chevy 350 tranny probs.

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  #1  
Old 09-01-04, 05:09 PM
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chevy 350 tranny probs.

Hello,

I have a 1985 chevy 1/2 ton truck with a TH350 w/lockup transmission. I rebuilt it 6-8 months ago and all has been well. Until now that is. Monday, it started shifting erratically, and trying to go in and out of third even at low speeds like 15mph. It would cycle from 1st th third with lockup engaged. It cycled fast...about twice per second. I got it home and pulled the governor. It was sticking, so I replaced it. Now when I drive normally, it stays in 1st longer than it should, then shifts into 3rd, and lockup immediately engages even if I'm still lightly accelerating and only going 25 mph. If I drive aggressively and hit the gas, it goes through all the gears fairly normally. I disabled lockup, but it still wants to skip second. Once I hit the gas a bit more, it will kick down and get second gear. Even more...it doesn't do this every time, but 80% or more of the time. When it doesn't do this, and second engages like it should, I can maintain my speed and stay in second for a while, but if I try to accelerate, the second I hit the gas it shifts into third.

As if that weren't enough, here's more info that might help you pinpoint my problem: When I come to a stop, the tranny doesn't want to shift back into first until after I have stopped. I can feel it thump when it does it. Sometimes it doesn't go back into first until I begin to take off from the stop. I try to get it going in second, then it drops into first with a little thud and goes.

I can manually shift the transmission just fine.

Any thoughts as to what could be the problem? I changed the modulator, checked the TV cable adjustment, checked vacuum at the modulator, checked fluid level. It is nice and red and full. I think a valve or something is sticking, but not sure what or where.

Sorry for such a long post, but I wanted to give as much detail of the symptoms as I could, in hopes for an accurate diagnosis from you pros!
 
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  #2  
Old 09-02-04, 06:50 AM
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sounds like...

...you have a valve or valves sticking in the valve body. When you rebuilt it, did you flush the cooler and/or replace the convertor? The most common cause of small particle contamination is from the convertor altho there are others. drop the pan and look for it. then drop the valve body and see if you have some sticking valves.

PS...the TV cable is really just a detent cable for forced downshift at cruise speeds. you could remove it completely and it wouldn't cause any shift concerns other than won't downshift at WOT
 
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Old 09-05-04, 12:27 AM
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I replaced the converter, and I can't remember what I did with the cooler. I may have flushed it, or I may have blown it out. I think I figured out what's going on though...the 1-2 shift valve is sticking. (I think...haven't puled the valve body to check yet...haven't had the time so I'm just having to drive it like it is for the moment). I think it is sticking, and won't come in when it should during acceleration and so when 1st gear gets wound all the way up, it just goes to 3rd. Then 2nd comes unstuck and finally works, but then when I slow down to a stop again, the valve for 2nd is stuck in the open position and hangs up, holding it in 2nd longer than it should and not downshifting into 1st. Hopefully I'll be able to check it out this weekend if the hurricane leaves us alone.
 
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Old 09-07-04, 04:05 AM
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sticky valves...

...aren't real common on cast iron valve bodies. that's why i asked about the convertor. even tho you replaced it when overhauled...it may be that it's failing. if you have small particle contamination from friction materiel, i'd suspect the convertor...small particles tend to get trapped in the cooler, so if you end up replacing the convertor again, also replace the cooler
 
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