Thread: Learning the 4T65E

OK folks, since I botched the rebuild of my 4T65E I decided to do what I should have done to begin with. I found a GM manual that explains exactly how this thing works, including hydraulic schematics. Since I work with electronics by trade, I am more comfortable reading these. In an attempt to figure out what went wrong with my rebuild, I am studying everything I can about this transmission. I will not be an expert by any stretch of the imagination, but at least I will be a well-informed idiot.

One thing I have learned so far is that due to the unbelievable complexity of this system, it is virtually impossible to troubleshoot/diagnose anything more than the basic problems that these transmissions are known for (i.e., bang shifting, sudden loss of 4^th, etc, etc…) over the Internet. I applaud those brave souls who try to help those like me as much as they can. Guys like Dave (Trannyman) at TEP and Billboost must pull their hair out at times. They have my utmost respect and admiration for what they try to do.

So, in my endeavor to understand how transmissions (well, the 4T65E anyway) work, I am going to pass on what my perceptions are as I understand it better. I find that I learn better if I write things down. Since I’m writing it all down, I thought I would share what I learn along the way.

IMPORTANT NOTE: I am NOT a transmission expert! I never will be. DO NOT take what is written here as transmission gospel.

Another IMPORTANT NOTE: If you decide to tear into your transmission based on what you read or saw in another thread or on a different forum, I caution you now…learn how the dang thing works before you even drop the pan. Just because someone else managed to pull it off without a hitch doesn’t mean your tranny rebuild will be as straight-forward. There are many tiny things that can be wrong. So, learn how it works, and learn what to look for as best as you can.

OK enough lecturing…just the dad in me coming out. Without further ado (did I really just say that???) Here is first gear from a hydraulic standpoint. I’ll do a mechanical standpoint (how the hard parts work) later…because I don’t know yet.

Buckle your seatbelt…here we go…

When the car is placed into drive, the manual valve routes line fluid into the D4 fluid circuit. If I understand the hydraulic schematics and operation correctly, D4 fluid is just a fancy name for the fluid circuit used to operate each individual gear set. In other words, D4 is line fluid after it passes through the manual valve.
The manual valve also stops PRN (Park, Reverse, Neutral) fluid and allows it to exhaust. Since PRN is holding the #3 check-ball open, this exhaust action allows input (1^st gear) clutch fluid to seat the #3 check-ball. Well, it has to do other things first, but I’ll get to that later.

Now, when Line fluid exits the manual valve as D4, it does a couple of things. For one, it goes to the TFP switch and activates D4. It also travels to the 2-3 shift valve and passes through the 2-3 valve as auxiliary input clutch apply fluid. This happens because the 2-3 shift valve is being held in the downshifted position by 2-3 signal fluid via the energized 2-3 shift solenoid.

At the same time, D4 fluid also seats the #6 check-ball and travels to the FWD band servo, applying the FWD band. If you are romping the gas when shifting to D, fluid is also routed to the FWD boost valve to help fill the FWD servo faster.

At this point, since the 1-2 shift solenoid is energized, it blocks 1-2, 3-4 signal fluid from exhausting, which forces the 1-2 shift valve against the spring and into the downshifted position.

Fluid now passes through the 1-2 shift valve where it now becomes LO/1^st/Input (whatever you want to call it) fluid. This input fluid then unseats the #10 check-ball, and passes through the 2-3 shift valve (which is still being held in the downshifted position), then through the 3-4 shift valve (also being held in the downshifted position by 2-3 signal fluid).

Now, finally, Input fluid unseats the #3 check-ball, travels through the CP to the driven sprocket support, through the support and into the input clutch hub (between two of those bastige Teflon seals) and into the input clutch housing where it forces the input clutch piston against the clutches.

Let me go through this one thing at time to make sure I understand what could go wrong here.

Starting with the Manual valve: If the Manual valve were leaking bad enough, Line fluid would exhaust along with PRN fluid (or leak into other channels, resulting in the same thing, a drop in D4 pressure). This would cause slipping of 1^st gear at the very least and no apply of 1^st at all in a worst case.

TFP: If the switch were bad, I would suspect the PCM would recognize a problem and set a code, although I cannot confirm that.

The #6 check-ball: If this check-ball is leaking there would be reduced or even no flow to the FWD servo. In other words, the FWD band would not apply and even though the input clutch is applied, you won’t go anywhere.

1-2 shift solenoid: Since this solenoid should be energized, and therefore closed, 1-2 3-4 signal fluid pressure should increase holding the 1-2 and 3-4 shift valves in the downshifted position. If this solenoid is de-energized (due to an electrical malfunction, etc) or is leaking, 1-2 3-4 signal fluid could exhaust allowing the 1-2 and 3-4 shift valves to move against spring pressure. If this happens, there would be no fluid pressure to the input clutch, hence, no first gear.

#10 check-ball: If this check-ball is stuck closed, again, no fluid to the input clutch and no 1^st gear.

2-3 shift valve: If this valve is leaking, I suspect there could be a problem (fluid could be allowed to exhaust). Since the valve opening is much larger than the leak could be, I wouldn’t expect to see a complete loss of 1^st gear though (there may be considerable slipping though). However, I’m no expert so don’t take my word for it. In any event, this would be the last place I would look for a problem with 1^st gear. On the other hand, if this valve is stuck in the up-shifted position, no fluid could get to the input clutches. If this were to happen, there would be more problems than just no first gear.

3-4 shift valve: See 2-3 shift valve above, basically the same scenario.

#3 check-ball: This check-ball could be a real problem if it’s leaking or stuck open. That would cause Input fluid to exhaust along with PRN fluid. I suspect this would cause a serious drop in line fluid and a badly slipping or no 1^st apply.

Driven sprocket support: Unless it’s badly worn, or the apply port is plugged up, I don’t see it being a real problem. Anything is possible though.

Input clutch hub: The potential for a problem is there. This hub/shaft uses Teflon seals to direct apply fluid to the input and 3^rd clutches. Leaking seals could easily cause slipping or no apply of 1^st gear.

Input clutch piston: This piston is famous for wearing out. Once this thing wears out enough, no 1^st gear, or at least bad slipping.

Here is my old one:


OK, that’s my understanding of how 1^st gear operates, from a hydraulic standpoint. If any part of this is not correct, please let me know so I can correct it.

EDIT: It occurred to me that there is no accumulator for 1st gear. My assumption is that the FWD servo sort of acts like an accumulator, if you think about it. As input fluid is being applied to the clutch housing, it is also going to the servo to apply the FWD band. I suppose if the servo piston became stuck, the input clutches would probably bang together. Of course, if the piston was stuck, you probably wouldn't feel the bang because the FWD band isn't applied (because the piston is stuck). Just my curious ramblings here...

I think you are going much further than most will ever understand w/o going into a trans or trying to understand the valve body. It's good though. I am enjoying reading most of it. Meanwhile..something you may also find very helpful is this thread.

http://www.grandprixforums.net/trans...nks-42428.html

The last link I had recently found and was amazed to say the least. Lots of very good info and maybe it helps you best understand what is happening.

I have no wish for you to stop. I'm simply saying..I know a little about transmissions and I had trouble reading halfway through. Might be that I'd do better with a picture/diagram of flow etc in front of me at the same time to help visualize it. Which is why I brought up that other linky link link link.. because as I started into it, I think it ran down with visuals a lot of what you are trying to go through.

BTW..what do you think happened in your trans?

I took the easy way out of the valve body/channel plate when I did my build. I sent them to Dave.

Isn't second the tough seal? Meaning you can't see it well

Originally Posted by BillBoost37

Isn't second the tough seal? Meaning you can't see it well

No, that's input. Second and third use a molded piston, third also has an inner lip seal that is very hard to see, but second does not..

Ahhh that's right you had the super special lip/seal guide thingee. I used freezer zip lock bags cut to the right length and then lubed with trans fluid to get past all the sharp edges.

Do you think maybe your high tech piece there allowed one of the seals to get torn on one of those sharp edges? That was my biggest concern going through the seals.

Driven sprocket is 4th gear? It's been a while since I looked up the names.

Well, the driven sprocket support holds the 4th gear piston and clutches, but it also has a shaft that passes input, 2nd and 3rd apply fluid to the appropriate clutch housings. On the outside of that shaft there are 2 plastic sealing rings for 2nd gear. Between those rings, there is an opening that feeds apply fluid to the 2nd clutch piston housing (via the holes in the 2nd housing that you can see in the pic above). In earlier model years, there were 2 sets of seals. One was a quad rubber spacer inserted into the groove with a plastic sealing ring over that. At some point before 2000 (mid '98 maybe?) that design was changed to just the plastic sealing rings. They have a wider locating tab on them so you can't accidentally use the wrong ones in the older version. I didn't like it because when I air tested the clutches, second seamed to leak like a sieve, but the clutch applied so I didn't pay it any mind.

Ahhh yes I thought that was what we were talking about and yes.. I also tested all the clutches seperately and then assembled the whole mess and tested through the various apply holes as well.

I just got off the phone with Dave (TEP). He gave me some interesting info on the 2nd clutch drum. He said that he has seen, on very rare occasions, the shaft inside the 2nd clutch drum is cracked along a weld. It seems that the hub is welded to the shaft, and that weld has been known to crack. He told me how to check for it, as it might not be completely obvious. That would explain a lot of the problems I have, especially when the tranny is hot (allowing the crack to expand). He also confirmed that is unlikely to be a VB problem because even after the tranny is hot, I still have strong reverse and 1st gears.

I tell you, that Dave guy is awesome. Did you tell him I said hi...you always should. it's kinda like a running joke between the two of us.

I'm with Bill on this one....I think you've either rolled a seal or accidently cut a seal when intstalling. With the car shifting fine and going throught all the gears I'd say the VB is fine also. Knowing and working with Dave on my trans rebuild I'm sure there is a good chance he'd be able to pull a rabbit out of a hat with that diagnosis. Let us know whatyou find out.

Bill I'm not sure if you can answer this one but Monty had said in his original post while rebuilding the trans that he tested all of the clutches seperately @150psi. I thought that was really high in terms of pressure. In the manual and posts that I've looked at it says to test with 10-15 psi max. Any chance that putting that much pressure to one of the seals caused it to fail?