HELP! 98 Sierra 5.7 Auto Trans will not downshift

Picked up the truck last Friday. Everything was normal and perfect. Gas guage was not working so I ran it out of gas. Filled the tank, now fuel pump was humming louder than normal (I know they hum anyways but it was loud) Next day merging onto the highway I put the pedal to the floor only to find it would basically go into nutreul and sit at about 2500 RPM but would shift back into gear once I let the gas off a little. Took it back to the dealership and they say they replaced the fuel pump but I was originally told they were only going to replace the fuel sending unit (I doubt they put a new pump in due to how loud it still is and how they treated me when I brought it back to be repaired) Still have the same problem, except my fuel guage now works.

Is it a tranny problem? solenoid, relay or such? Fluid is not burnt or discolored.

Something to do with the computer? It has had some unknown upgrades done to it. (have already disconnected the battery in an attempt to reset it and nothing)

Is it still the fuel pump?

Any help would be great. I'm still able to drive it just can't rip on it like I'd like to.

Is it true that a fuel sending unit cannot be purchases seprate from the pump?

Not downshifting is very different from what you have described. It sounds like your tranny is slipping under load. Does it start to slip when it shifts?

Proper diagnosis should be done with a pressure gauge and a trans capable scanner hooked up to the tranny on a test drive.

//2000CAYukon

Not downshifting is very different from what you have described. It sounds like your tranny is slipping under load. Does it start to slip when it shifts?



It started the not downshifting problem right after I ran out of gas, this leads me to believe that it has something to do with it.

It only slips when I mash the pedal to the floor, normal acceleration shifts fine. I can manualy shift through the gears and get the same problem when I mash the pedal to the floor.

Also, I am wondering if any type of programming has been done that may effect the shift pattern. I know it has had some work done, wether it be a chip upgrade or what I am unsure but on the computer itself there is a sticker with a company name and number like an advertisement of sorts and another stick that says "92 Octance or higher only". That same sticker is also on the gas cap. The dealership did not know to much about it so as of right now I am putting 93 in only. IS there any way to tell what has been done without taking it to the shop?


I find it weird that the only problem is the downshifting, normal shifting is perfect! Feels like a brand new tranny outside of the downshift. I highly doubt any major servicing is needed, I have had transmission problems with an older vehicle that had the 4L60e and I am experianced nothing now like I did then.

Shift points and shift firmness can be reprogrammed with an aftermarket programmer. However, if the transmission is slipping at all, something is wrong. Either, there is a mechanical issue when increased line pressure is not working properly. I guess it is possible that the reprogramming messed up the line pressure when the throttle position is increased.

Driving it while the tranny is slipping can cause the clutches to wear out and a rebuild will be required.

A pressure gauge will tell you this.

//2000CAYukon

The shop that installed the aftermarket chip and reprogrammed it is really close to were I live so I'm going to run it in there to see whats going on.

Also found out that a freind had the unit to test the tranny's electric stuff so hopefully I'll have it all figured out in no time.

Buddy said it may be a solenoid. I hope so.

Just got through sending a nasty e-mail to the vise president of the dealership I bought the truck from. She is more than willing to fix the problem free of charge and was actually worried that I would take the issue further, like to court.