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As already stated, the HUGE leak that would result in D4 Fluid (applies Forward Clutch in all Forward Ranges) would prevent the Transmission from moving the Vehicle.
This is called D4 Fluid because it is the Fluid that leaves the Manual-Valve when the Gear-Selector is in the D4/ OD Position.
Line-Pressure enters the Manual-Valve, and the Fluid that leaves the Valve is D4.
D4 goes to apply the Forward Clutch and supply Fluid to the 1-2 Shift Valve-Train, so that the 2nd-Gear Shift and others can occur.
Low D4 Pressure, or NO D4 Pressure = No Transmission movement... Not selective Transmission movement.
As already stated, the HUGE leak that would result in D4 Fluid (applies Forward Clutch in all Forward Ranges) would prevent the Transmission from moving the Vehicle.
This is called D4 Fluid because it is the Fluid that leaves the Manual-Valve when the Gear-Selector is in the D4/ OD Position.
Line-Pressure enters the Manual-Valve, and the Fluid that leaves the Valve is D4.
D4 goes to apply the Forward Clutch and supply Fluid to the 1-2 Shift Valve-Train, so that the 2nd-Gear Shift and others can occur.
Low D4 Pressure, or NO D4 Pressure = No Transmission movement... Not selective Transmission movement.
You still have your overruns. That explains why i had a normal pressures and transmission acted normally in Manual 1 and 2. And while you're around 40 km/h, you shift to manual 3 and it works also(With the biggest leak possible on Fwd acum piston). You can decelerate and accelerate. It's just dies in dead stop.
Imagine a 1/4" hole in fwd acum where the pin seals. Maybe commanded pressures vs speed had overcome that leak at a certain speed.
that leak is going to be present in every forward range.
D4,D3,D2,D1
selecting a different position on the shifter would make no difference, you would just have a failure to apply the forward clutch
IF the leak was such that pump volume could overcome said leak and apply the clutch...that would be true for all ranges still. You wouldn't need to select D3
Selecting D3 doesn't change that leak
If you don't know what you are talking about...which is clear...just maybe don't talk
that leak is going to be present in every forward range.
D4,D3,D2,D1
selecting a different position on the shifter would make no difference, you would just have a failure to apply the forward clutch
IF the leak was such that pump volume could overcome said leak and apply the clutch...that would be true for all ranges still. You wouldn't need to select D3
Selecting D3 doesn't change that leak
If you don't know what you are talking about...which is clear...just maybe don't talk
Re-read my posts above. 3 becomes neutral from a dead stop, but if you shift manually from D1 to D2 and your car will drive normally, using the overrun clutches only, get around 40 km/h, you would be able to shift to D3.
Remove your fwd piston acum and it's pin, with that you create a huge 1/4" hole with a leak, you would be able to manually shift all the gears from 1st to D.
It's a fact. My only guess is PCM commands higher line pressure VS speed.Thats what overcomes the leak. Just maybe you think you know too much.
Tested on a stock P12 PCM.
This is from my T42 TCM. And yeh, i can confirm that technically.
Last edited by Edward Stark; Oct 26, 2021 at 05:47 PM.
The way that the Gear-Selector and Manual-Valve operate in regard to fluid is as such:
-OD Position = D4 Fluid Only.
-D3 Position = D4 and D3 Fluids.
-D2 Position = D4, D3, and D2 Fluids.
-D1 Position = D4, D3, D2, and D1 Fluids.
So as already stated; D4 applies the Forward Clutch Apply-Piston in all Forward moving Gear-Ranges.
Once i installed a sonnax pinless forward acum piston and forgot to bang that pin hole with my 2 ba!!s that Sonnax provides because my wife called me and confused me by yelling at me how a bad of a person i am and how i ruined her life. So i assembled the valve body pan, filled fluid and this is what happened :
1. Drives good only in D1 and D2.
2. D3 once engages and once is neutral from dead stop. Once you raise that pressure up with RPM, the vehicle moves.
3. Once you shift from D2 to D3 while driving, it shifts pefectly.
4. D Doesnt work at all from dead stop.
5. D Works when you shift manually while driving fromD2 to D3 then to D.
You've learned a lot of theory and in practic, but even an old dog can learn new things.
This is a Verified experiment by mistake i had. And you can argue with these facts however you want.
6TDD 4L60e from a 2006 Trailblazer 4.2L.
There you go Mr.
I'm a bit late here on this one.
Ordinarily, I'd do anything that's possible before dropping the entire transmission. I have also pulled a transmission out and done a full rebuild expecting to find failed hard parts, only to find massive leaks that could've been repaired without removing the transmission from the vehicle.
And, you've done a decent job of explaining how a big leak at the accumulator piston might cause the shifting issues.
That said, I also think it's a broken sprag. The one symptom your "big leak at the accumulator piston" doesn't explain is this one:
Originally Posted by apex2112
as the truck goes to do the 2-3 shift it does a very harsh bang like it was stuck in two gears.
I'm not sure how a big leak would develop in a way that causes a "very harsh bang like it was stuck in two gears."
Without that symptom, someone might get lucky and find a big leak behind the valve body and fix it with the transmission still in the vehicle. Actually, I wouldn't pull the transmission out without taking off the pan and inspecting things first, so pulling the valve body and taking a look doesn't waste much time, and sometimes you win the lottery and find an issue you can fix without pulling the transmission out.
However, given all the symptoms in the opening post, I think the "big leak" theory is very unlikely on this particular unit.
Another detail, my driveway has a very slight incline and if I put selector in (D) the truck will start to roll down the incline if I take my foot off the brake.
It will not roll with selector in (D3) with brake released.
At the time this happened I never tried driving the truck in any other selected gear except (D) and (D3) .
I doesn't appear to have any of the symptoms described by Edward.
By the weekend we will know what failed...
Another detail, my driveway has a very slight incline and if I put selector in (D) the truck will start to roll down the incline if I take my foot off the brake.
It will not roll with selector in (D3) with brake released.
At the time this happened I never tried driving the truck in any other selected gear except (D) and (D3) .
I doesn't appear to have any of the symptoms described by Edward.
By the weekend we will know what failed...
Another detail, my driveway has a very slight incline and if I put selector in (D) the truck will start to roll down the incline if I take my foot off the brake.
It will not roll with selector in (D3) with brake released.
.....this is because the 2-3 shift valve that has been installed is applying overrun clutches in D3 but not D4
Well finally got the trans out and started to disassemble, when I pulled the pump out then the drums this is what I found
here are some pictures of the sprag
All the snap rings appear to be in good condition and were fully engaged.
one picture of each side
Everything seems to be in place expect for the sprag itself.
The inner race has a pretty deep wear groove on one side as shown in the earlier picture.
That Dog-Bone/ Spag had some wear on it...
It was past do for a replacement.
Overrun Clutch use should be made to be part of ones normal driving habits.
For those of you who can not modify their own Valve-Body...
A Sonnax 2-3 Valve will get the job done, as long as the Gear Selector stays in the D3 Position (excluding expressway driving, use D4).
