Hard Shift

[ramu3527]

Hey folks,

Just picked up an '08 GMC Sierra SLT Crew 5.3 4WD with the All Terrain package. I'll start by saying WOW! What a sweet truck.

One thing though. I am starting to notice (or I think I'm starting to notice) a hard shift going mainly from first to second under soft acceleration. Other than that the trans seems to work great.

This is my first GM so I'm not sure what to think.

The truck is under warranty, should I take it in? Maybe it needs a flush and a filter.

Thoughts?

Thanks
Ray

 

[Molson02536]

Hey folks,

Just picked up an '08 GMC Sierra SLT Crew 5.3 4WD with the All Terrain package. I'll start by saying WOW! What a sweet truck.

One thing though. I am starting to notice (or I think I'm starting to notice) a hard shift going mainly from first to second under soft acceleration. Other than that the trans seems to work great.

This is my first GM so I'm not sure what to think.

The truck is under warranty, should I take it in? Maybe it needs a flush and a filter.

Thoughts?

Thanks
Ray

Is your tow mode button on??? the tow mode button brings up the transmission line pressure which is controlled by the EPC valve in your transmission. The 1-2 shift has always been firm with the GM's transmission, but if your transmission has not been service yet. It would be best you do have them service it and replace the boost valve.

I have always replaced my boost valve every transmission service and it's running 365,000 miles now. The boost valve is what regulates the transmission line pressure and low pressure leads to clutch slippage which turns in to a burnout transmission. Boost valve is only an extra $50.00 and takes an extra 10 Min to replace when the transmission pan is dropped and filter changed.

As for flushing your transmission, NEVER FLUSH YOUR TRANSMISSION. The flushing process could dislodge and O ring or check valve in your transmission which will cause you to have transmission failure. Just give it a normal oil and filter service and try to us synthetic transmission fluid compatible to the fluid in yours now so you don't have to drain the whole transmission fluid from the transmission.

http://www.amsoil.com/storefront/atf.aspx

 

[ramu3527]

Sounds like great info, thanks. The Tow Mode is not on so thats not it. I was thinking in the back of my mind this just may be how GM's shift but sometimes it's pretty hard with soft acceleration. I just got the truck Sat. so I haven't looked at the main. schedule yet. It has just over 30,000 on it. I guess a trans oil and filter change wouldn't hurt. It's time to build a relationship with the GM dealer anyway. Anymore thoughts?

 

[Molson02536]

Sounds like great info, thanks. The Tow Mode is not on so thats not it. I was thinking in the back of my mind this just may be how GM's shift but sometimes it's pretty hard with soft acceleration. I just got the truck Sat. so I haven't looked at the main. schedule yet. It has just over 30,000 on it. I guess a trans oil and filter change wouldn't hurt. It's time to build a relationship with the GM dealer anyway. Anymore thoughts?

Any difference in your tow mode button on or off? If no difference can be felt the two setting, EPC (Electronic Pressure Control) solenoid failed. Easy fix in pan. Fix as soon as possible or hard parts will eventually break.

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).

Here is a link to your transmission on the TSB that has been out for it. Good luck and hope this helps. http://www.aboutautomobile.com/TSB/2008/GMC/Sierra/Automatic+Transmission

 

[ramu3527]

thanks buddy, i'll let you know. much thanks