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Shifting problems

Tiny
DRIVERBING
  • MEMBER
  • 1988 CHEVROLET TRUCK
Transmission problem V8 Four Wheel Drive Automatic

I have the above truck and have owned it for about 2 years now and has ran great until now. I hooked up my small trailer the other day and drove about 20 miles up the highway and dropped off my wife in town. All the way there I never had any problems with the tranny but after I dropped of my wife off I started to. When I left it took a while for it to shift into 2nd so I stopped to make sure I was in the right gear and I was. So I tried again and after I got the RPM's higher it shifted into 2nd. And the same for it to get in to 3rd gear. And it keeps getting worse but otherwise no slipping between gears and it hooks up and goes on the highway. A friend of mine told me to change the fuel filter because the silverado was know for for this problem with the tranny being all electronic what do you suggest to do? I have to get the speed up to about 30mph for it to shift into 2nd. And into 3rd about 45 or 50 it feels like. And normally it shift's into 2nd at about 15mph. And then into 3rd at about 30mph. And i've even tried to shift it manually but that doesn't help either.
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Tuesday, June 1st, 2010 AT 12:06 AM

10 Replies

Tiny
BLACKOP555
  • MECHANIC
  • 10,371 POSTS
Sounds like the shift solenoids are getting a bit lazy inside the transmission. How good are you with electrical testing?
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Wednesday, June 2nd, 2010 AT 9:53 AM
Tiny
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I can do some electrical testing work it depends on what tools I would need and what's involved.
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Wednesday, June 2nd, 2010 AT 9:14 PM
Tiny
BLACKOP555
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What model pickup and well get started
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Wednesday, June 2nd, 2010 AT 11:16 PM
Tiny
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It's a 88 Chevy Silverado 4x4 extended cab with the 5.7 liter motor and 4 speed tranny.
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Thursday, June 3rd, 2010 AT 10:09 AM
Tiny
BLACKOP555
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Well your shift points seem on time. I must have missread your question, sorry.

Do you feel trans is shifting later or that the transmission is slipping?

Or you need higher rpms to maintain a certain speed.
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Thursday, June 3rd, 2010 AT 10:24 AM
Tiny
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It seem's to be shifting a lot later than it was before. Because I have to really have to have the RPM's a lot higher now for it to shift and I have to have it going like 30 or a little bit more for it to shift into 2nd. I even tried letting off the throttle a little bit when the RPM's are high to get it to shift but that doesn't work either or by shifting it by hand it won't shift into 2nd. And I haven't drivin it that far since it started except to the store which is only like 3/4 mile from the house and it didn't shift at all. And i'm afraid that if I keep trying to force it by upping the RPM's that I might damage the motor in which I can't afford either.
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Thursday, June 3rd, 2010 AT 10:35 AM
Tiny
BLACKOP555
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Is ur trans a thm 400 or is it a different module. I belive its a 400 and that has a vaccum modulator. Are teh shifts harsh to like really hits when it engages? If the modulator is bad it thnks your accelerating at a high throttle and it will shift later and harder.
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Thursday, June 3rd, 2010 AT 11:18 AM
Tiny
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I don't know for sure what model tranny it is but as far as I can tell there is no vacume lines going to the tranny. It does shift hard when it changes gears unless it's when I let off the gas a little then not to bad.
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Thursday, June 3rd, 2010 AT 11:52 AM
Tiny
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I did some checking and I do believe it is a THM400 transmission in my truck but it has only 1 3wire connector on it which is on the drivers side of the tranny and it has a 16 bolt pan on it. The casing number on it which is on the passenger side just above the pan near the back corner of the tranny and the number is 8TXM292A
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Friday, June 4th, 2010 AT 9:10 PM
Tiny
JOE1111111111
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Hello all, I really can use some advice, I have a 1988 gmc 3500 box van its a 350 and a automatic tranny, the tranny works and shifts great my big problem is at 50 mph and up it does not shift into a higher gear, so from 50mph and up the rpm are way to high, so yes I don't go over 50 mph. Please anyone. Will a speed sensor fix this. Or I notice their is some kind of a vacume hook up on the passenger side of the tranny, thank you all
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Wednesday, February 10th, 2016 AT 6:05 PM

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