I know it gets discussed quite often but I cant really determine how to properly check my pin length. The vehicle is a 1991 suburban that recently lost reverse. shift it into R and it gives you a heavy clunk with no motion. Rev it up to 1500rpm and it slams into reverse, sometimes, often times not.

I pulled the reverse servo and welded/added .300 worth of extension to the end of the rod. with everything assembled, I can barely get the piston flush with the chamfered edge. bolt the cover down and the band seems bound up tight. I knocked the apply rod down to 3.610"(from where it seats in the piston to the tip) for a total extension of .200 and it still seems too long. this time however, with the cover on, the band has some free play.

my major concern here is the fact that I can not push the band tight by hand. with all servo components removed the band sits super loose where i can wobble it back and forth with a screwdriver. put the rod in and push up and i can see it take the slack up but not to the point where its tight. it reaches a stopping point where I can bench press the piston as hard as I can with one hand, and still wiggle the band with a screwdriver with the other hand.

is this just normal because of how much pressure it takes to apply this band, or should I suspect a broken band?

any input is much appreciated! im in way over my head here!

 

Best to assemble without the seal or the 1-2 accumulator spring.
Check from the chamfer edge to the top edge of the piston where it would support the seal.
That will give you a ballpark estimation.

Better yet, get another servo cover, cut a large hole or a notch in it, and weld a nut to the top of it.
Put a bolt in the nut and use it to push on the pin. Tighten to 80 in-lbs then setup a dial indicator to touch the piston through your hole in the cover.
Then unwind the screw and watch the dial indicator. That will show you the true travel/clearance that you have

 

appreciate the reply.

sorry I did not clarify in the initial post. I have already done just what you described in the first paragraph of your reply. the problem I'm having is I'm not sure if I'm measuring from the point where i can no longer push, or the point where the band is actually holding. because I can not get it to hold based off of me simply pushing on the piston. that's what concerns me and makes me think the band is out of place, broken anchor, something way off.

So I just went out there and assembled it with a washer(.120") between the cup and the piston, band hold's rock solid. remove washer and re install assembly. band still dragging. grind the pin back down till the drum just barely drags, good to go. check one last time with the washer and now it wont hold with the .120 washer pushing it up.

I guess i will have to add a little more with the welder and and try again. It does not seem that I can achieve any amount of hold, and still have it spin free with the travel spec that I'm finding on the internet. question is, do I setup for a strong hold and a bit of drag. or make sure there's no drag and just assume its going to take a good 2 or .300 of stroke to hold the band?

 

Setup properly, you should have strong hold and not much drag

Is this an old used band original to the 91 year?
Do the ends of the band butt before the band can clamp the drum?

Checking with your hands is inconsequential. You cannot push hard enough to simulate the hydraulics so don't bother.
The shade tree check is from the chamfer in the case, push with hands until you can't push anymore, and check distance from top of piston seal support to the chamfer. Typically a measurement of .050 to .070 is good for that chamfer to piston distance

 

Its all original to my knowledge. I'm doing this under the vehicle with the pan off, trans still in the truck. I just bought this truck sight unseen, 160k miles. I really figured I was getting a th400 in a squarebody truck. I was pretty excited to see an 80e... now I'm not so excited.

ill add some more washers and determine just exactly how much throw it takes to get a true hold and if its going to be feasible to have no drag and still get it to hold. I really wish i would have gone through and determined all of this BEFORE i just watched a youtube video and started welding to the damn reverse rod without measuring anything prior!

last thing to note, when I'm spinning the output shaft by hand, as the drum does finally hold or start to hold the output shaft does not stop turning. it just becomes difficult to turn and i can see the low reverse drum is staying put. I assume I'm working against a sprag.... or do i have a bad sprag.... or better yet, do I have no idea wtf I'm talking about in any capacity at all...

 

it seems to take about .220 to get it from a slight drag to a complete solid hold.

at this point I think I'm good to go, the band wiggles very free with it al assembled although it does still have some drag. with an old distributor hold down between the piston and cup (.180-.220 depending on how it settles in between the 2) i can tighten the cup up and get the band rock solid tight.

maybe shave a hair off to keep it from dragging??

 

.220 is huge travel. .120 would be on the high side of enough.

 

well what would you suggest i do?

add .120 back to the rod and stop worrying so much about the band dragging?

at .220 its dead loose no drag at all.

 

When the top of the servo piston seal ring support is at the start of the chamfer...that's your "zero"

push the band in until it won't push any further...and measure to the chamfer again.

That's your clearance

I feel there is a misunderstanding here as you're referencing slight drag to complete hold etc.

If you can't get a consistent measurement or don't understand...then get a spare cover and an indicator and do as I described earlier to get a true and valid measurement.
Or...post some photos

but I can tell you that if you're measuring this accurately...it's not going to work at .220.
If you're measuring it inaccurately then I can't make any sense of your numbers anyway

 

ok. let me try to be clear because i can see how confusing I'm making this. and I will add some photos as soon as I get a chance.

when I install the assembly completely, springs, pistons, cup, gasket and all, that is what I'm referring to as loose band/ slight drag. that's my starting point.

I then remove the assembly and shim the piston off the bottom of the cup about .220 and re install. it takes some torque to get it all the way up and the band is seriously tight.

if i do the same process at .120, the band does not get tight, it does however get as tight or maybe tighter then I can do by hand. what I'm trying to determined is what type of travel it takes to actually get a hold on the drum. Since your first response regarding my ability to push the piston by hand I've put no concern into the "eyeball how much you can push it up from the flush edge of the chamfer" trick. sonnax has some literature referring to the total travel being around 3/16", which sort of adds up with what I'm experiencing. all the information on the internet about this .70-.120 range seems to only refer to a hand test, not an actual test with the j-tool or home made cup/nut doo jiggy where we are getting the band to actually hold?? does this make sense or have I effectively confused the situation further?

or is there something seriously wrong with my transmission because it should not take so much force to get it to a hold, which was my initial thought.

 

did the hand job style. from the flush edge of the chamfer to the point of drag is something like .070, maybe less. the piston has more then enough stroke to get that band tight. why is this rocket science?

 

You're NOT going to be able to press the servo assy in adequately with all the springs and seals on it.
Take ALL the springs off. You only need the servo, pin, and washer to check for clearance.

Look at Youtube under Jake's Performance for a 400 servo check.

 

correct, by hand i mean with no springs. I did leave the seals on though. although I've tried it every which way.

I've watched that video repeatedly. looks about like mine when doing by hand. I'm certainly not too far in the hole like some of the other videos of the no reverse club. this would all be much simpler had I just checked it prior to firing up the welder.

We are talking in circles at this point. I think I understand the process thanks to all the help, I really appreciate it!

Now for my final question. I have no idea how many check ***** fell on the driveway but I know I've only got 8 available to me now. can I leave the #2 second accumulator ball out weather mine was "some models" or not.

 

still no reverse. off to the transmission shop it goes