Rebuilt THM425 and Related Products for the GMC TZE


My Procedure for Removing the TH-425 from a TZE

The way I approach removal of the transmission from the motorhome is by raising the front end with a floor jack. A 20-inch clearance is needed under the engine cross member. I also place two jack stands under the frame where the front clip bolts to support the motorhome so the floor jack can be removed and put out of the way.

Next I check to see if it has a Ragusa pan is in place. If a Ragusa pan is there, I drain the fluid. If the regular pan is there, I’ll drain it later after the transmission is removed.

I ask the owner if he has a battery disconnect. If one exists, it’s simple. If not, I disconnect the starter cable from the solenoid post on the firewall.

A 3/8 ratchet with a 5/8 socket removes the two bolts holding the starter to the tranny. A 9/16 open end undoes the nut holding the electrical cable. A pair of lineman’s pliers, removes the nut for the 12v signal on the starter solenoid. After that the starter can be lowered and put aside.

Three 1/4-inch bolts hold the flywheel shield in place. I either have the owner clean it or put it in the parts washer if the water is hot.

A 3/8 ratchet with a 9/16 socket will remove the three flywheel bolts on the converter. A long flat screwdriver prying on the flywheel teeth against the bottom of the boss, where the starter bolts in, will rotate the flywheel to get to the next bolt.

While in the same area, but above, with the same 3/8 ratchet and 9/16 socket, the two tranny bellhousing-to-engine bolts can be removed. The vacuum tube and kickdown wire can be moved over the exhaust pipe and out of the way.

Now, with an open end 9/16 wrench, remove the hidden nut while holding the tranny to the final drive. Sometimes a flashlight helps in locating things so have one handy.

Moving to the other side, the speedometer cable is disconnected, the pin holding the shift linkage from sliding out is pulled out. This will leave the shift rod dangling.

To unbolt the rest of the tranny-to-final drive bolts I’ll use the 9/16 open-end wrench. I start with the bolt holding the fill tube first. When the bolt is out I wiggle the fill tube out and up easily. Next, the top bolt is removed. From there, the rest of the bolts come out in no particular order. If the tranny was drained, probably no fluid will come out. If the tranny was not drained, some fluid may come out so have a shallow tray handy unless you enjoy puddles on the floor.

The next two bolts will require a 3/8 ratchet with a 2-inch extension with a 3/4 socket to loosen the these bolts holding the tranny mounts. The bolts are accessed through the crossmember. There is one on each side.

If the owner is inside the coach unbolting the bellhousing and disconnecting the cooler lines, it’s almost time to drop the tranny. If not, that’s the next step – read on.

If I don’t have help from the owner, I start with loosening the cooler lines. If the original lines are 5/16-inch and the engine has headers, I’ll cut the lines at the bend to replace them with 3/8-inch lines and rout them along the frame or over the bellhousing if the aluminum radiator is there.
Next is the bellhousing bolts. If the cruise control diaphragm is mounted, it may not go back if the cruise control is not working, neither will the transducer mounted on the firewall.

I use a chain to wrap around the tranny to lift/lower with a come-along winch. I use another chain to support the engine with a second come-along. I use two of the bellhousing bolt on the end of the heads through each end of the chain.

Over the engine hatch, I use two tees spanning from side to side with a 3/4 pipe nipple 12-inch long threaded on each side. The vertical member of the tee is 24 inches high the bottom is 34 1/4 inches.

The first tee has a 3/4-inch pipe cap welded on top, with the side facing the front of the coach so the 12-inch pipe nipple can be threaded into. The other tee has a 1-inch pipe nipple 2-inch long welded on top so the 12-inch nipple can slide into. Once the 12-inch nipple is screwed into the tee, slide one come-along, then the other. Slide the other end of the nipple into the second tee. Now it’s just a matter taking the slack out of the chains holding the engine and supporting the tranny.

The next step is to unbolt the three bolts holding the tranny to the mount support. I try to use an air ratchet to do this but it can be done without one. If the owner is helping, he works the come-along and I use a long screwdriver to push/separate the tranny from the engine and final drive; guide the tranny down. If the owner is not present, it’s up/down, up/down, a number of times with the screwdriver.

That’s it, this is how I do it. The transmission is removed and ready to be rebuilt or replaced.


THM425_SideView