in process, it will sling fluid and make the hole almost impossible to locate again. After fluid has been drained, wipe clean all areas with a disposable wipe, and replace plug.

If this is done on a totally drained transmission, fill to appropriate level, and run engine for a few minutes, while switching through ALL AVAILABLE shift settings. This fills accumulators in the transmission. And let the Transmission warm up to operating temperature. Be sure to check the fluid while doing this so as not to activate the pump. An additional fluid add is inevitable.

If this is done on a transmission that has only been partially drained and refilled, the above procedure is not recommended because, A) the fluid is already mixed, B) the fluid will try to drain the entire transmission through the convertor (read: a million years to drain).

Hope it helps,

-Joel Brodsky

'76 IHC Scout II 345/tf727
'75 IHC Travelall 4wd 392/tf727 - (All auto trans.)
'72 Chev Carryall 3dr 4wd 350/th350


That was an excellent post on the torque converter draining procedure. Just want to make sure that everyone understands that there is no TH400 4-speed transmission. The TH400, a three-speed HD light truck trans is also known as the 3L80. The 700R4 transmission is also known as the 4L80E in its most recent configuration. It is a four-speed overdrive, locking converter, electronic-controlled trans which bears no relationship to its older much heavier-duty relative.

Glenn 93 4drht

Pete Leenhouts wrote:
A fellow here in B.C. Canada has developed and patented a machine that ties into the transmission cooler lines that drains and fills the transmission at the same time, basically exchanges the fluid, apparently works great.

FYI, Jiffy Lube (a 5-minute oil change shop) does that down here in Texas. They connect it to the oil cooler & hose line from the transmission. Has a window that shows you the old fluid and the new stuff going in. All of the Jiffy Lube shops that I have seen have it (Texas area).

93 Red Wagon - 6.2L Diesel 3 Speed
Allan Madar

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