Control freewheeling in GM transmissions (4L60-E) 5

[gerhardf]

Hi everybody! I'm more of an electronics guy, so please bear with me here...

I'm trying to shift the gears in a GM 4L60E transmission. I have managed to control the shift and torque converter solenoids, and everything works pretty nicely -- almost. The problem is that with the shift lever in position D, the gears 1, 2 and 3 freewheel. (By this I mean that the motor rpm can drop below the rpm that corresponds to the wheel rotation.) In 4th gear this is not the case, it is coupled tightly between the motor and the wheels.

When the shift lever is in position 3, it is the 3rd gear that's coupled tightly, and gears 1 and 2 freewheel.

What I would like to get to is that all gears are coupled tightly while the shift lever remains in position D, so that I can shift the actual gears with the solenoid valves only and have a behavior that is similar to a stick shift transmission.

Can anybody give me a hint where to start looking for answers here? It seems to me that the only way is to change the control flow in the valve body. Is this correct? It seems that there is a clutch or something in the transmission that controls the freewheeling, and that it can be activated by applying pressure to it. My idea is to find this and add a solenoid to the valve body that controls the freewheeling. Is there a place where I can get a drawing of the hydraulic control flows in the transmission? Or am I completely off base?

Thanks for any help!

 

[GregLocock]

I don't know about that trans in particular, but I'd get the proper workshop manual for the vehicle, it should have the power flows through the trans for each gear. I'd add that they are VERY complex at first sight, but you can usually figure it out in the end.

Word of warning - hard shifts are a quick way to stretch your band brakes.



Cheers

Greg Locock

 

[gerhardf]

I have the GM manuals for the vehicle, which contain a section about the various transmissions. There are all kinds of drawings in there, but not the diagrams for the hydraulic controls. Are there any specific sources where I could look for these?

 

[GregLocock]

I don't know what GM call these books, Ford call them Repair Manuals, and they include the valving, at least as a schematic, showing both the hydraulic and physical power flows.



Cheers

Greg Locock

 

[patdaly]

gerhardf, I am a bit confused here. How could the remaining gears be locked in at the same time? Look to a manual transmission, if at any time, more than one gear happens to be engaged, you have instant lockup. I am not that familiar with Automatics, but would that be what the overrunning clutch is for in the 4L60E?

 

[gerhardf]

Patdaly, I'm not sure you're talking about the same thing. I don't want to engage all gears at the same time (I think that's what you call "locked in&quot. What I want is basically engine braking in all gears, even when the gears are controlled by solenoids rather than by moving the shift lever.

Automatic transmissions seem to generally freewheel and only provide engine braking in the highest gear that is possible in any position of the shift lever. This is usually the desired behavior when shifting automatically, but it's not in my case.

I'm not sure what the overrun clutch does. In any case, it seem to be something that can be controlled through hydraulics, because the shift lever controls it through the manual valve.

 

[GregLocock]

if you want engine braking you need to look at the lockup clutch as well, otherwise your torque converter will be spinning. This (L/U) is another component that is easy to burn out through over enthusiasm.

Cheers

Greg Locock

 

[gerhardf]

Greg, I'm pretty sure I have the TCC lockup under control. I can see the drop in rpm when I engage it, I can see the increase in rpm when I disengage it.

In 4th gear (in D position) with the TCC engaged, the rpm is tied to the wheel speed like with a manual transmission. Very little slip in either direction. This is what I would like to have in the other gears, too.

I'm pretty sure it's not the torque converter that's causing the freewheeling. When the TCC is disengaged, the TC allows the engine to run at higher rpm than the wheels would require, creating additional torque. When it is engaged, this doesn't happen. That's how it's supposed to be, according to my understanding. But no matter whether the TCC is engaged or not, the transmission allows the engine to run at lower rpm than the wheel rotation would require (in gears 1, 2 and 3), and so there is no engine braking in those gears.

In the recent trucks with a "tow/haul" switch that sets the shift points higher, you can feel that freewheeling in 2nd and 3rd gears (always talking about the D position). With the normal shift points, you usually don't get to feel it, because it shifts in the higher gears before you really feel any freewheeling.

 

[patdaly]

Ok, I guess thinking back, what I thought was not possible. At least the old TH350 ( what the 4L60E is based upon ) had full engine braking, in each gear as you pulled the lever back. LOL, the reason I know is the old Pontiacs made a wonderful sound with Glasspacks as you let off in low or second.
Next, I would say you may just be on to something, as I know the old TH400's did the same thing, yet the TH400 I have in my current car will freewheel in both 1st and 2nd, with the only major change to the Transmission being a reverse pattern transbrake valve body. Just how they accomplish that however, I do not know. Perhaps you could contact one of the Major aftermarket Transmission people ( B&M, TCI, JW Performance, etc.) and find out how they accomplish it.

Greg, on my 4L80E in my Truck, I do have engine braking in 3rd, obviously without TC lockup, are you sure about having to have the TC clutch locked?

 

[GregLocock]

You don't /have/ to have the LUC engaged, but it does help. You are driving the TC backwards so it probably isn't all that good at torque multiplication (I don't understand TC performance diagrams so I can't confirm that).

Our gearbox has at least two one way clutches in. These are called OWCs. Again I don't understand their function, I've never had to mess with them. I suspect they control the free wheeling.




Cheers

Greg Locock

 

[crashbox455]

the only way to get engine braking in the lower gears is through the manual valve, which is controlled by the shift lever. the ECU controls (A and B solenoids, TCC solenoid) only control the "normal" upshifts and downshifts.

maybe you could "MacGuyver up" some stepper motor control for the manual spool valve?

 

[crashbox455]

also, the 4L60e doesn't use the TCC in any gear except 4th, and engine-brakes just fine without it in 1-3.

 

[gerhardf]

<< the 4L60e doesn't use the TCC in any gear except 4th >>

Crashbox, this is not quite correct, in two respects. It is not the transmission that controls the TCC, it is the PCM. At least in the Tahoe, the PCM engages the TCC in 3rd gear when the shift lever is in the 3 position. This basically can be summarized in saying that they engage the TCC only in the highest gear available. (And that's always only above a certain speed, around 35-40 mph.)

I think there's nothing that says that they couldn't engage in in the lower gears, too -- it's just that they didn't go that extra mile in complexity in the PCM, probably figuring that the time the car spends in the lower gears is too short to worry about. This might not be true though for towing, for example, or for extended uphills (which they don't have in Detroit .

 

[GregLocock]

As an example of that in our calibration is that in Drive, in third gear, the lockup schedule is less aggressive than it is in manual third. The idea is that when towing people will probably use 3 rather than D, and are more interested in cool running and fuel consumption rather than 0-60 times and refinement.

Cheers

Greg Locock

 

[crashbox455]

yes, i understand that the PCM controls the solenoids. that's what i'm trying to tell you. the PCM ONLY controls the solenoids, which ONLY handle part of the transmission controls.

i forgot about TCC in manual 3rd, but no, you cannot say that it &quot;will engage in highest gear possible&quot;. 3rd and 4th only. the point of my statement is that you don't need TCC lockup to have the gears &quot;coupled tightly&quot;, you need to actuate the manual 1st thru 3rd options, so as to lock up the overrunning clutch and input sprag. so the internal workings of the trans portion of the PCM don't really mean diddly.

i realize this post sounds a bit snotty, but it's because i'm in a hurry and typing fast. i hope my input was worth something.

 

[gerhardf]

Chrashbox, thanks for the input. I knew already that the solenoids only can do so much; that's the whole point of this post here. What I can do with the solenoids, I think I already have explored. Now I need to get to the rest...

I think I get to a drawing of the hydraulic control flow soon, and I'll check out the overrun clutch and input sprag controls. (I'll also try to understand what exactly they do Is this all that's related to the freewheeling?


PS What means &quot;MacGuyver up&quot;? I've been thinking about the stepper thing, but if I can add just a little solenoid that applies pressure to a clutch or sprag, that could be easier.

 

[crashbox455]

MacGuyver was a TV show back in the 1980's here in the US about a guy with a bad haircut (Richard Dean Anderson) who could make lots of useful devices out of common things. Americans make fun of it today, like most 1980's TV because it was so silly and unrealistic.

for example, when my throttle cable broke on my camaro, i pulled out one of my shoelaces, and used it to move the throttle by pulling it through my driver's side window. that was a 'MacGuyver'.

as for the input sprag and overrunning clutch, they are opposites in effect on the transmission. the input sprag allows the engine to put torque through the transmission, while letting the engine idle during deceleration- if you know what a &quot;sprag&quot; clutch does, it should be obvious. the overrunning clutch is the opposite, when engaged, it keeps the output from &quot;overrunning&quot;, which means it keeps the engien from idling under decelleration, which means ENGINE BRAKING. they are opposite in direction, and in &quot;DRIVE&quot; or &quot;D&quot;, only the input is engaged. in manual 1st thru 3rd, they are both engaged, which gives you engine braking. the overrunning clutch is only available through the &quot;manual valve&quot;, which is actuated by the shifter. this is why you can't get &quot;tightly coupled&quot; gears using only the solenoids.

i assume from the 'MacGuyver' question that you are not from the US, so hopefully i am crossing the language gap effectively. let me know if you need any clarifications.

 

[gerhardf]

Hey, thanks, that was quite instructive. This probably helps me a lot when I have the diagrams and try to figure out what the different valves control and how it all works.

You're right, I'm not originally from the US (I'm from Germany), but I've been here long enough that the language gap is not the real problem anymore -- except for such things like the MacGuyver show. I haven't been around long enough to know about it

 

[bvasa]

An aftermarket shift kit would be the easiest way to do this project, if you didn't want to just figure it out for yourself. These kits usually consist of a couple new check balls and a new metering plate that gets sandwiched between the wormtrails. This changes the shift logic and, consequently, the transmission behavior.

A large part of the reason one way clutches are used is to simplify the act of shifting gears. As patdaly noted above, you can't have two gears of different ratios engaged at the same time without risking a very quick stop. This implies that each gear ratio needs a clutch so it can be individually engaged. In a manual transmission, that would be the synchronizer or a dog clutch. In a planetary automatic (gotta be specific these days, what with all the powershift and automated manuals out there), you have friction clutches (plates or brake bands) and one-way clutches. Until recently, the OEMs have had great difficulty in making shifts from one friction clutch to another smoothly. Too much delay between release of the lower clutch and engagement of the higher ratio clutch, and you're in neutral (engine flare). Not enough delay, and you're back to the two-gears-at-once scenario (tie-up).

However, if you put an overrunning clutch on the lower ratio, then you can engage the friction clutch for the higher ratio without releasing the friction clutch for the lower ratio, because the overrunning clutch will effectively release the lower ratio gear when the higher ratio gear takes up the torque.

When you're looking through the service manual, you want to find the &quot;clutch schedule&quot;. This is a text chart that will show the gear selections on one axis, individual clutches on the other, and the field will indicate which clutch is involved in selecting which gear. There's often an adjacent chart that indicates the status of the one-way clutches under engine braking. Once you understand what's needed mechanically, then you can go hunting through the worm trails to figure out how to make it happen.
Happy trails!

 

[JayMaechtlen]

I happen to have a service manual here for a 91 Buick, which used a 4T60.
I suspect the power flow/shift scheme is similar to the 4L series...

In Drive range: first gear, the input clutch and 1-2 band are both applied, and the input sprag is holding. Release the gas, the input sprag will overrun.
In Manual Lo, the input clutch and third clutch are applied, along with the 1-2 band.

So, for engine braking in first, you need the third clutch applied.

In Drive range, second gear: input clutch, 1-2 band, and second clutch are all applied.
In manual 2nd, you have the same. (???) that means that we should have engine braking in Drive range second gear- but it would normally just upshift instead...

In Drive range, third gear: - input clutch and 1-2 band are both now released, and you have second clutch and third clutch applied.
In manual 3rd: second clutch, third clutch, and input clutch are applied. hmm- so apply input clutch for engine braking in 3rd?

In Drive range 4th gear- the fourth clutch, third clutch, and second clutches are applied.

This is from the GM Service Manual for the vehicle (which was mine until a few months ago...) Beautiful full color hydraulic circuit diagrams, full description of shift swquences, etc. Hopfully you can find one like this for your vehicle!
happy hunting.
jay

Jay Maechtlen