Why does my 1998 gmc sierra shitft hard from 2nd to 3rd gear?

[C.F maintenance]

This truck only had 118,000 miles when i bought it I believe it has the 4l60e transmission 4.3 liter vortec engine nice truck no other issues has been fleet maintained. when i first bought it I test drove it and there were no issues at all I had my mechanic look at it and everything. One month into the truck it started shifting kinda hard from 2nd to 3rd gear. So I took it back to my mechanic had him perform a tranny service just to see if there was any shavings he said there were none. Theres no codes or anything that are coming up Ive been driving it like this for close to a year now and it doesnt seem to be getting worse the only thing i can think to say is that the guy who I bought it off of had a aftermarket cruise control installed at some point i dont know if that could be faulty or not please any advice would be good advice. Thanks in advance

 

[Molson02536]

This truck only had 118,000 miles when i bought it I believe it has the 4l60e transmission 4.3 liter vortec engine nice truck no other issues has been fleet maintained. when i first bought it I test drove it and there were no issues at all I had my mechanic look at it and everything. One month into the truck it started shifting kinda hard from 2nd to 3rd gear. So I took it back to my mechanic had him perform a tranny service just to see if there was any shavings he said there were none. Theres no codes or anything that are coming up Ive been driving it like this for close to a year now and it doesnt seem to be getting worse the only thing i can think to say is that the guy who I bought it off of had a aftermarket cruise control installed at some point i dont know if that could be faulty or not please any advice would be good advice. Thanks in advance

Put a line pressure gauge ($55 shipped on Ebay, or visit a local trans shop) on the pressure test port and go for a drive to verify proper line pressure. Base pressure at idle in P,N,OD should be around 70psi and around 140psi in R. Max line pressure should be at least 190psi at WOT in 1st or 2nd, 225psi is optimum. If base pressure is low, trans or pump is worn out. If max pressure is low, check EPC solenoid. If reverse is slow to engage, and line pressure does not come up to at least 130psi in reverse, replace the boost valve with a Transgo 0.490" boost valve (easy fix in pan).

Also check TPS for smooth electrical response. If the response is jumpy or erratic at all, replace TPS. Start with these few things, I personally replace my boost valve every time i get my transmission serviced. A worn boost valve will cause the EPC to raise the transmission fluid to compensate for the worn out boost valve.

Good luck and keep us posted. http://www.compperformancegroupstor...Screen=PROD&Store_Code=TC&Product_Code=374420