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Hi Everyone,
I have a 2004 Rado Z71 LS 2wd crew cab with 270k miles on a 4-speed (I believe) auto trans and have been having a hard shift issue come up in the last few months and would greatly appreciate any insight. I love this truck (bought it in 2006) and it is my daily and while I do most of the maintenance myself, transmissions are way out of my scope.

During normal acceleration between 50 to 60 mph, I get a pretty noticeable shift into either 4th or overdrive (not 100% sure) but shifts up and down fine through all the other gears. However, it has no problem shifting under hard, light or feathered acceleration. I had the transmission flushed but the problem continues.

I was talking to a buddy of mine that was a mechanic in the army and he was telling me that the problem is probably a shift servo that is either gummed up, needs to be replaced, or there is a vacuum leak, not allowing it to shift correctly.

So the question is,
1) Is this likely the servo being gummed up or a vacuum leak, or is this a deeper problem?
2) Is this something that a novice mechanic could do? If not, any guesses on cost?

Thanks for everyone's help.
 

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My advice is to simply get it serviced - new fluid, filter, etc., by a pro. 270k is a lot of miles, but the transmission can last significantly longer if cared for. It could be a low pressure condition that's easily fixed.
 

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It may be that solenoids or shift accumulator are failed . They can be replaced by dropping the pan to get to them . You can do a YouTube search on how to swap them out .
 

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I don't think I've seen a transmission with a vacuum line on it since my late 60's muscle car days! I'm pretty sure it's all electronic now, and to be perfectly honest I don't actually remember what the vacuum line was for on transmissions back in the 60's. I do know that a 2 speed "powerglide" transmission (used for drag racing) would not change gears if the vacuum line came loose. Anyway, I've replaced parts on automatic transmissions in the past few years to correct problems. I have no idea how they work or what the parts do, but a google search of the problem an purchase and swap out of the recommended part has worked well for fixing transmission shifting issues. To me, Whatever takes place between the motor an the driveshaft is some sort of strange "VOODOO", so grab some garlic and a little bottle of holy water and maybe have a wooden stake close by and go for it!! Obviously, I'd prefer working on things I have a basic understanding of, but I just don't have the time or interest to learn about automatic transmissions. I will recommend that if you decide to go for it that you go ahead and get all the replaceable parts and get them all replaced together. My thought is, if one has gone bad, the rest will soon follow. I replaced all the demons and evil spirits inside my '01 Pontiac Bonneville transmission for under $100 including the filter, fluid, gasket and all the plug-in actuators, accumulators, doohickeys, and I think they came in a kit. I may have had to remove the "valve body" to get to some of them. That's just inside the pan and looks like some kind of toy race track that all the demon-filled fluids run around in the grooves and make the voodoo happen. If I remember correctly, it's held in place by about a dozen bolts with 10mm heads that were very tight and some were longer than others. All said though, it was pretty simple after watching a couple of you tube videos and drinking a couple of beers to help me not think too much about what I was working on.
 

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A bit of a tangent - my 66 Corvair with a 2-speed Powerglide has a "vacuum modulator" that smooths the upshift based on acceleration. The drag race guys would actually pull the vacuum line off, which would maximize the transmission hydraulic pressure and make the hardest/firmest shift. Fun is when they start leaking and your sucking transmission fluid into the intake manifold and you can't figure out where the transmission fluid is going...

Our transmissions are completely electronically controlled via solenoid valves.
 

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Yes, that's it! I remember now that the first time I learned about that vacuum line was when my '69 4 door Malibu with 307 and powerglide started billowing out the most dreadful cloud of smelly smoke going down the road! I was working at a service station at the time and one of the mechanics laughed at me when I came to work with a huge cloud behind me! I was cussing and fussing and told him I needed to start looking for a good rebuildable engine AND transmission! "Not only are my piston rings completely shot, but I've blown my transmission too!" He told me he could fix both problems for $100. This was in 1976 and I was working for $3/hr. I was a high school kit at the time, and minimum wage was $2.10/hr! That was more than a week's take home pay for me, but I told him to go for it. He went to the rack of spools of different sizes of hose and pulled off about a 5' piece of 1/4" vacuum line and connected one end to the intake manifold or carburetor (can't remember which) then let the other end fall on the ground under the car. He put the car on the lift and raised it up and put the other end on a nipple on the side of the transmission after removing a crusty burnt and brittle line that was already there. "There you go! That'll be $100 sir!". We joked about that for months! He insisted that I owed him the money because the repair was about $15 for time and labor, but the other $85 was for his knowledge and experience! He never got the $100, but I learned a lot that day. I learned to be patient and take time to figure things out. I learned if I had two separate problems that occurred at the same time - not matter how illogical it seemed - were probably related. I learned that experience and accumulated knowledge truly was worth a lot to others that didn't have it.
 

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'05 crew cab Canyon, swapping to 6.0 L96Vortex
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Hi Everyone,
I have a 2004 Rado Z71 LS 2wd crew cab with 270k miles on a 4-speed (I believe) auto trans and have been having a hard shift issue come up in the last few months and would greatly appreciate any insight. I love this truck (bought it in 2006) and it is my daily and while I do most of the maintenance myself, transmissions are way out of my scope.

During normal acceleration between 50 to 60 mph, I get a pretty noticeable shift into either 4th or overdrive (not 100% sure) but shifts up and down fine through all the other gears. However, it has no problem shifting under hard, light or feathered acceleration. I had the transmission flushed but the problem continues.

I was talking to a buddy of mine that was a mechanic in the army and he was telling me that the problem is probably a shift servo that is either gummed up, needs to be replaced, or there is a vacuum leak, not allowing it to shift correctly.

So the question is,
1) Is this likely the servo being gummed up or a vacuum leak, or is this a deeper problem?
2) Is this something that a novice mechanic could do? If not, any guesses on cost?

Thanks for everyone's help.
I would put in a bottle of Trans Fix before any major repairs are done.
 
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