As the title says. Had a call from a guy that says his truck is displaying this symptom. Says reverse is strong and his son lost all forward gears sitting at a redlight. Like randomly. So I told him to check the fluid level in park with the engine both running and not. This is to varify proper fill and observe the drop in fluid level to confirm that the pump is at least trying to work. That checks out so I tell him to unplug the harness to dummy down to 3rd gear only and max line pressure. Said he tried D4,D1, then D2 and got nothing. Then he felt a little bump on the way to park so he tried reverse again and it still worked, D4 still didn't work but lo........D3 bumped into gear! 3rd gear but a forward gear. Says it felt ok but didnt want to do much other than go a little forward since he doesnt know what it could be. I was a little stumped that only D3 made the difference. D1-3 should have the overrun clutches running if it has the HD valve installed so it couldnt be a forward sprag issue. The forward clutch is alive in any forward gear so that shouldnt be it right? Maybe the lower roller clutch or a stuck checkball. I don't know so I figured I would present this specific issue here for the master minds. Enjoy!

 

Could be sprag or fwd clucth or fwd clutch acum broke as the overuns would take up for fwd and sprag in default 3rd gear.in manual 3 position.
With the HD vavle the overuns would replace forward and sprag in d1 2 3 or with my overun mod even D4.
So could be as simple and busted fwd accum to sprag or fwd clutch.

 

ok so lets assume there is a sonnax hd valve in place that activates the over runs in D3 and below. I would think that the there would be forward motion in those selections whether or not the harness was pulled. So I am assuming that the forward sprag is not it. No the forward clutches, wouldn't they work in 1, 2 and 3 if pulling the harness gave enough of a pressure spike to overcome a leak? What is the difference hydraulically that allows forward motion only in D3?

 

Line goes to max, could apply even fried frictions if enough pressure but if that was it would move in d2 also . Low rollers is also out of the loop but should also move in if that was it,
In D3 UNPLUGGED the 3-4 clutch, then forward clutch and the overun clutch is applied
In manual 2 the overun clutch on if it has the sonnax valve and the 2nd band .
But everything points to not having the sonnax valve and or has it and the gasket was not cut or optionally VB drilled .

If you have vehicle i would drop pan and look at forward accumulator first.

 

Update!!

ok so I got this truck in and checked for the forward scum failure and the piston seems intact with a good print and pin fitment. The fluid is very dark with some clutch material but no burnt smell. Magnet has slight filmy layer. Pressure tested with the harness unplugged confirmed over 200psi of line pressure. Plugged in saw good line rise in drive and reverse. Up to 300 and over 200 respectively. Idle pressure at 60-70psi. Pulled the transmission and the pump was really hard to remove. No obvious reason so far. Band and reverse drum appear brand new. As in off the shelf condition. Reverse clutches, perfect. Input shaft air checks fine I think. I’ll have to review proper air checking to confirm but the top two holes will do this. If I blow in the middle hole, no leak and puff when released. Blow in the top, I get air out of the middle. Hold middle and blow in the top, no audible leak. Release the top and the middle will puff out when released. If I air it back up and release the middle, both circuits will vent. Is this correct? Also the sprag was free wheeling. It wasn’t rolled…. It was stuck in the release/free spin position. The cage had bound up on the one way clutches and had the frozen in place. All clutches in the input drum showed signed of use but not abuse if that makes sense. Steels had rotation lines but no heat or gouges or marks that catch a nail. Clutches showed no signs of heat at all. All had plenty of material on them. I was surprised that I could not make out any writing on them. That is as far as I got yesterday. Video of sprag to follow.

 

 

Yup sprag is bad and would explain symptoms you mentioned so seems you have your issue.
Odd for a sprag to do that with no visible damage . I would replace the whole assembly , In my case I would also do my overun mod but then I do that to all units , Stock or not, I like the extra insurance.

 

Interesting.....

 

Back together with different sprag assembly with new bw 29 element deal. First test drive went ok until I tried to downshift from 4th to 3rd. It just revved like d3 was neutral. Back to d4 and overdrive engaged and tcc locked up fine. No temperature increase. So I tried with just pressing the throttle and same thing at first then it engaged with a bang. I immediately let off the throttle and started coasting. Looked at the data and overdrive was back on so i came to a stop. Moving the shifter engaged forward and reverse like normal. Normal temp and no codes so I started back driving. Now it downshift from 4-3-2-1 and back up again fine. Slight bump 3-2 coming to a stop with the overrun mid but that’s it. Perfect lockup. Temps around 175-180. No shift flare or evident slip. Going to drive it again this morning when I get back from the chiropractor. I hoping it was a stuck valve that is now moving ok.

 

Sounds like the 3-4 signal valve was sticking which they are known to do. Also the end cap leaks badly . If the issue repeats cold I would check that valve. I even stretch the spring a little and replace end cap with new or better yet oringed cap It would seem the leak at cap would get worse hot . But it is opposite even though the valve body and end cap are both aluminum. they are different grades and heated the cap expands and seals tighter, I noted this when disassembling a hot VB after cleaning. I could not get that end cap out. Got frustrated and walked away came back it was cool and the cap all but fell out . the 3-2 bump is normal with the overun mod.

 

Thank you, Mr. Frank!

 

Well after two successful drives, the issue has come back and is here to stay looks like. Now when you go from overdrive to 3rd in any capacity it will free rev until you let off the throttle. Then and only then will 3rd engage. Passing down shift, manual downshift, coasting downshift are all the same. Taking the valve body out tomorrow

 

Look at that valve, also the spring behind it is also known to break.

 

Was the Sprag Element-Assembly Only replaced?
The Sprag Races do experience wear as well, and do at times also need to be replaced.
Especially after an exceedingly worn-out Sprag Element-Assembly has failed.

It is no where near as common to have to replace the Races, but it does indeed need to happen.

On a separate note, the Sprag Element-Assemblies should not really be re-used on an Overhaul.

There has been Note of the (Complete) Sprag Assembly appearing to Operate Properly "in hand", but Actually NOT Operate Properly installed in the Transmission.
ATSG and ATRA have both Published this Warning.

Extreme attention to Detail should be taken when examining Sprag (Dog-Bone Type) Assemblies and more often than not, should be replaced on all Transmission Overhauls.

 

I had this happen to me with a 440-T4, (very similar to a 4L60 sprag). The sprag worked great in my hand but once install would not hold. I replace all sprags now when rebuilding. On a 4L60 if the races are worn the sprag will slip or not hold at all.

 

He actually I think replaced both the sprag and the races I am pretty sure .

 

Yes that is correct I replaced the races and the element itself as there was no apparent fault with the races or the element other than sticking in the released position. Literally flicked back to operational in my hand. Could not get the issue to repeat itself in my hand so I just sat the whole assembly on the shelf for later tinkering.


On another note, I pulled this truck in the shop today and noticed a 2-3 second delay going into forward gear. No delay in reverse. This was not present yesterday. Tried blipping the throttle and this made no difference. So I took the vb out and checked the 4-3 sequence and 34 relay valve bore. Vacuum checked over 18”. So I took another valve body and it checked the same way. I decided to put this vb in and try it. Same delay going into forward gear. Got on the highway and the shifts were all good and so is lockup. Apply throttle for passing gear and the flare going back to third is the same or worse. Just revs with no forward pull until the revs drop all the way to idle. Which at that time 3rd engages as noted by engine breaking apply throttle and tcc locks up and away we go. No further slip or revs just accelerates. Even and 3/4 it just pulls away no problem. Upshifts to 4th no problem. I’m assuming I have to have a forward clutch issue or something to be causing this as the second vb changed nothing.

 

Also both vb had more than 16” of vacuum on the forward abuse bore. Forward accumulator, although plastic, was in good shape with new orings.

 

It is time to obtain a 300Psi Line-Pressure Gauge and a rather long Hose, so that the Gauge can be taped-on to the Windshield.
This way you can Drive the Vehicle and see the behavior of the Line-Pressure.

You need to be able to observe the Line-Pressure while the Transmission is not Operating Properly.
Then Post your findings.

 

I have spent literally hours thinking on this and no good answers, The sprag seemed like the logical thing and obviously was bad. When coming down from 4th the 3-4 clutch is already on, The forward clutch holding. The overun mod you added does come back on but relatively slow depending on how you made connection to the FWD circuit but the sprag should be holding anyway . The only thing i can think is you have a large leak in the fwd circuit and it would need to be pretty big considering for instance with my band cooling mod I actually introduce a .037 leak with no issues . Of course I do enlarge the FWD feed to compensate to .093 .
I think you have a leak in the FWD circuit internally, look at inside of stator and check for leaks between stator and pump and even pump halves. I have literally done air checks through pump body only to find massive amounts of air coming out the where they join a few times. A leak in the fwd clutch circuit would even prevent the overun mod from working properly. A pressure gage should show you a leak as mentioned though not where.
Check your drum to hot , but use an oven to heat it so will be even to about 200f then air check and observe shaft to drum seal. The overun mod would actually improve things if it was that as no dump would exist of the overun clutch in 3rd gear D4 position like would happen stock through the D3 feed.

Reading you taken the VB off the table and the sprag which leaves us with pump, pump stator or input drum issues or unikley case.