I said in a few posts I was going to do a write up on the teardown and rebuild of the 47RE overdrive unit. Here it is!
Note, this thread is not meant to replace the need for a factory shop manual. It is intended to supplement the manual, since the manual has no pictures, only black and white line drawings.
Also, this is really for those who ONLY have overdrive mechanical issues. It assumes you have done troubleshooting already, and all indicators indicate that everything is fine in the transmission and valve body, that the PCM is commanding overdrive and you just aren't getting it, etc. IF YOU HAVE QUESTIONS FEEL FREE TO POST IN THIS THREAD, HOWEVER IN THE INTEREST IN PRESERVIING THE INTENT OF THIS THREAD, DON'T POST "HEY MY OD IS DROPPING OUT WHATS WRONG?". I understand I don't have control over this thread but I'd rather not have it turn into 3 pages on your specific problem. Thats why there is a "Post new thread" button on the page.
The background leading up to MY issue. Bought the truck and it shifted fine but had absolutely no OD. Reverse was fine, 1 2, 3 was fine. Shifted normal and all, it was just like driving a 3 speed. I troubleshooted and found that the PCM was in fact commanding overdrive which ruled out APPS, noise, temp, and speed sensor issues. I did a full set of hydraulic pressure tests per the shop manual and it passed with flying colors except the OD had almost no pressure when OD was commanded. I checked the appropriate valves in the valve body and found no issue, which pointed to the only thing it could be, something in the OD unit...
FIRST I am going to explain how the power makes it from the intermediate shaft(output shaft of the 1-3 gear section of the trans) to the output shaft of the OD unit in 3rd and 4th. Print this if you need to, so you understand what all these parts are for and what they do and how they interact.
Intermediate shaft is turning clockwise, which is splined to the overrunning clutch and planetary.
During regular power transmission in 3rd, power is transmitted from the intermediate shaft to the planetary. The Planetary turns the sun gear, which is splined to the sliding hub. In 3rd with no OD piston pressure, spring pressure applies the direct clutch packs which are splined to the sliding hub. The direct steels transmit power to the direct drum. The direct clutch drum is splined to the annulus gear, which is splined to the output shaft.
Between 3-4 shift, after the direct clutch disengages and before the overdrive clutch engages, power is transmitted to output shaft directly through the overrunning clutch.
After the 3-4 shift:
Overdrive steels are splined to the case; the overdrive clutches are splined to the sliding hub. With the overdrive clutch engaged, the sliding hub stops turning, which is splined to the sun gear which also stops turning. Direct clutch is disengaged, allowing the direct clutch drum, annulus and output shaft to spin at a different rate from the sun gear and sliding hub.
With the sun gear not rotating, power is transmitted from the intermediate shaft into the planetary gear set. Since the sun gear is held stationary, the annulus begins to turn at a faster rate, spinning the direct drum(which spins the disengaged direct clutch steels), as well as the annulus gear turns the output shaft. Over running clutch is overrunning.
What Im going to do in this thread is help you remove and tear down and rebuild your OD unit. No, you do not need to remove the trans from the truck. Yes, you will need a factory shop manual.
This thread starts out with the vehicle jacked up, tranny drained and the valve body removed. The shop manual is pretty straight forward on that.
First, the shop manual calls out all the ports to air check the clutches EXCEPT for the OD clutch. To test the OD port, apply air pressure to the rectangular port highlighted in the following photo. It wont "thunk" like the other clutches, but you shouldn't hear a ton of air pressure pissing out. But it does have a bleed back to the accumulator so it will bleed some.
Ok, on to the work...
Looking into the hole where the park rod is going you can actually see what the problem(or one of them) was with mine. The black wire looking things should not be there, thats the lip seal for the OD piston..
Ok, enough stalling, here we go.. Heres what it looks like starting out down there.. Make your life easier.. Whenever you can, put the bolts back in the holes they came from. Put nuts back on the studs after you remove the parts. Dont just throw everything on the floor.
Remove the 4wd actuator line from the truck by disconnecting the hoses. Note which hoses went to which tube.
Remove the two nuts from the tranny mount.
Re-install the pan, and support the weight of the trans and transfer case with a tranny jack or floor jack, raising it 1/4" or so off the crossmember. Then remove the bolts from the cross member to the frame. It wont come out. You have to spread the frame rails with a porta power.
Remove the driveshafts, and disconnect the vacuum lines from the top of the transfer case. If I was to do it again, at this time I would also remove the vaccum switch that the vacuum lines plug into. Its easily damaged once the transfer case comes out. If you dont and need to buy a new one, at Napa its called a "transfer case lockout switch" and is about 11 dollars.
Disconnect the Vehicle speed sensor.
Pop the linkage for the transfer case shifter from the transfer case
Remove the nuts from the studs holding the transfer case to the trans. I dropped the case just by holding it up with my knees. To reinstall it I used the tranny jack
The next case shown is the OD unit. All things OD are mostly contained in this box. Remove the tranny mount from the unit, and then remove the 7 bolts that hold it to the tranny.
Remove the VSS if like me, you forgot.
Remove the OD unit from the transmission. This is a video. Click it to see the OD unit being removed. The thing that falls out of the OD unit is the park rod.
Once the OD unit is out, here is what it looks like. This shaft is what would be called the output shaft and connected to the driveshaft if this was a 3 speed transmission. In a 46/47RE or RH its reffered to as the intermediate shaft. The round thing on the back of the transmission case is called the OD piston, which sits inside of the OD retainer. When the valve body commands overdrive it sends fluid to the retainer which pushes the piston towards the back of the truck, which engages the OD clutch and disengages the direct clutch.
In the video below you can see the piston being removed, as well as my blown out lip seal.