Transmission wont shift when accelerating

I have a 99 Silverado z71 5.3L. Occasionally (2-3 times per hour) my transmission refuses to shift into the next gear as I accelerate around 45-60 mph. I peg the gas, down shift, then work my way through the gears again without problem. Any idea why this might be happening?

Other info that might be valuable: I have 175k on the truck with original transmission. I had the transmission fluid replaced and it has been slightly overfilled. No grinding noises or anything like that.

Talked to a transmission mechanic. He said that there weren't any codes for transmission problems and he would have to take it apart to find out what's wrong. Said 18 hours of labor minimum. Does that sound fishy to you?

Appreciate your help! :)

ya don't sound correct...If these vehicles have any tranny problems the drive train code scanner will reveal the DTC'S...if you have the correct scanner that is capable of doing this scan ! most non dealers do not..

get a tranny shop that does have a scanner and get the failure codes...this can be something minor...

do not operate with the fluid over the full mark...when the fluid gets full of air bubbles its all over for the tranny...use a hand pump with a long plastic hose to suck out the excess fluid..out the fill tube........

Aight.... thanks for the tips. The shop i took it took specialized in transmissions but claimed not to find a code. I'm skeptical. I was just trying to avoid taking it in to the stealership for codes.

Aight.... thanks for the tips. The shop i took it took specialized in transmissions but claimed not to find a code. I'm skeptical. I was just trying to avoid taking it in to the stealership for codes.

how much is he gonna make giving you a code report ? how much to give you a rebuild ...he is a crook ..

go to a shop that can pull these codes ..with you shift issues they will be there..

An internal transmission problem will rarely show a code. With 175k on your trans, and knowing your trans it sounds as though you are having problems shifting from 3rd to 4th. If the computer is showing no codes then most likely your problem is in the band servo. Since from what you have described it sounds like you are getting 2nd gear but are having problems getting 4th. This is most likely a servo problem. The trans guy sounds like he is approaching the problem correctly. Dont be so quick to call the guy a crook if you dont understand the inner workings of an automatic transmission.


transman

Dont be so quick to call the guy a crook if you dont understand the inner workings of an automatic transmission.


transman[/quote]

with this statement: "Talked to a transmission mechanic. He said that there weren't any codes for transmission problems and he would have to take it apart to find out what's wrong. Said 18 hours of labor minimum. Does that sound fishy to you? "


what I stated was not fully correct as to my opinion of this tranny mechanic ....he is either a crook or unskilled ...

18 hours with this type of problem to JUST find out what is wrong ??????

In order to remove anything but the 2-4 servo, lower valve body, and separator plate the trans would have to be removed from the vehicle. If it's a 4WD/AWD you can add some more hours.

You can't even get a good look at the band without removing the pump assembly. As TransMan remarked, there are too many possibilities, including accumulators, sprag, skived rings, sprags, steels and plates, apply pistons, and so on that won't set a code. Computers are only a diagnostic aid, not the final word in diagnostics. Too many shops rely solely upon the PCM for their diagnoses and their resultant failure rates are higher than at an actual SKILLED shop.

As an example, this stuck apply piston set NO trouble codes but the truck wouldn't shift to 4th as a result:

http://www.wwdsltd.com/files/4L60OverRunForwardPistons.jpg

All the solenoids worked fine. All the pressure transducers detected the correct pressures and the correct ports, but the stuck piston wouldn't release the direct clutch so it would never overdrive.

PCMs just don't catch that, nor do they catch every other problem. This didn't even set a "general transmission fault" error. The sooner people stop operating under the common misconception that the PCM monitors everything and learn diagnostic skills, the sooner we will have smarter people.

your explaination as to what may be his issue and the PCM'S limited code failures to locate the exact problem I agree ...

what I still object to is , 18hrs labor to locate the problem ...not to repair the problem ....

this shop / mechanic must not be that experienced with this transmission..

OK, I've been analyzing this problem a little closer. When I'm accelerating it feels as if the truck is attempting a shift when it shouldn't be, and not finding the correct gear. That's usually when I peg it to downshift, then work my way through the gears correctly.

I have not taken it in to a shop or a dealership yet. I want to find out as much as I can from people that may have had similar problems before I hand my wallet over to the stealership.

OK, I've been analyzing this problem a little closer. When I'm accelerating it feels as if the truck is attempting a shift when it shouldn't be, and not finding the correct gear. That's usually when I peg it to downshift, then work my way through the gears correctly.

I have not taken it in to a shop or a dealership yet. I want to find out as much as I can from people that may have had similar problems before I hand my wallet over to the stealership.

need better discription from you . speed,gear, throttle position rpm's.