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Hard downshift....any idea why?

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reece13

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Well, just wondering if anyone has an idea on what i may be experiencing.

Currently have an 89 blazer, 350 tbi, 700r4, now on 35x12.50s with 3.42 gears. (have a 14sf i still havnt put in).:doah: Transmission is original and has 165k on it. I drive it easy, have a trans cooler, and change the filter and top off fluid about once a year. It always shifts smooth up thru the gears, reverse works fine, and i usually just drive around town in D (not overdrive) because i dont want to stress the transmission with big tires and a weaker gear. Within the past couple of weeks, i have noticed that when it downshifts (out of lockup i think, or 3rd to 2nd perhaps), it shifts hard. and hard enough to know that it doesnt feel like a normal operating transmission. The fluid looks good and is full. What is the cause of this? is this the start of something bad??...gulp
 
You could try checking the TV cable. But honestly, its a modern miracle that it lasted 165k. Try dropping the pan, see if anything is in it. Then report back.
 
Ok. I'll check tv cable and maybe drop the pan, but i did changed the filter and topped off about 4-5 weeks ago. Everthing looked normal. Fluid wasnt even bad looking either.
 
There were some years of 700s that had this hard downshift in D (3), but not in OD (4). Have you tried moving the selector before coming to a stop to confirm? I thought this was like 84-86, but maybe found on other years or maybe yours was swapped at some point. I thought it was the 2-1 downshift, but maybe I forgot and it's the 3-2. I've heard there is some valve-body fix, I searched for it, but couldn't find it.
 
Sounds like line pressure is too High. Tv cable could be loose and letting the engine rev too high before engaging the tv and changing gears at too high a rpm. Feels kinda like a shift kit. Does it do it when going to 1-2 or2-3? Or just 3-2?
 
I have heard of the 700r4 problems but i thought they were pre 1987. The only hard shift it has is 3 to 2, or actually just out of lock up. All manual shifts work good as well as all upshifts. Could it b just the tv cable starting to get out of adjustment?
 
Nearly all 700's will exibit a harsh down shift coming to a stop when left in the D range. The servo piston bottoms out against the cover over the third accumulator spring when shifting down to second due to routing of accumulator oil in third gear. You can usually soften this by cutting down the center of the servo piston under the cover so the spring will compress further absorbing the hit instead of the piston to cover contact.
 
When shifting from 3-2, you are applying the inner piston of the servo. It moves back out when shifting from 2-1. How is the oil routed differently in here in D vs. OD? There are 3 springs in this diagram. Are you talking about making more room for the right side of the left spring?

SERVO700.gif
 
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talking about cutting that shoulder of the 2nd gear apply piston in the center that the band pin rides through, on the side of the 3rd accumulator spring and cover.
 
So cut where the red lines are, but not so much that the accumulator cover bottoms out around the outside diameter?

Servomod_zps618484a1.jpg


This is very interesting because the whole thing can be fixed without even pulling the pan. Nice info!:waytogo:

And to Reece, have you confirmed yet that the problem stops when you run in OD range?

Servo mod.jpg
 
Take about an eighth inch to three sixteenths off the area in red, I usually cut it down to the bottom of the "chamfer" on the center hole the pin goes through. I do it on a table lathe but you can use a grinder, it won't hurt if it is ugly and uneven, just so it keeps the piston from bottoming against the cover. Where did you find those sweet illustrations by the way?
 
Don't you guys usually put it on the OD gear anyways? I would assume it would all be OD unless towing or something...
 
I do use OD if im on the interstate or doing over 55, but with my current setup (35s and 3.42 gears), its not optimal. The rpms would be way to low, especially when in od and lockup. small block chevy motors dont like to run at 1300 rpms! Now if i had 4.10s or 4.56s, then yes, OD all the way. Numerically low gears and big tires arent good on a 700r4
 
33s and stock gears...i think you'll be fine. It also depends on what "stock" gears you have. Most likely its a 3.42 or 3.73. which should be fine with 33s. But i have seen people that want more power at takeoff so they step up to 4.10s and 33s. Depends on your preference. If you have OD, you can def take advantage of using a lower gear if you want more power and not have to worry about the rpms screaming when u drive down the highway.
 
To Greg: this is great information that I searched for in the past but until now nobody was able to tell me.

To reece: Now you just have a quick one evening project ahead to fix this. Yes, a lathe is not required as this is not a rotating assembly. Even in the slim chance the piston got messed up, they are available at any local transmission shop or dealer parts counter.

To willpetty: As for 33's and stock gears, that is OK if you have 3.73. With 3.08s (which were actually pretty common) the transmission will not be happy. Why would you not find out your ratio? Even if you don't want to look at the gears (are you sure you couldn't use fresh gear oil?) you can count driveshaft revolutions or if you have a tach, use an online calculator.
 
I need too but I didn't know how so I will have to see...I've been just trying to get motor good before looking at anything else...I'm not going to hurt anything as I'm only using the blazer to go around the block a couple times a week. Fluids were changed and I have a rebuilt tranny (I was told) so I should be good. Checking and making sure is always better though
 
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