727 sometimes not going in reverse

[Knapp870]

I bought this vehicle and found that sometimes it will not go into reverse. It has a cable shifter on it and often, especially if at an incline, you cant get reverse to grab. I rebuilt the transmission and installed and it went into reverse more than it used to but still sometimes wouldn't! I then thought it had to be the valve body so I swapped the valve body from one I knew worked previously and it still acts funny sometimes. It will lock up like it is going in reverse but pushing the accelerator does nothing. However it doesn't roll forward like it's in neutral. Also it seems to mainly happen when it's at an incline. Ant suggestions?

 

[AJ/FormS]

HeeHee,Here we go again..... What are we working on? Oh wait, 727; I see it now.
Well then the first thing to do is prove the tranny is in reverse when the cable says it is. This is called synchronizing the shifter.
Then you need to understand how the tranny works.
To make Reverse; the L/R band is applied plus the Hi-drum.
To make any forward gear only, the Forward clutch is engaged.
To make Manual Low; the L/R band is again applied, and with the F-clutch.
To make Drive-Low; the L/Rband is not used. Instead the roller clutch is used.And again the F-clutch.
To make Drive-High, both clutches are applied.
What does all this mean? It means we have a built-in diagnostic system.
If you study this. you will see that Since you do NOT complain about any other gears, that tends to mean that the both the clutches and all the bands are working. But hold on there. In Manual Low, L/R band could be Not working, and the low could still be working cuz the roller clutch would be doing the work instead. The result is that you cannot know the band is applying; until you find no reverse. Then you know.
All this leads up to me saying;something about the L/R apply circuit is not working.
It could be the Band adjustment. It could be the servo. Could be a broken band.Or the case, or the VB, or a gasket.Since the rear servo has an external port,the easiest thing to do is is an oil-pressure test.
-If the servo pressure is good, next would be the band adjust.If the band is adjusted right, next would be to drop the VB and do an air test.While the VB is off you can check the gasket.If the air test is ok, and the oil-press is good, then it's time to inspect the band.
-But if the servo pressure is bad, then you will need to check the line pressure as well. If that guy is good, while the servo pressure was bad, then the VB would be suspect. Since you have already swapped VBs, I would then suspect the case, or the servo piston, or again, the gasket.
So don't drop the pan until the oil-pressure results are in.
You will need a service manual to check the pressures
.

 

[Knapp870]

HeeHee,Here we go again..... What are we working on? Oh wait, 727; I see it now.
Well then the first thing to do is prove the tranny is in reverse when the cable says it is. This is called synchronizing the shifter.
Then you need to understand how the tranny works.
To make Reverse; the L/R band is applied plus the Hi-drum.
To make any forward gear only, the Forward clutch is engaged.
To make Manual Low; the L/R band is again applied, and with the F-clutch.
To make Drive-Low; the L/Rband is not used. Instead the roller clutch is used.And again the F-clutch.
To make Drive-High, both clutches are applied.
What does all this mean? It means we have a built-in diagnostic system.
If you study this. you will see that Since you do NOT complain about any other gears, that tends to mean that the both the clutches and all the bands are working. But hold on there. The Manual Low L/R band could be Not working, and the low could still be working cuz the roller clutch would still be working. The result is you cannot know the band is applying.; until you find no reverse. Then you know.
All this leads up to me saying;something about the L/R apply circuit is not working.
It could be the Band adjustment. It could be the servo. Could be a broken band.Or the case, or the VB, or a gasket.Since the rear servo has an external port,the easiest thing to do is is an oil-pressure test.
-If the servo pressure is good, next would be the band adjust.If the band is adjusted right, next would be to drop the VB and do an air test.While the VB is off you can check the gasket.If the air test is ok, and the oil-press is good, then it's time to inspect the band.
-But if the servo pressure is bad, then you will need to check the line pressure as well. If that guy is good, while the servo pressure was bad, then the VB would be suspect. Since you have already swapped VBs, I would then suspect the case, or the servo piston, or again, the gasket.
So don't drop the pan until the oil-pressure results are in.
You will need a service manual to check the pressures
.

Thank you. However I did put a brand new band in and billet servo. I also air checked upon reassembly after rebuild. What in the case causes issues? What gasket are you referring to? The metal between the two valve body halves? Thanks!

 

[AJ/FormS]

The brand new band, doesn't automatically guarantee that it's any good.
No; I'm referring to the VB gasket that covers the ports that you air-tested in the accumulator piston area.
As to case issues, that is highly unlikely, but Ima thinking porosity or a crack, or a bad/cracked servo.If you are confident about the air-pressure test results, it would automatically rule all that out.All being; the gasket, the passage,the servo, and the case.
I should also mention that in reverse, the maximum pump pressure is directly routed to the servo and the Hi-drum, via the manual valve. The pressure regulator is out of the circuit.There are no other circuit branches involved.With maximum pressure,the only thing preventing reverse from slamming in, is the cushioning built into the Hi-drum.Most guys take that cushioning out for a slam-bang 2-3 shift.What I'm suggesting is, there should never be a hydraulic issue engaging reverse.If the oil-pressure test shows that, then,with all the info you have supplied,I'm already suspecting the band or the band-adjustment.
So oil-pressure test would be next.

 

[winstoninwisc]

pull the pin out make sure it shifts all the way to reverse and then to park that's when adjustment is right on for adjustment . any thing else could be in trans

 

[Knapp870]

Sounds like I need to dig out my pressure Guage. There is no gasket between the valve body and transmission on a 727 though. I think you may have your order of operations mixed up with something else as well or we are speaking different terminology. Reverse is the same as low except reverse uses the front clutch.

My understanding:
Low-rear band and rear clutch
Reverse-front clutch and rear band
Second - front band and rear clutch
drive-front and rear clutches

I will say that after I rebuilt it I tested and found this issue, dropped pan and checked the rear band, servo and readjusted the band (2 turns off of 72 inch pounds).

Also I have a pretty sketchy shifter in right now that does not have any notches for each gear, it is essentially a lever on a cable, I typically count the notches I feel to get in/out of each gear.

 

[AJ/FormS]

That is correct, Ma calls the Hi-drum "front clutch", and she calls the Forward clutch "rear clutch". I may have forgotten to mention that.

And OOPS you are also correct that the TFs do NOT have a VB to case gasket.Sorry to be so confused, ignore all references to the gasket, and instead, substitute, machined surfaces or something to that effect.

As to the 2Turns spec, I have never really,( after the first couple of trannys) paid much attention to it. I set all bands by a 1/4 inch rule, That is to say, there should be about,(oh boy), 1/4 inch clearance under the apply arm to the pin.I've never had a complaint as to band action.When I was rebuilding these back in the day, there was no way of knowing what these finished units were ever going to end up in.Since there were several different specs for various car lines and engines and years and even band types; the 1/4 inch rule was established, for every one.Just saying.The big thing is to ensure that the band retracts off the drum while still staying in the working range of the servo when fully applied, and still have some room left to ensure you won't have to drop the pan every couple of months to reset the band due to wear..Sheesh; that right there, is a pretty long run-on sentence.
Love your shifter!

 

[Knapp870]

I just did a pressure test on the rear servo and it looks to be in spec. The Guage was 250-275 psi but the needle was jumping back and forth a decent amount? Based on what you suggested above I guess the next step is to drop the pan? I will. Mention again that reverse is not always not working, only about 7-8 times out of 10 does it not work and usually on an incline with nose pointed down. Thoughts?

 

[AJ/FormS]

IIRC you said the oil level was correct. Now I'm wondering if the dipstick is the correct one. Before I would take off the pan, I would "overfill" the pan. a half a qt atta time to maybe 1.5qts total, and see what happens.
Just maybe the pump is sucking air, as the jumpy needle seems to indicate.
However, with full hydraulic pressure in the rear servo, and with third gear(Drive) working,The only other conclusions I can come to are 1) There is something wrong with the band or its struts, or2) the planetary splines are stripped out. But since low works,and reverse sometimes works, I'd have to think the splines are ok.

Just hang on for a bit, maybe someone else will chime in.

 

[Knapp870]

Wouldn't I see a drop in pressure if not at the right level? I compared the dipstick to the other I have and lengths appear to be the same. I'll have to get even more fluid to try what you suggest though. This is quite the gremlin

 

[AJ/FormS]

Well you say the needle is jumpy. This usually indicates air. Pinch the line a little at a time to reduce the jumpiness. The needle will stabilize at a more accurate pressure reading.The pump is not a piston pump. The pressure should be steady at a steady input rpm. IIRC the test is to be done at 1600 engine rpm.
I suppose your high-pressure relief valve, or spring, could be bad.BTW, this valve is a ball, but it is a 3/8 ball, while all the others are 5/16.
Buy a qt of ATF then, and add a pint right off, and see what happens. If some improvement add the other half. If problem disappears recalibrate your stick.

 

[Knapp870]

Still seeing tons of needle jump. It seemed like the more I added and Warner it got the pressure dropped but still in spec though it was still jumping like crazy. When the reverse seemed like it wasn't activating I could give it alot of throttle and then it would grab....

 

[AJ/FormS]

Still seeing tons of needle jump. It seemed like the more I added and Warner it got the pressure dropped but still in spec though it was still jumping like crazy. When the reverse seemed like it wasn't activating I could give it alot of throttle and then it would grab....

That sure sounds like the pump is sucking air.I don't know what to tell you, except If the pump is sucking air, the fluid will be aerated. You will have to check it immediately after trying reverse with warm fluid. Just pull the stick and check out what's there. If it looks the same as what you just poured in there, then its normal and the pump circuit is fine.If it's foamy with lots of bubbles, that would be no good. ATF has anti-foaming agents in it, so you will need to be speedy in the checking. Leave the engine running.
If the oil is fine; I would be dropping the pan, and checking the band and the servo. I would pressure test that bad boy to within an inch of it's life and if there was any doubt at all;I would pull the servo right out for inspection. But before that, I would study the band as you work that servo. Use a blow-gun with a rubber tip and at least 100psi. Watch the band for monkey business.If it is one of those triple-wrap bands, pay extra attention.If it split off one side of the anchor you'll see it.
If that's all normal, I would check the manual valve operation.

Stop! I think it just came to me.At least the jumpy needle. Here's my theory;
The pressure builds up in the circuit. Then there is a sudden drop in pressure, right? That's what you are seeing on the gauge; and then the pressure builds back up again. This happens continuously.So the needle is jumpy.
Ok so what could cause this? First off, it did this before the rebuild. I assume you replaced all the sealing parts in this circuit, which ain't much. So it has to be in the hard parts. Well you swapped out the VB so that ain't it. That just leaves the servo, the band, and the case.
Since we're having an issue with the clamping of the rear drum, at the same time, but it's a different band....... I'm gonna go with servo and case.
So drop the VB and crank up the air compressor to the max.
Put the pressure to that circuit and hold it. Listen. Something has to be leaking,suddenly, like a blow-off valve, Nothing? Back off the band adjustment and let the servo come all the way to the full applied position and retest. Still nothing? Swap in a different servo.Set the band and put it all back together. I know this is getting old. Fill'er up and retest. Same problem? replace the case, it has to be porous or cracked in the servo house, Problem gone? go find that old servo, and a big hammer and you can guess the rest.
But if you didn't replace the servo seal, or you nicked it on the install, your compressor may not have had enough pressure to make it show up. Remember the pressure on the gauge was hitting 250 to 275 before blowing off.
Now before you do any of this, wait a couple of days. There is a chance I'm wrong . Maybe someone else will chime in with a new thought or experience.
Just bump your thread back to the top every now and then.Don't be shy.

 

[Knapp870]

Thank you for your help! When pressure testing I stuck the air on front clutch and listened for the engage sound. This case has always seemed a little weak there, but then again it was dry. On the rear servo test I basically just hit it with air to ensure the servo engaged the rear band. You're saying I should hold it and see what air comes out eh?

Also on the needle jumping that sucked bounces up and down Like a pogo stick continuously. The rear servo is one of those billet jobs. Is it possible that rear servo hole is worn out to where it's not always getting a good enough seal even though it has a new seal?

 

[AJ/FormS]

No, the servo bore will be fine.There is no metal to metal there, and the servo is only used in reverse, and manual low, so for most of the tranny's life, it was probably seldom used.
Well you didn't complain about high gear dropping out, so I've been concentrating my thinking on the L/R circuit. There is every chance that the front clutch actually has the issue. It is even more likely. So while the VB is off, air-test that as well, but limit the pressure to 100psi. Apply,hold and listen. Now this is a tough test, cuz you have nothing to compare it to. This test always produces a hissing noise, as the cast seal-rings are lousy at sealing air. So you have to put on your thinking cap, and picture in your mind how it works. When OIL enters the circuit, it pushes the big piston out to clamp the clutches.Until the piston stops moving, this is a low pressure circuit. Once the piston stops, the oil pressure rises to clamp hard. The oil pressure backs up and some of it leaks out the seal-rings. No biggie, cuz there is lots of extra oil flow to maintain adequate pressure. That's how it works with oil.
Now with air, it's a little different.The second the air enters the circuit, it immediately starts leaking out the cast-rings. You will hear it blow the oil out. It sounds like a kid blowing his nose.Then the pressure builds up and blows the piston out to it's limit and it makes that nice thunking noise. Now the pressure backs up and really starts to hiss as it leaks out the rings.As the pressure in the piston rises, you will hear the hissing change pitch, and finally stabilize. At that point, you can release the pressure, but keep the rubber tip in the hole to prevent air leaking out there. Keep listening.As the spring forces the piston back to it's parking spot, the air will continue to leak out the cast rings.
Here's the tricky part.If the cast rings are bad all the air in the circuit will almost instantly be dumped.But if the rings are good, it will take maybe a quarter to a half a second for the air to blow out of the circuit, and it will sound like the last bit of air escaping from a balloon. Kindof a sigh.You will have to listen for the sigh.
Bad seal rings are not the only thing that can cause instant pressure release. A miss-match of parts in the front can do it too, as can improper washer locations or excessive end play.For this reason I always set up the complete front end on the bench and pressure test it through the front housing.By the time it gets installed into the case,I have every confidence that it will work.
It may be that in the distant past somebody waited too long for 2nd gear band adjust, and burned up the front drum. If someone replaced it with the wrong part, the tranny would have had this issue from day one after the rebuild.
Lucky you!