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-   -   700R4 2nd gear start/4th gear bind up? (http://www.hotrodders.com/forum/700r4-2nd-gear-start-4th-gear-bind-up-61382.html)

myclone1 04-15-2005 01:13 PM

700R4 2nd gear start/4th gear bind up?
 
I have been rebuilding 700s out of my home garage for a while now and have had fairly good results but a 1990 unit out of a full size Blazer w/350 TBI that I recently rebuilt has issues that I cant find a solution to.

The problem(s) are that when the gear selector is put into OD the trans engauges 2nd gear until the throttle is slightly applied which causes it to immediately down shift into 1st with a "clunk". The vehicle doesnt have to be moving to do this either as you can make it happen by holding the brakes, selecting OD, and slightly applying the throttle. If you manually select 3rd, 2nd, or 1st this problem doesnt occur just when selecting OD.

The other problem (related to the above?) is that while road testing the vehicle when the trans shifts from 3rd to 4th it binds up causing the vehicle to slow until the speed is slow enough that the trans down shifts back to 3rd then all is fine again.

Other than the two problems listed the trans works fine, shift points are right on the money, shift feel is fine, TCC apply is good, etc. I rebuilt the unit using a Transtar kit, new Daaco TC, pump clearance mic'd fine, total end play is .029", all clutch stack clearances were well within spec, air checked input drum (no leaks), etc. The only mods that were done to the trans, which I perform on all trans's that are destined for full size trucks, is the TC check ball in the input shaft was deleted, PR valve land ground off, .420" boost valve (unit had a .400" originally), load release springs deleted and #44 hole in seperator plate drilled to .093" per the ATSG manual when deleting these springs.

I swapped the unit out at the customers house for another fresh unit I had on hand that was built the same and brought the defective unit home for tear down. Upon tear down the only thing I can find is that the 2-4 band is burned which I kind of expected. Other than that the clutches look new, all seals/gaskets are fine, no valves in the VB were stuck/sticky, all servo/accume springs fine, etc. Other than the burnt band the trans looks new.

Anything thoughts on what to check? I thought that maybe I put the forward sprag togeather backwards after cleaning it but after checking it the sprag was assembled correctly. Im kinda stumped on whats wrong. :confused:

TIA for any help.

BTW, the unit in question was a replacement from the salvage yard that burnt completely up due to the customer not adjusting the TV cable when he installed it (he thought it was the "kick down" cable...UGH). He removed the trans and brought it to me for rebuilding and once he had reinstalled it I adjusted the TV cable for him. Once that was done and upon testing is when I found the 2nd gear start/4th bind up issues. I forgot my line pressure gauge at home (DOH) so I never got to check the line pressure while the unit was running and the customer was over an hours drive from my garage so getting line pressure readings didnt happen.

Crosley 04-17-2005 10:52 AM

check balls left out or installed where they are not needed.

gasket mismatch.

leaky accumilator pistons, worn pins , worn pistons, scuffed walls of the accumilator bore, old style pistons and seals re-used

cut lip seals, lip seals left out.

Trash in the valve body area causing a cross leak , a leak in the governor circuit..... all can pioint to problems.

What I do not understand is this; You say 2nd gear applies, you touch the throttle it jumps to first with clunk. It sounds like the forward clutch does not apply initially.

If the forward clutch is applied and the band... the tranny is in second gear as it sits still. When throttle is applied IF the band comes off, there should be no 'clunk' if the gear train is loaded with the forward clutch applied. This leads me to think the forward clutch is not applied. just an idea without seeing the vehicle.

Forward clutch is applied in all forward gears... I am sure you know this, others reading may not know. When you move the shifter to "OD" at a full stop... the forward clutch applies..... you are in first gear

A bind up in OD would involve another clutch pack applied that should be OFF.

in OVer drive; The band is applied, 3-4 clutches applied, forward clutch pack applied.

If the over runs are applied during OD ... they burn.

IF the low/reverse are applied during OD... they burn

myclone1 04-17-2005 07:32 PM

Quote:

Originally Posted by Crosley
check balls left out or installed where they are not needed.

Only check ball left out was the TV exh that everyone leaves out (the "big" one).

Quote:

gasket mismatch.
I put the new gaskets up against the old ones to make sure I had a match before installation so I think Im good to go there.

Quote:

leaky accumilator pistons, worn pins , worn pistons, scuffed walls of the accumilator bore, old style pistons and seals re-used
The 1-2 accum was scuffed and the pin hole was wallowed out so I installed a new accum as well a new 1-2 accum housing. I checked the accum bore in the case and it wasnt egg shaped so I just installed a new piston since I had one handy. All seals replaced and bores were scotch brited/cleaned before assembly.

Quote:

cut lip seals, lip seals left out.
Lip seals ok and all where theyre supposed to be.

Quote:

Trash in the valve body area causing a cross leak , a leak in the governor circuit..... all can pioint to problems.
Possible I guess but after tear down I didnt find anything obvious. Before assembly everything is cleaned spotless with brake clean then recleaned right before assembly with ether (starting fluid) and blown dry with shop air.

Quote:

What I do not understand is this; You say 2nd gear applies, you touch the throttle it jumps to first with clunk.
Thats what happens but then again the vehicle has so much driveline slop that its hard to tell where the "clunk" noise is originating. However, no matter where the noise is coming from you can feel the 2nd to 1st down shift when light throttle is applied. While road testing at low speeds you can tell by the accelleration that the truck is taking off in second gear though by VERY gently easing into the throttle from a stop. Give the truck normal throttle input from a stop it moves ~6", clunks into 1st, then accellerates normally. I thought about a gov prob but since Im able to duplicate the issue with the brakes applied and the vehicle not moving I think there may be other issues though.

Quote:

If the forward clutch is applied and the band... the tranny is in second gear as it sits still. When throttle is applied IF the band comes off, there should be no 'clunk' if the gear train is loaded with the forward clutch applied. This leads me to think the forward clutch is not applied. just an idea without seeing the vehicle.
Understood and I will look closer at the input drum assembly to see if there is anything amiss. I tore it down but didnt have time to go over it with a fine toothed comb. I did note that all the clutches looked brand spanking new with absolutely no evidence of heat damage. Just the band was cooked.

Quote:

A bind up in OD would involve another clutch pack applied that should be OFF.

in OVer drive; The band is applied, 3-4 clutches applied, forward clutch pack applied.

If the over runs are applied during OD ... they burn.
As was mentioned all the clutches in the input drum and rev drum look brand new. Thats whats kinda got me scratching my head as I would think Id find more than just the band burnt but then again see the next quote.

Quote:

IF the low/reverse are applied during OD... they burn
I havent gotten the tear down past the sun shell yet as I was pressed for time but I plan to tear the unit the rest of the way down this coming week.

BTW, thanks for taking a minute to help out and pass along your ideas. Much appreciated.

myclone1 04-19-2005 02:51 PM

FWIW.. I think I might have found the problem. I finally got the chance to take a close look at everything and once I got to the seperator plate I noticed severe wear on the #8 check ball hole (2nd/1-2). I didnt notice how wallowed out it was before so this time I put a check ball in the hole and it would almost fall through but if you put a little pressure on the check ball it would get stuck. I dunno how I missed that on the first assembly (complacency maybe?) but lesson learned to check those buggers closely every build.

Other than the seperator plate hole being worn severely (and the previously mentioned band) the rest of the trans looks new so I replaced the seperator plate, replaced the band, reassembled the unit, and it patiently awaits installation for road testing.


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