Having problems with your GMC Sierra 1500 ?

The 6 speed transmission slips going from 2nd to 3rd or 3rd to 4th,it races just before shifting 2nd to 3rdwhen slowing down it feels like it sticks in 3rd beforelike a clunking noise then it will shift 2nd and then 1st

Posted / edited by AnonymousUser on : 12-03-2018

Answers :

I suggest trying a lucas additive to you transmission. If that don't work you need to let a transmission shop, you can trust, run a scanner on it. If you have a lot of miles on it I would not recommend changing trans. fluid.
Before guessing at transmission problems, check the fluid level (car warmed up, engine running in Park on level ground) and have the PCM scanned for trouble codes FIRST. Secondly, put a line pressure gauge (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 0.500" boost valve (easy fix in pan).\015\012
\015\012Soft shifting, gradual performance degradation: Poor line pressure rise due to leaking boost valve, clogged EPC filter screen, failing EPC solenoid, or worst case: leaky seals throughout. Also check TPS for smooth electrical response. If the response is jumpy or erratic at all, replace TPS.\015\012
\015\012Good luck and keep us posted, thank you for using fixya
Repair Help & Product Troubleshooting for GMC Sierra 1500

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