xmission trouble?

dsusca · January 28, 2004

I have a 99 Sierra 4wd 4.8 ECSB w/Overdrive automatic that has 48k miles and for the past few days I've noticed it is shifting weird. the normal shift point at around 30mph seems to be elevated and I'm not sure if my overdrive (lockup convertor) is working correctly. Last night I took it for about the 30 mile ride on the highway and at 70mph it was tachin' at 2000rpm. It felt as if the motor was revving too high. I tried repeatedly to depress/release the tow/haul switch and did not feel any shift out of overdrive. I thought I remembered the tow/haul doing that pretty much anytime you hit it. At slower speeds around town, I'm not sure if the overdrive is kicking in either. Usually, I can hear/feel the overdrive at slow speeds cause it drops the tach and gives you that kind of rumble resonance through the drivetrain until you acclerate enough for it to disengage. Had my trans fluid powerflush about 6k miles ago with Castrol brand fluid at one of those Express Lube joints. I checked the level upon returning home and it was fine (in between two holes on the dipstick while running and warm) and it is pinkish and smells ok. I unplugged the battery negative for 5 min. to reset the computer, but no difference this morning. Anyone have any ideas? Thanks! Dana

robsawatsky · January 28, 2004

What's your axle ratio and what size tires do you have because 2000 RPM at 70 mph sounds about right.

dsusca · January 28, 2004

According to the RPO codes, I have the G80 gov-lock with a 3.73 ratio and P245 tires. Now what?

GM GIMP · January 29, 2004

2,000 rpm at 70 should be ok with 3.73.

Also, tow-haul mode doesn't eliminate overdrive

unless you're towing pretty heavy.

You could still have a problem though.

Shaners · January 29, 2004

If it's been excessively cold where you live, then the truck could be running in it's cold engine shift mode.

The tranny holds gears longer, and OD won't engage until the engine/tranny are warm.

dsusca · January 29, 2004

Thanks for responses! It has been really cold here in Northern CT. Although on the trip the other night, the truck was good and warm. Maybe I'm just paranoid! I recently had to fork out $1600 to have the wife's AWD Minivan tranny replaced so I'm not looking forward to another huge repair bill if I can help it. I also read on the engine/tranny forum that someone with a 4L80E had a similar problem and due to slippage a code was being thrown that prevented shift into overdrive. I bought my truck used and didn't know all the specifics about it until I learned about the RPO codes. Does anyone know where (other than the stealership!) that I could get my codes read for free/cheap? I heard maybe Autozone? Thanks again for your responses. This board rocks!!!! Dana