[Mike Davis]

Paul- nice cars on your home page!!-lol

[TTS]

Before too long there should be some better pics of those cars.

[Will]

I dunno much about transmissions, it's the one component of my cars, along with rearends, that I've always paid to have done by someone else.

That said, I had this exact problem in the past when the vac. modulator was connected to ported rather than manifold vacuum, as another poster mentioned.

Furthermore, I don't understand the comments TTS is making about no engine braking in D. I have NEVER owned a car, any car, that did not have engine braking in D. The trans is still engaged when you let off the gas and the engine will drag the speed of the car down. Most noticeable at freeway speeds and faster. This is one reason why you don't want to just go from full throttle to zero throttle at the track if you're running low pressure slicks and running fast through the lights. The sudden drag on the rear of the car can give you speed wobbles and get you out of control, so best to ease out of the throttle. Seen that happen too - nothing like watching the car in the lane next to get out of shape at 120 then hit the wall in front of you on your side of the track.

[TTS]

This conversation has been about a TH400 and it's braking, correct?

D3 gear ratio is 1:1. How can high gear provide braking? The braking is in the differential- not the transmission.

[TTS]

Originally Posted by Will

That said, I had this exact problem in the past when the vac. modulator was connected to ported rather than manifold vacuum, as another poster mentioned.

Low braking would work best with ported vacuum- the pressure only goes up. I don't know if you reread the posts but the only problem the transmission has now is low braking.

[Will]

Originally Posted by TTS

This conversation has been about a TH400 and it's braking, correct?

D3 gear ratio is 1:1. How can high gear provide braking? The braking is in the differential- not the transmission.

Perhaps we're suffering from a terminology problem. If the transmission is engaged and I'm travelling at a high rate of speed and let off the throttle then the compression of the engine will drag the car's speed down faster than if the transmission was in neutral or freewheeling, disengaging the engine from the drivetrain. This is what I'm calling engine braking, and is the same thing that happens in lower gears only it's more pronounced because of the gear reduction in the transmission that causes the engine to spin faster for any given speed.

I have never owned and automatic transmission vehicle, whether with a TH400 or whatever that disengaged the transmission whe you let off the throttle in D.

[screamingchief]

Originally Posted by TTS

The band holds reverse just fine. The S&R is there for comfort, it allows a coast in D1.

Gear ratio for reverse is only 2.08,low is 2.48,the increased gear reduction and the likelyhood that one will require a greater amount of power application in L1 than they would in reverse would explain the reasoning for more holding power for that function.

Only the front gear-set is used for reverse,while L1 requires the the input of both the front and rear gear-sets.

Reverse simply does'nt need more holding power,not unless the driver is some psycho trying out his best Jim Rockford moves...

Originally Posted by TTS

If the band slips on a decel, how will an overruning clutch change anything?

That was just an open ended "wise guy" comment,meant to imply that if the band is'nt applying properly for any odd reason,what you essentially have then is D1,and then that "goofed" comment was basically correct in that narrow context.

Sorry if that was confusing...

Originally Posted by TTS

Does it show low oil behind the servo in L2?

No it does'nt.

Originally Posted by TTS

I know it does because there is nothing between the manual valve and the band, that's my point.

I dont know what to say,I'm not even 100% sure what this comment is getting at?

Are you suggesting there should be low oil behind the servo in L2???

If so then what your basically saying is the circuit diagram in the Ron Sessions TH-400 book is incorrect.

I dunno,that reads like it was a typo or something...

Originally Posted by TTS

The manual valve is the only low brake control- period.

Then please do explain to us laymen how the trans prevents a high speed band apply when the trans is shifted into L1,and further where does the line pressure boost for the low ranges comes from?

Do you not agree the line pressure is usually higher in the low ranges???

And higher line pressures surely affect the operation of most of the other hydraulic circuits do they not?

How do those things happen by way of only the manual valve???

Originally Posted by TTS

Low oil isn't going all over the place. Drive oil is going all over the place- low only goes 3 places.

I agree with that for the most part,but those other circuits often affect how those 3 circuits operate as well,the drive oil among them,the drive oil runs to the govenor and after that it tells the 1-2 shift valve if the vehicle speed is slow enough to allow the band to apply safely.

So sorry if I dont consider those others hydraulic circuits as completely unimportant or seperate for the sake of this discussion like you choose to do.

Yes the low oil specifically goes to only three places,the detent regulator valve,the 1-2 accumulator valve,and the 1-2 shift valve.

But there are still a hell of a lot of other active hydraulic circuits in L1,and this may just be a long shot,but I'm guessing they all need to be that way for things to work properly.

Originally Posted by TTS

Low oil can't have higher pressure than acc.- it's the same supply. Low is applying a larger surface area, that's all it needs.

Yeah that's true,but the end result is basically the same,that's sorta arguing over semantics,it's still happening by design,the two forces fight for control of that band regardless,if either of those pressures drop enough,the other side takes over and controls the band operation.

The way my mind thinks of this is it's a battle for control of the transmission,the various hydraulic circuits fight each other to tell the trans what to do,the manual control valve is pretty much the mastermind of the battle for all those opposing forces in my view.

Originally Posted by TTS

I mean we can go on for days about a leaking circuit. Hell, I could drill a random hole in the VB could you find it on your circuit diagrams? Of coarse not, so I tend to problem solve from a reasonable foundation. It's impossible to think of everything someone could do wrong.

I agree completely,and that was pretty much what I said earlier,that this deal can end up being a PITA to try and track down the root cause when things get hairy.

It's cool TTS,I get what your saying,but we just look at the same basic information in different ways,and this forum likely just complicates that exchange of views even further yet.

But it's been fun giving the old grey matter a workout!

[TTS]

Originally Posted by Will

Perhaps we're suffering from a terminology problem. If the transmission is engaged and I'm travelling at a high rate of speed and let off the throttle then the compression of the engine will drag the car's speed down faster than if the transmission was in neutral or freewheeling, disengaging the engine from the drivetrain. This is what I'm calling engine braking, and is the same thing that happens in lower gears only it's more pronounced because of the gear reduction in the transmission that causes the engine to spin faster for any given speed.

I have never owned and automatic transmission vehicle, whether with a TH400 or whatever that disengaged the transmission whe you let off the throttle in D.

Yes, you are suffering from a terminology problem . The transmission, in 3rd gear is 1:1. That's like and extension of the crankshaft all the way back to the R&P. To have engine braking, there must be some gear reduction. If you neutral the transmission from 3rd you lose braking because of disconnecting the differential.

[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?

[Schurkey]

Originally Posted by TTS

Yes, you are suffering from a terminology problem . The transmission, in 3rd gear is 1:1. That's like and extension of the crankshaft all the way back to the R&P. To have engine braking, there must be some gear reduction. If you neutral the transmission from 3rd you lose braking because of disconnecting the differential.

When the modulator was connected to ported vacuum, releasing the gas pedal at 40 miles an hour still allowed the engine to instantly return to curb idle. Applying more pressure to the throttle would increase the RPM, but the vehicle still lost speed. A bit more power--and the RPM came up enough to engage the one-way clutch so the vehicle was, again, under power. Except for the smoothness of the re-engagement, it really did act like you threw the trans into neutral every time you backed off the gas pedal.

SO--while adding gear reduction will be more effective at engine braking, releasing the gas pedal should NOT allow the engine to drop completely and immediately to curb idle--the drive wheels will still turn the engine faster than that. The engine then slows the car because it's fighting it's own manifold vacuum--and that, to me, is engine braking.

[TTS]

Originally Posted by screamingchief

Are you suggesting there should be low oil behind the servo in L2???

No I'm not suggesting it, it's a fact!!!

Originally Posted by screamingchief

If so then what your basically saying is the circuit diagram in the Ron Sessions TH-400 book is incorrect.

Yes, it's incorrect if it doesn't show it.

Originally Posted by screamingchief

I dunno,that reads like it was a typo or something...
Then please do explain to us laymen how the trans prevents a high speed band apply when the trans is shifted into L1

Happily ! Starting with the shift lever in S2, with a stock 1-2 shift valve; In front of the servo there is 2nd accumulator oil and there is second oil between the accumulator and the servo. The entire front side of the servo is pressurized. When the shift lever is moved to L1, regardless of the 1-2 shift valve position, the apply side of the servo will be pressurized via the manual valve. If the shift valve has not moved for a 2-1 downshift nothing will happen because 2nd oil is still present and the servo cannot move forward. As soon as the shift valve moves, second oil is vented, and the servo moves forward applying the band.

So, if the RS 400 book does not show oil at the low servo apply side in L2, he failed to color that passage in.

Your turn. Using the RS book, you tell me how it says low braking is achieved. And more importantly in L2 where is this magical apply oil? If it works better for you, just come up with your own tale of how it happens. Maybe I'm off my rocker, show me some facts.

Originally Posted by screamingchief

,and further where does the line pressure boost for the low ranges comes from?
Do you not agree the line pressure is usually higher in the low ranges???

And higher line pressures surely affect the operation of most of the other hydraulic circuits do they not?

I need a circuit diagram to tell you how pressure is increased in low range, would assume through the detent regulator valve. Instead of speculation maybe you could research that since you have a diagram. It's getting old being the only one to bring facts to the discussion. It's not a concern of mine, so I haven't studied it. No, the higher pressures do not effect anything other than apply.

Originally Posted by screamingchief

How do those things happen by way of only the manual valve???

What else have you changed when shifting to low? Did you push some other switches, rotate something, pull another lever?

Originally Posted by screamingchief

I agree with that for the most part,but those other circuits often affect how those 3 circuits operate as well,the drive oil among them,the drive oil runs to the govenor and after that it tells the 1-2 shift valve if the vehicle speed is slow enough to allow the band to apply safely.

So sorry if I dont consider those others hydraulic circuits as completely unimportant or seperate for the sake of this discussion like you choose to do.

Yes the low oil specifically goes to only three places,the detent regulator valve,the 1-2 accumulator valve,and the 1-2 shift valve.

But there are still a hell of a lot of other active hydraulic circuits in L1,and this may just be a long shot,but I'm guessing they all need to be that way for things to work properly.

The discussion was about no low braking. Not when the downshift takes place, not about unable to achieve manual 1, not about the price of gas. It was about no low braking in L1, correct? In that scenario there are only 3 things to look at in the oil circuit.

If you disagree, fine. If you want to argue, get some facts together that are different than what I'm telling you, and prove them.

[TTS]

Originally Posted by Schurkey

When the modulator was connected to ported vacuum, releasing the gas pedal at 40 miles an hour still allowed the engine to instantly return to curb idle. Applying more pressure to the throttle would increase the RPM, but the vehicle still lost speed. A bit more power--and the RPM came up enough to engage the one-way clutch so the vehicle was, again, under power. Except for the smoothness of the re-engagement, it really did act like you threw the trans into neutral every time you backed off the gas pedal.

Correct. When the transmission is hooked to a ported vacuum source the line pressure is maximum at closed throttle then again at WOT! When you released the pedal at 40 mph the pressure increased and downshifted the transmission instantly. That's why you lost speed and gained power, you where in second gear. It felt like neutral when you where off the gas because second was overruning, that's basically neutral until you regain rpm to out accelerate the driveline.

Had the 2nd brake been on, the transmission wouldn't have been overruning and the engine would've increased rpm (braking).

[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.

[Half-Inch Stud]

As I said in Page 2, yea the 1-2 Shift valve VB hole needs plugged if the shift valve landing is modified with a flat.
As others said.
Elec Detent vale can be clogged open with metal debris (from Center-Support screw) but that would lose 3rd, and sometimes 2nd engine braking.

So, a reasonable person would drop the pan & take a look at the BB pluggin of the 1-2 Shift Valve EXhaust. Take a look at the VB pluggins & report back.

FUNCTIONALLY: FWD & DIR packs both have to "let go" to "Catch neutral".
So "loss of Line pressure" while in Manual 1st MIGHT is the only functional way to make this happen,while the other ranges work fine.

Anybody besides me suspect the "got no engine braking is 1st" is a bogus problem?...my BS meter a bit jumpy.

[TTS]

H.I., yes, the description is bogus. But the description of the problem says low line in L. Read my very first post, suspected low line then and I still do (missing plug).

[71Formula]

What do you mean "bogus problem"??? Are you accusing me of making up this problem? Why would I make something like this up? That chaps me! Well as much as a half-inch could anyhow!

Believe me it's real. TTS and ScreamingChief have some good explanations of possible causes. Thanks!

I'll have the V.B. pulled and checked for the modified valve and plug.

[TTS]

For those who may be interested, the boost valve signal is from the manual valve in L or 2. It's routed from the 2 position that's active in both ranges.

I had to get some tools from the shop, thought I'd look and share .

[screamingchief]

Originally Posted by TTS

It's getting old being the only one to bring facts to the discussion.

No offense intended,but that comes off sounding kinda arrogant on your part,,,what brought that out???

This has been friendly enough,I'm not looking for a dust up,and if that's where this is going I'll be outta here as I really dont need the grief either.

Originally Posted by TTS

No, the higher pressures do not effect anything other than apply.

I sorta disagree with this,pressure affects how many things operate.

If not then why change it?
Why not just keep the pressure the same everywhere all the time???
Does not shift timing change with more pressure?
Does not shift firmness change?

Or do you include those in that "apply" comment??

And then why do we also often change certain springs when we bump up the line pressures?

Originally Posted by TTS

I need a circuit diagram to tell you how pressure is increased in low range, would assume through the detent regulator valve. Instead of speculation maybe you could research that since you have a diagram.

Originally Posted by TTS

For those who may be interested, the boost valve signal is from the manual valve in L or 2. It's routed from the 2 position that's active in both ranges.

Yep,intermediate oil is indeed responsible for the line pressure boost in low ranges,the actual boost happens at the pressure regulator valve,that intermediate oil applys line pressure to the boost valve inside the pressure regulator assembly and that increases the boost springs tension and that inturn bumps up the line pressure.

The boost pistons diameter is pretty small so the effect is controlled somewhat.

Pretty much the same as shimming the spring or a stiffer spring raises the line pressure.

Sure the intermediate oil starts at the manual valve,I'll readily admit that,but that's not the only link in that chain,the fluid path for that intermedaite oil circuit also goes to the 2-3 mod valve,where it has other circuits fighting to tell that valve what to do.

If there were a leak in the intermediate oil circuit at any point after the manual valve or the pressure regulator/boost valve,or some other ailment has occured like a stuck valve or such,it's entirely possible that no line pressure boost will occur,thus the manual valve alone is not the sole controling feature of that flow path.

In hydraulic systems pressure is exerted equally thru-out the entire circuit,that is Pascal's law,look it up,if pressure is lost anywhere in a hydraulic circuit,it is lost equally thru-out that entire circuit,so if a given hydraulic circuit extends past a given valve or such,there lies the possibillity for a leak in another area that may not be directly related to that given point one is looking at.

That's been my point from the start here,TTS you seem to suggest these circuits begin and end with the manual valve,I just disagree with that narrow view,I feel the manual valve just tries to tell the rest of the valves what to do,and then they get to work fighting with all these other forces trying to be sure they are doing the right thing at the right time.

A circuit leak can happen most anywhere,but we cleared that point up already I believe.

I'd post the hydraulic circuit diagrams here for all to see if I could find a way,but they just are'nt readable when I shrink them down enough to fit the forum size posting limits.

I'll keep trying though.

BTW,,,those hydraulic circuit charts in the Ron Sessions book,,,they're from GM,,,I trust their accuracy...

Originally Posted by TTS

So, if the RS 400 book does not show oil at the low servo apply side in L2, he failed to color that passage in.

See above,,,your saying GM failed to do that???

It does'nt show low apply oil at the servo because there is none L2 (or S2 as you seem to prefer that terminology) because there is no need anymore,the trans does'nt want the low-reverse band to apply in manual second,that low oil path has been lost at the valve body,by that all powerful manual valve of yours.

Originally Posted by screamingchief

Then please do explain to us laymen how the trans prevents a high speed band apply when the trans is shifted into L1

That was meant as a rhetorical question,but seeing as it was taken outta context and used here we'll run with it.

Originally Posted by TTS

Happily! Starting with the shift lever in S2, with a stock 1-2 shift valve; In front of the servo there is 2nd accumulator oil and there is second oil between the accumulator and the servo. The entire front side of the servo is pressurized. When the shift lever is moved to L1, regardless of the 1-2 shift valve position, the apply side of the servo will be pressurized via the manual valve. If the shift valve has not moved for a 2-1 downshift nothing will happen because 2nd oil is still present and the servo cannot move forward. As soon as the shift valve moves, second oil is vented, and the servo moves forward applying the band.

This I agree with,never tried to say otherwise,my problem was with the statement that there was low apply oil behind the low-reverse band apply servo in S2 (L2) when the diagram clearly shows there is none.

I agree there may be some low oil trying to apply the servo momentarily while the car is still in S2 (L2) after the initial downshift to L1 at the manual valve,this charges the servo to prepare for the band apply,meanwhile the valvebody is sorting out what exactly to do based on the govenor input to the 1-2 shift valve and such,but that low oil does'nt stay there long if the manual valve is moved back to the S2 (L2) position as it will vent the circuit to the pan via the manual valves relative position.

What would be the purpose of having low oil trying to apply low-reverse band servo in S2 (L2)?

It's not gonna affect the operation of the accumulator much at all.

And where is this low apply oil coming from if the manual valve has "dropped" that circuit completely when the shifter was moved to S2 (L2).

At times I feel were saying the same things,just in very different ways.

Ask me this is getting to be like a "who's on first" routine...

Here's how I see S2 (L2) as it concerns the servo/accumulator,there is 1-2 oil on the one side of the accumulator piston trying to push the accumulator piston to the bottom of it's bore's (one bore also happens to be the inner bore of the servo apply piston),when it gets there it also will try to push the servo to full release as well (as long as there is no low apply oil fighting it),when the range selector was moved to S2 (L2) that opened up an exhaust path for the low oil circuit,the action of the the 1-2 apply oil on the accumulator piston bottomed out in it's bore just pushes the low-reverse servo to full release and exhausts that low oil circuit.

The delay caused by that process (ie. exhausting the low oil) is how the accumulator has slowed the apply of that circuit (1-2) and cushions the apply of that circuit,during that process it is also "charging" the 1-2 accumulator in preperation for it's continued shift timing duties.

Meanwhile the intermediate oil has begun pushing on the other side of the servo trying to push that accumulator piston back up the bores (this also happens to be filling the inside chamber of the servo piston),but it also has to be pushing against a heavy spring as well,and seeing as it can necessarily "beat" the 1-2 apply oil and that spring it will in essence just "cushion" the apply of that circuit as that action just delays the apply of full pressure momentarily.

Those two circuits time the apply of the intermediate clutch and band dependant on input from the remainder of the valves in those circuits.

I can see zero reason for any low-reverse servo "apply" action in S2 (L2).

Likely that's why there is none show in the diagrams as well.

Originally Posted by TTS

Your turn. Using the RS book, you tell me how it says low braking is achieved. And more importantly in L2 where is this magical apply oil? If it works better for you, just come up with your own tale of how it happens. Maybe I'm off my rocker, show me some facts.

Engine braking in L2 (S2) is from the intermediate band,not the low-reverse band,entirely different servo from the low-reverse servo...

The low-reverse band is not applied in L2 (S2).

Low-reverse oil controls the low-reverse (rear) servo.

Intermediate (cl) oil controls the intermediate band (front) servo.

We must not be on the same page with this whole L1-L2-S2 deal or something???

Again I expect a lot of the back and forth is more about terminology and reading comprehension and writing composition on both our parts,and this forum is not the ideal place for straightening out the differing issues of such at times.

But I dont expect that will end anything here...

I'm not sure how much more I feel like going on on about this topic here.

It seems like the language/terminology differences alone could take a lifetime to work out,some times two different fellas just cant get on the same page even when they both know what's going on,that barrier is often just too high to climb,and neither side seems to want to bother to work that deal out.

I'm pretty much done with this...

I'm gonna go have a beer or two!

[71Formula]

Holly$h!t I think I'm going to go put some eye drops in after ready all that... and have another couple beers.

[TTS]

Look, I stopped reading about half way through. I don't think you read most of my statements right. I'll reread again fully in a minute, but we're redoing the same thing over and over. About the facts comment, It's not meant to be anything other than what it is. I'm telling you the facts the way they are, I've done nothing but transmissions for over 20 years. I didn't make this stuff up, and I'm not sure it's worth convincing anyone anymore. I'm asking you to tell me how the low circuit works since I have it wrong.

I said oil is behind the servo anytime the lever is in L. No matter the gear 1st or 2nd. My example started with Second position, 2nd gear into a manual downshift to low position 2nd gear at high speed, then to low position low gear. And it's exactly how the circuit works.

Bottom line, if the diagrams don't show pressure on the apply side of the servo they are wrong. Arrogant or not, It's a fact.

Can you scan a photo of the low position second gear, low position low gear diagrams?

BTW, D3, D2, D1 shifter in Drive position in 3 gears- agreed?
S1, S2, S3 in Second position in 3 gears- agreed?
L1, L2 in Low position in 2 gears- agreed?