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Discussion Starter · #1 ·
We have a 97 Tahoe with 100k miles,which 99% of the time is driven by my wife to and from work,20 mile round trip,wife drives like a minister.Trans is fine in this routine,but if we go on a trip the trans warms up,it will shift very hard once you stop and go to take off again.Also the check engine light comes on after about 1/2 hour,and doesn't go out again for a couple of weeks.Once the car is parked and things cool off its fine again and then in about 30 minutes it will start shifting hard again.This has been going on for years.I want to take a long trip with this car this spring and I wonder if any techs here can suggest what to do with this trans.Also,what model is it?Thanks.
 

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A friend had the same problem, He had been changing the fluid every 30 thousand miles .Which is recommened. This time he had to change the modulator and it went away. Not to say it will help you but it might be a quick fix rather than going to tranmission place and they telling you that you need a rebuild.
 

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Find out what codes are stored before you jump into any repairs. GM put some defaults into the trans software, that is most likely what is going on with the trans. May be a simple repair as others have indicated.
 

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Doug,
My 99 Tahoe has done it once or twice. Apparently if the ecm senses an excessive amount of torque converter lock up clutch slippage, it will put the trans in a default mode which is very high line pressure to keep it from slipping. I think mine is ok now after a fluid and filter change. There is a valve that can stick and not apply enough pressure to the torque converter clutch and cause the problem.

This is probably an oversimplified version of the problem. Like T-man said, get a scanner on it and see what codes it is setting, they should be in memory.

I will see if I can get this thread moved to the tranny forum, you will probably get some better answers there.
 

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Doug, next time the check engine light comes on, take it to your local Autozone, they can scan your codes for ya.
 

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Discussion Starter · #7 ·
Thanks guys,feel free to move it Bill.
 

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Sound likes the torque converter has gone bad . The code you have is probably 1870. When they replace your converter they will have to change the valve in the valve body that caused it .

Dave
 

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The information on the torque converter and temperature are key. My 99 Venture has gone through the same problems beginning at 60K, and now has 121K on it with no repairs needed. If it gets too hot, according to the management system, it will go into limp-home mode and build maximum line pressure to shift. Also check your engine tune. If you have a poor tune, the length of time the trans takes to shift may be affected and it will go into limp-home mode. Any of these things will turn on the check engine light. The recommendation for a trans fluid/filter change and a radiator flush to ensure better trans cooling are good things to do.
 

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I have done repaired many of these and never had to replace the converter. A trans shop can ream the bore in the valve body and install an oversize valve in it. Sonnax makes the tools and valve kits. The torque converter valve bore wears, then the valve sticks, preventing TCC lock up, after the pcm detects lack of lock up it will set a P1870 code and will go to high line pressure resulting in your harsh shifts. This mostly happened in the 96-97 models, fullsize and s-10 styles. Same trans 4l60E....Also the valve usually only sticks after everything is good and warm. Then after it cools it will work again.
Brian
 

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Discussion Starter · #12 ·
Brian,I took this to a small trans shop several years ago,and he said just what you said,but he didn't have the reaming tool.How big a job is it and what should I expect to pay?Thanks
 

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It takes a couple hours and I would expect to pay no more than $500, at the very most. Call around and see what they tell you.
You can also get a complete reman valvebody from gm. And do it yourself if you want.
 

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Discussion Starter · #14 ·
I,m a manual trans guy,how much is a reman valve body and how big a job to put in?All I've ever done to automatics is fluid and filter changes.
 

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I have done repaired many of these and never had to replace the converter. A trans shop can ream the bore in the valve body and install an oversize valve in it. Sonnax makes the tools and valve kits. The torque converter valve bore wears, then the valve sticks, preventing TCC lock up, after the pcm detects lack of lock up it will set a P1870 code and will go to high line pressure resulting in your harsh shifts. This mostly happened in the 96-97 models, fullsize and s-10 styles. Same trans 4l60E....Also the valve usually only sticks after everything is good and warm. Then after it cools it will work again.
Brian
I would repace the converter because whats causing the over heating is the torque converter lock up is slipping .

Dave
 

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I don't want to argue over this, but the problem here is not overheating. The problem is a sticking valve after the trans reaches operating temperature.
 
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