[TTS]

71 Formula, your transmission guy is an idiot, or he's hoping you are.

When low is selected, oil travels directly behind the reverse servo from the manual valve. There is drive oil at the front side of the servo via the 1-2 accumulator control valve, reducing the apply area to that of the 1-2 accumulator bore diameter. So, there is a much smaller apply area than in reverse, but the band shape or servo pin length isn't going to stop low from happening.

I don't have a guess at this point, there's more wrong here than meets the eye.

[screamingchief]

As should be obvious at this point,many things can cause similar issues and still be completely different issues to try and solve or diagnose,so I'm not gonna mislead ya'll into thinking I'm giving you the best answer to this problem,I'm just giving a few thoughts to ponder.

That said I've run into this sorta issue with the very first TH-400 I rebuilt myself,so I can only tell you what caused my problem of this nature.

Many sources recommend removing the inner lip seal from the forward clutch apply for more holding power in high performance TH-400 builds,this is a very common high-perf mod for that trans,what many sources forget to tell you is that you also need to block/seal the passage for the reverse feed circuit when you do that (either in the VB plate or at the center support) as that circuit will backfeed and you'll either loose reverse completely,or in my case I lost the engine braking in manual low.

When that was going with my trans,reverse still worked fine,drive still worked fine,and only the low (1st gear) engine braking was not working properly,and yes it acted very odd when using the manual low ranges,it freewheeled when it should have been engine braking,then if you apply the gas it goes right back into 1st gear,occasionally with some mild drama involved.

Only later on after rebuilding a few more TH-400's did I learn of the cause of that issue with that trans,it was a simple mistake caused by following a magazine article that told only a portion of the needed info for a proper modification...

So my "best guess" is either the forward clutch lip seal mod was attempted and not done correctly,or the inner lip seal on the forward clutch was damaged when assembled or something to that affect.

If you look into the hydraulic flow diagrams of these circuits like those in the Ron Sessions TH-400 handbook you can clearly see why this happens,the pressure that is bypassed/backfeeds from the forward clutch pack directly disengages the rear servo much the way it does when the car is in the D ranges.

HTH.

[71Formula]

Originally Posted by screamingchief

When that was going with my trans,reverse still worked fine,drive still worked fine,and only the low (1st gear) engine braking was not working properly,and yes it acted very odd when using the manual low ranges,it freewheeled when it should have been engine braking,then if you apply the gas it goes right back into 1st gear,occasionally with some mild drama involved.

HTH.

This does sound like the same thing mine is doing.


Would a pressure test find the missing plug needed for the 1-2 shift mod? How about the leak screamingChief had after the lip seal mod, would a pressure test find that?

[TTS]

Originally Posted by 71Formula

This does sound like the same thing mine is doing.


Would a pressure test find the missing plug needed for the 1-2 shift mod? How about the leak screamingChief had after the lip seal mod, would a pressure test find that?

Yes, a pressure test would find the missing plug for the 1-2 shift mod. Best way is to put compressed air with a gauge into the pressure tap. Compare 3,2, and 1. 1 should sound like you're blowing almost directly in the case and the pressure will drop if the plug's missing.

Didn't you say this was a problem before the new shop?

If so, have the shop pull the vb and look for the two items in the pictures. They owe you that.

As far as the dual feed mod plug. No, I don't think it can be pressure tested.

[TTS]

Make that the Direct clutch inner seal, not forward.

[screamingchief]

Originally Posted by TTS

Make that the Direct clutch inner seal, not forward.

Doh,,,your right about that,,,my mistake...

I'd correct that if the forum would only let me.

I had initially written those comments stating the direct clutch,but for some reason that got changed to state the forward clutch when I rewrote these comments to post here,I occasionally do such when I dont post on a topic right away,I do a rough draft in MS notepad and then I work from that rough draft to post here later.

I stopped to have supper,I guess I mixed those two up in the process.

I'll have to go back and double check the feed circuit diagrams and see if this still makes any sense.

I just know I had the same type of problem when I tried that lip seal deal and did'nt block the reverse feed in the center support.

And it would stand to reason that if that seal had been overlooked or damaged that too could cause the same condtion.

The direct only applies in Drive (3rd gear) and reverse,so the back-feed and low-reverse band apply connection is a bit fuzzy I agree.

But that was how my trans acted with that issue.

That trans had the TCI trans scat super II valvebody kit in it.

I think what was happening is the pressure intended to operate the low-reverse servo in manual low was infact exhausting itself via the direct clutch circuit back-feed deal as that circuit is not active in manual low,and the TCI valve body kit might have contributed to the situation.

Hope that clears my comments up a little...

[TTS]

Low is a function of the manual valve and the manual valve only. If the manual valve is in low, and the band is capable of applying it has to. The low shifter position is there for 1 reason- low braking. There are no spool valves, springs, checkballs (well one checkball but reverse wouldn't work without it.)

If there was a massive leak at the servo cover, reverse could work and low not because of the apply area difference.

[71Formula]

Got the car back today, but not fixed yet. He ordered the parts he needs and they will be here next week. I will take the car back in after Memorial Day to have it finished.

To clear things up a bit. Everything is actually working properly except for manual low. Manual low does not engine brake, but as I apply the gas the RPMs go up and first gear smoothly engages and works normal. Sounds exactly like ScreamingChiefs description of the problem he once had.
My transmission guy is saying the rear band is not able to apply all the way in low 1, but in reverse it is. The flow is all there, but the band is not able to apply all the way because the apply rod is not long enough. I don't know if he is right, but I am going to let him give it a shot.

The shop is an actual automotive repair shop, not just transmissions.
This has been a problem from the very beginning...

[screamingchief]

Originally Posted by TTS

Low is a function of the manual valve and the manual valve only.

The hydraulic flow routing chart in my manuals would seem to disagree with that comment.

The fluid path goes to a whole lot more than just the manual valve.

There are like a dozen other elements in the hydraulic fluid flow path for manual-low besides the manual valve itself.

The only real difference between the mechanical power flow thru the trans when looking at manual-low and reverse is that the forward clutch is engaged to get manual low,and for reverse gear,the direct clutch is engaged instead,that in turn overides the low-roller clutch and makes it ineffective when reverse gear is selected and that changes the direction of rotation,where-as for manual-low the low roller clutch is effective and that keeps the rotation in the forward mode.

Without the band apply in manual low with the forward clutch applied what you get is basically 1st gear in the D range.

All you really loose is the engine braking for the most part.

Now dont misread my comments,I've said from the get-go I'm not swearing to my deal being this guys problem,and I'm not 100% sure how best to explain why that rebuild acted the way it did,all I can say is that is how it was built,and that was how it acted after the rebuild,and I later learned why that must have been so.

The trans rebuild itself was necessary due to shearing of all the teeth off the rear ring gear,which in and of itself was a pretty damn odd trans faliure...

That was a PQ code TH-400 in my '72 WW5 455 H-O GTO.

I attribute the ring gear wear/damage to excessive backlash in the rear diff that put a lot of strain on the trans geartrain.

Most of the text on that lip seal mod warn of the rear band applying in drive if you leave that circuit unblocked,though I never had that issue that I recall,and it seems pretty certain I would have had remembered such if that had happened,as it surely would have smoked the band,and I would likely have lost reverse as well,and that never happened.

So it seems obvious that hydraulic pressure was being exhausted somewhere that was not causing additional issues other than the loss of band apply in manual-low.

My opinion holds that could have happened back thru either the direct or reverse clutch circuits as that circuit "plug" was not there to stop a "back-feed" type situation when the low-reverse servo was applied.

A long shot guess I agree!

I do know this for a fact,,,somewhere in that maze of fluid passages something just was not "right"...

What I'm getting at here,to simplify the intent of my comments,is that when a fella goes changing the fluid flow-paths in an auto trans all willy nilly,unexpected end results often happen,sometimes it's painfully obvious as the car simply wont move,other times the issues are much less noticable,and barely register on many folks radar.

And even seasoned builders run into problems from time to time,but they usually have a much better idea how to tackle the problem,as long as they have a decent diagnostic process for solving problems like this.

You zero into areas to check more closely,look for smoking guns,then if you dont find one,go looking for the "long shot" candidates.

I know this,I learned a lot from that first TH-400 trans,and have'nt had issue one since.

But I also make real sure about what mod does what,and how and why it does so.

No more "trans building tips from car magazines" after that deal...

[TTS]

Originally Posted by screamingchief

The hydraulic flow routing chart in my manuals would seem to disagree with that comment.

Pay very close attention to what you're looking at. There are 3 oil paths opened when low is selected; 3,2, and 1. They are all engaged by the manual valve. 3 or D is still the oil circuit that makes things happen, ie shift control, governor, modulator, accumulators etc...

2 and 1 ONLY ADD THEIR ONE PIECE TO THE PUZZLE... braking. 2 does very little in that there is a front "band off" circuit opened when in 3 that is closed when the manual valve is moved lower into 2 allowing the front brake to actuate.

In 1 the low band is on with nothing else needed to do so.

Think of the manual valve like this, instead of D 2 1 it's D, D+2 and D+2+1

Originally Posted by screamingchief

The fluid path goes to a whole lot more than just the manual valve.

There are like a dozen other elements in the hydraulic fluid flow path for manual-low besides the manual valve itself.

The only real difference between the mechanical power flow thru the trans when looking at manual-low and reverse is that the forward clutch is engaged to get manual low,and for reverse gear,the direct clutch is engaged instead,that in turn overides the low-roller clutch and makes it ineffective when reverse gear is selected and that changes the direction of rotation,where-as for manual-low the low roller clutch is effective and that keeps the rotation in the forward mode.

In manual low the low roller is INEFFECTIVE (braking), in auto low it's effective. Reverse and low have another difference in the servo. Low doesn't have the area reverse does. When in D+2+1, D oil is in front of the servo and 1 oil is behind it. D and 1 oil are equal but 1 oil has a larger surface so the band comes on. The band is forced off when 2 oil floods the accumulator during the shift. 1 oil is dumped behind the manual valve and the servo is off. The transmission is now in D+2.

Originally Posted by screamingchief

Without the band apply in manual low with the forward clutch applied what you get is basically 1st gear in the D range.

All you really loose is the engine braking for the most part.

And there is a pressure increase in manual low, the boost valve is actuated. Low and reverse pressures are similar.

Originally Posted by screamingchief

Now dont misread my comments,I've said from the get-go I'm not swearing to my deal being this guys problem,and I'm not 100% sure how best to explain why that rebuild acted the way it did,all I can say is that is how it was built,and that was how it acted after the rebuild,and I later learned why that must have been so.

The trans rebuild itself was necessary due to shearing of all the teeth off the rear ring gear,which in and of itself was a pretty damn odd trans faliure...

That was a PQ code TH-400 in my '72 WW5 455 H-O GTO.

I attribute the ring gear wear/damage to excessive backlash in the rear diff that put a lot of strain on the trans geartrain.

Most of the text on that lip seal mod warn of the rear band applying in drive if you leave that circuit unblocked,though I never had that issue that I recall,and it seems pretty certain I would have had remembered such if that had happened,as it surely would have smoked the band,and I would likely have lost reverse as well,and that never happened.

So it seems obvious that hydraulic pressure was being exhausted somewhere that was not causing additional issues other than the loss of band apply in manual-low.

My opinion holds that could have happened back thru either the direct or reverse clutch circuits as that circuit "plug" was not there to stop a "back-feed" type situation when the low-reverse servo was applied.

A long shot guess I agree!

I do know this for a fact,,,somewhere in that maze of fluid passages something just was not "right"...

What I'm getting at here,to simplify the intent of my comments,is that when a fella goes changing the fluid flow-paths in an auto trans all willy nilly,unexpected end results often happen,sometimes it's painfully obvious as the car simply wont move,other times the issues are much less noticable,and barely register on many folks radar.

And even seasoned builders run into problems from time to time,but they usually have a much better idea how to tackle the problem,as long as they have a decent diagnostic process for solving problems like this.

You zero into areas to check more closely,look for smoking guns,then if you dont find one,go looking for the "long shot" candidates.

I know this,I learned a lot from that first TH-400 trans,and have'nt had issue one since.

But I also make real sure about what mod does what,and how and why it does so.

No more "trans building tips from car magazines" after that deal...

[TTS]

Originally Posted by 71Formula

To clear things up a bit. Everything is actually working properly except for manual low. Manual low does not engine brake, but as I apply the gas the RPMs go up and first gear smoothly engages and works normal. Sounds exactly like ScreamingChiefs description of the problem he once had.

It sounds exactly the opposite. You have low line pressure, there is either a big hole or your pump has problems.

Originally Posted by 71Formula

My transmission guy is saying the rear band is not able to apply all the way in low 1, but in reverse it is. The flow is all there, but the band is not able to apply all the way because the apply rod is not long enough. I don't know if he is right, but I am going to let him give it a shot.

The shop is an actual automotive repair shop, not just transmissions.
This has been a problem from the very beginning...

Sorry to hear that. It's painfully obvious they are not transmission specialists.

[stevep]

I still believe that he did something wrong while he was installing the shift kit.
If he did actually install a Trans-Go kit, if he calls Trans-Go and describes the problem, their tech dept. should be able to get to the bottom of this.

[TTS]

I think I have it figured out, assuming low has the only problem.

If they installed a modified 1-2 shift valve- for low at any speed, they didn't install the plug in the VB casting. When the valve is installed it leaves a BIG leak in the low circuit unless the hole is plugged.

I'll post some pics if I have them.

[TTS]

This is where the plug goes.

[TTS]

This is a modified 1-2 shift valve from a transmission with a Transgo Shift Kit. Many others use this valve or similar.

Stock valves have a 3rd larger spool on them. If someone ground flats on the largest spool (would be located at the top of this one) it works the same as the one shown and needs a plug.

[screamingchief]

Originally Posted by TTS

In manual low the low roller is INEFFECTIVE (braking), in auto low it's effective.

That's not exactly how it is according to both my trans manuals...

The only time the low-roller clutch is ineffective in manual low is when the engine is decellerating and the trans is engine braking,then I agree the low-roller de-clutches (ineffective) and over-runs,but as soon as your back on the gas it is indeed effective again,as it is in auto low as well.

Originally Posted by TTS

Reverse and low have another difference in the servo. Low doesn't have the area reverse does.

Agree with this,and the comments afterwards too,I did'nt mention that difference as it was'nt really relative in the context of my comment,I was discussing the manual power flow thru the trans and gear train,not the hydraulic circuits that activate the elements needed to achieve that power flow.

Originally Posted by TTS

And there is a pressure increase in manual low, the boost valve is actuated. Low and reverse pressures are similar.

That was one of the other dozen or so elements I was refering to in my comments.

So you agree that more than the manual valve is being used in manual low now?

I'm not arguing that some of the true relevence of some of those could be considered mostly unimportant in the grand scheme of things here though,just stating that they are other elements involved in the deal...

Originally Posted by TTS

Think of the manual valve like this, instead of D 2 1 it's D, D+2 and D+2+1.

Pay very close attention to what you're looking at. There are 3 oil paths opened when low is selected; 3,2, and 1. They are all engaged by the manual valve. 3 or D is still the oil circuit that makes things happen, ie shift control, governor, modulator, accumulators etc...

Looking at the flow path,I agree with these comments completely.

That's why I said there are like a dozen other elements in the hydraulic flow path in manual low.

Originally Posted by TTS

I think I have it figured out, assuming low has the only problem.

If they installed a modified 1-2 shift valve- for low at any speed, they didn't install the plug in the VB casting. When the valve is installed it leaves a BIG leak in the low circuit unless the hole is plugged.

I could see that,with the TCI V/B kit I used on that TH-400 I mentioned earlier it indeed used a modified 1-2 shift valve,and the kit included a pressed in expansion plug for that hole,so could be that plug was'nt a good fit and it was pushed out by the hydraulic pressure or such.

But that's pretty much like I said earlier,somewhere pressure was being exhausted that should not have been,I just never figured out 100% where that was with that specific trans,I only had some inklings where the issue may have been.

I was just never really worried enough about the loss of engine braking at the time to chase down the root cause of that issue...

More or less were trying to say the same things TTS,just in very,very different ways...

That's part of the problem for those not familiar with the workings of these deals,the confusion factor,even the manuals dont always agree on how best to describe the same things,even terminology varies on certain things,so when the non-tranny guy hears the tranny guy(s) spouting off what's going on,he's often lost as to what those words actually mean.

[TTS]

Originally Posted by screamingchief

That's not exactly how it is according to both my trans manuals...

The only time the low-roller clutch is ineffective in manual low is when the engine is decellerating and the trans is engine braking,then I agree the low-roller de-clutches (ineffective) and over-runs,but as soon as your back on the gas it is indeed effective again,as it is in auto low as well.

If the low band is ON the low roller is INEFFECTIVE. The low band is ON anytime low has been selected, it doesn't change with acceleration or deceleration. The carrier will not turn any direction when held by the band, I don't know how else to explain it. Explain how the carrier can spin through the one way clutch with the band applied .

Originally Posted by screamingchief

Agree with this,and the comments afterwards too,I did'nt mention that difference as it was'nt really relative in the context of my comment,I was discussing the manual power flow thru the trans and gear train,not the hydraulic circuits that activate the elements needed to achieve that power flow.

Understood. I added these comments so someone didn't add to the post that I had left them out. And for general discussion- when all the facts are out there problems are easier to understand.

Originally Posted by screamingchief

That was one of the other dozen or so elements I was refering to in my comments.

So you agree that more than the manual valve is being used in manual low now?

No! Put down the oil circuit diagrams and look at the transmission. There are exactly 3 things that happen directly related to low being selected @ the manual valve.
1. Oil travels to the large spool of the 1-2 shift valve to force a downshift if possible. There is no brake control here, just signal oil to move the shift valve for a 2-1 shift. If the valve can move against modulator oil a shift will happen.

2. Oil travels to the outboard end of the accumulator control valve to position it open for servo control. This is backside oil- an accumulator if you will- to reduce apply area of the oncoming servo. If this valve was stuck in the bore anywhere else along it's travel the band is coming on, just firmer.

3. Oil travels to apply the servo. Oil is going to the servo hell or high water, if there are leaks in the circuit or not. The band may not be able to hold with the reduced
pressure, but if there's enough pressure to move the car there's enough to apply the servo. Low oil travels to the servo if it makes the downshift or not!!! Are you with me? The only thing that keeps it from applying is the presence of D oil on the front side, and the addition of 2nd accumulator oil. Both sides of the servo are equal. Does your book show 2nd gear with low selected?

Originally Posted by screamingchief

I'm not arguing that some of the true relevence of some of those could be considered mostly unimportant in the grand scheme of things here though,just stating that they are other elements involved in the deal...


Looking at the flow path,I agree with these comments completely.

That's why I said there are like a dozen other elements in the hydraulic flow path in manual low.


I could see that,with the TCI V/B kit I used on that TH-400 I mentioned earlier it indeed used a modified 1-2 shift valve,and the kit included a pressed in expansion plug for that hole,so could be that plug was'nt a good fit and it was pushed out by the hydraulic pressure or such.

But that's pretty much like I said earlier,somewhere pressure was being exhausted that should not have been,I just never figured out 100% where that was with that specific trans,I only had some inklings where the issue may have been.

I was just never really worried enough about the loss of engine braking at the time to chase down the root cause of that issue...

If that plug is blown out the forward clutch and reverse band are not going to last long as low range is selected. I don't have any diagrams, you may want to see if that causes a leak in auto as well. I don't plan on leaving the plug out so I'd prefer to not research it .

Originally Posted by screamingchief

More or less were trying to say the same things TTS,just in very,very different ways...

That's part of the problem for those not familiar with the workings of these deals,the confusion factor,even the manuals dont always agree on how best to describe the same things,even terminology varies on certain things,so when the non-tranny guy hears the tranny guy(s) spouting off what's going on,he's often lost as to what those words actually mean.

I like a good transmission discussion, they're hard to come by.

[screamingchief]

Originally Posted by TTS

If the low band is ON the low roller is INEFFECTIVE. The low band is ON anytime low has been selected, it doesn't change with acceleration or deceleration. The carrier will not turn any direction when held by the band, I don't know how else to explain it. Explain how the carrier can spin through the one way clutch with the band applied.

Ok I got that comment a bit mixed up,I'll admit that,in drive low the low-roller clutch changes on accel and decel,the low-roller clutch unlocks and overides on decel allowing the front gear set to freewheel,and yes,indeed in manual low the carrier is held static in both situations,that's where the engine braking comes from.

Apparently I'm not all that used to having to write all this out like this and explain the nitty gritty details as I may have thought I was,it's a lot clearer in my minds eye than it is to put into words for others.

But the fact remains that the low-roller clutch is also locked in manual low,and it indeed helps hold the carrier static,as much so as the band does in manual low,and that is how the trans manuals define them as being "effective",in their descriptions both elements are holding the carrier from rotating in manual low,this is almost certainly due to the increased torque load seen as a result of the added gear ratio in the gear train...

Likely the band alone would not hold the carrier static on it's own merit.

Though I imagine if for any reason the low-reverse band were to slip any under decel in manual low (engine braking),it would also stand to reason that the low-roller clutch would inturn still over-ride slightly and freewheel as it would in drive low as well to avoid any potentially serious driveline damage.

Originally Posted by TTS

Does your book show 2nd gear with low selected?

Sure it does,it shows the low oil supply to the servo apply as being shutoff completely at the valvebody in L2.

I agree it shows 1-2 accum. oil to the back side of the accumulator in L2,and int. oil on the top side of the accumulator,logically that functions to "time" the shift firmness of the 1-2 shift.

In L1 it indeed has low oil to the back side (apply) of the servo,and it also shows 1-2 accum. oil to the back side of the accumulator (which also happens to be the front side of the servo),no int. oil to the front side of the accumulator,so the 1-2 accum oil is logically there to control the band apply and hold it off till the speed is low enough so as not to cause damage from premature band apply in L1,only when the low oil pressure is higher than the 1-2 accum. oil pressure should the band apply at all.

I understand that just fine.

That does'nt much change the fact that there is oil going all over the place in L1,even if much of that flow path is wholly unimportant in how one views this deal.

L2 has what appears to be a somewhat shorter flow path,and few less elements involved in it's operation compared to L1.

Originally Posted by TTS

If that plug is blown out the forward clutch and reverse band are not going to last long as low range is selected. I don't have any diagrams, you may want to see if that causes a leak in auto as well. I don't plan on leaving the plug out so I'd prefer to not research it.

Like I said,I cant swear to that being the issue with that trans,the cause was never determined,the car was sold before that was necessary,though it easily had 5000+ miles on that rebuild with no major trans issues of note besides the lack of engine braking in L1.

The TCI saturday night special converter was easily the biggest mistake of that build,never again...

I was like 21-22 years old at that time,and that was the third auto tranny I had rebuilt,first was a C-4 in vo-tech auto class,the second was a th-350,then that th-400,since then many more A/T rebuilds,no major issues to mention.

Surely not as many rebuilds as you've done though TTS...

Maybe 2-3 dozen or so builds,not more than 50,various makes,models.

Just enough to be dangerous! LOL.

Were cool TTS.

[Half-Inch Stud]

Sounds like the 1-2 Shift valve was replaced with a modified valve(landing cut to allow flow) to "catch 1st at any speed". VB casting needs hole plugged to complete the modification. Drop-pan, inspect casting for hole, hammer a suitable metal plug into hole or get nutty & tap it and install a short screw.

I do not recommend the " catch 1st at any MPH modification" it is profoundly stupid. I tried it and do not like it. Actually NICE to drop shifter into 1st at engine brake in 2nd and THEN and auto-downshift occurs ~35MPH due to the factory shift vale design. Factory made a good design, and no practical reason comes for removing that benefit.

[toomanybirds]

TTS, I take back my statement that you are knowledgeable.

If drive is selected, and the trans is in high gear, there is engine braking. The forward, direct and intermediate clutches are applied. The input shaft is directly connected to the output shaft through the applied clutches.

Try my suggestion with the kickdown switch, you will learn something.