'99 sierra 1500 Trans shifting problems

[hopsing415]

I have a 4.3l with a tow package from the factory. Auto Transmission fluid is full and has 220,000 miles. Problem is after 75-100 highway miles and I come to a stop. Shifting very hard from 1st to 2nd. Any ideas what would cause this? Just bought the truck and have no history 'cept the 2000 miles I put on it.
Thanks in advance. Patrick

 

[Molson02536]

I have a 4.3l with a tow package from the factory. Auto Transmission fluid is full and has 220,000 miles. Problem is after 75-100 highway miles and I come to a stop. Shifting very hard from 1st to 2nd. Any ideas what would cause this? Just bought the truck and have no history 'cept the 2000 miles I put on it.
Thanks in advance. Patrick

Check TPS for smooth electrical response. If the response is jumpy or erratic at all, replace TPS. The other issues also would be a worn boost valve and sleeves which the EPC is trying to compensate by bringing the transmission line pressure up.

Put a line pressure gauge ($55 shipped 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 0.500" boost valve (easy fix in pan).

Good luck and keep us posted.

 

[ChrisArnold]

I had the same problem with my 97 350 Sierra. More than likely the valve body in the tranny needs rebuilt. It may or may not give a code. If this is the case and it is not fixed it could cause the torque converter to go out. At most Transmission shops the valve body is only a couple hundred dollar fix. Just have to drop the pan and rebuild valve body.

 

[kniki10]

You can try changing the transmission fluid, if it has tunred brown.