URGENT Transmission help needed (1985 GMC Sierra Classic, 2.8 , 3 speed OD AUTO

Discussion in 'Chevy S10 Forum (GMC Sonoma)' started by b717doc, Apr 9, 2013.

  1. b717doc

    b717doc New Member

    Really in a bind with my 1985 GMC Sierra Classic 2.8 liter 3 speed automatic overdrive....I bought the truck about a week ago, and the previous owner, who bought the truck new in 1985 (he had the receipt from the dealership), told me that he pulled the engine and transmission for overhaul about 7 years ago, and the vehicle has been sitting pretty much since. The problem is that the transmission will only go into overdrive at a very high RPM....or around 57 mph / 5000 rpm. It shifts perfectly up until that point....Im assuming its around 5000 rpm when it finally goes into overdrive... The interstate is HORRIBLE....If I encounter even the slightest incline on the interstate, the thing downshifts into 3rd gear and about 6000 rpm....I've already adjusted the TV cable on the carb per the Haynes manual, and info I found on the internet, with no change....I just don't know enough about the shifting principles of this transmission, but seems like it's vacuum related....There are a couple of connectors hanging from the firewall, that I have no Idea where they go, and some vacuum hoses are either plugged, or empty nipples, but no apparent vacuum leaks....Is there some sort of vacuum switch that may control the vacuum input to the transmission? Please call me with any ideas, as Im 72 miles of all interstate from home, and it's the only ride I have*********Nevermind....taking truck to AAMCO transmission shop tomorrow...manager says he knows the truck and transmission, says the valve body either needs repair or replacement...maximum cost...$375, guaranteed...previous owner (seller) says he'll pay half for me.....
  2. tbplus10

    tbplus10 Epic Member Staff Member 5+ Years 5000 Posts Platinum Contributor

    Good to see you found a shop that knows that trans. They were notorious from day one for being a pain. Theres an updated valve body that fixes most of the ailements the trans suffered from. GM did a TSB at one time to change the valve body but only on vehicles specifically brought in for trans issues.
    Watch that 2.8 for oil pan leaks another issue they suffered from and dont expect a lot of power from it. And no the only way to ever make the truck more powerful is to swap in a 4.3. The 2.8 just never had any future.
  3. b717doc

    b717doc New Member

    UPDATE: The fine gentleman at the AAMCO shop called later in the afternoon, says his 30 year transmission specialist can't find anything wrong with the transmission....valve body works fine, as advertised, the tech drove the truck, says I definitely have a problem, as the transmission simply will not go into overdrive at the specified speed. 1st, 2nd, and 3rd gear is as smooth as butter, but REALLY takes a lot of RPM's to shift into 4th, or overdrive.....around 57 mph with a slight release of the gas pedal.....The tech strongly suspects that the rear end has been changed out with a manual rear end, in an effort to give the truck a better "bottom end" around town, but simply won't work on the interstate environment...She will shift into 4th, but any incline on the interstate, or an attempt to maintain 65 mph on the incline, kicks her down into 3rd at about 6000 rpm Im guessing....Back to the seller I go, but to only do Gods work, as his wife's funeral was only a few hours ago, and hand him containers of fresh Ham and Potato salad...I encourage him to call me if he needs ANYTHING, and try to avoid discussing our really bad transaction regarding the truck I just paid $3900 for... He asks me about the truck, and I tell him the AAMCO shop couldn't find anything wrong with the tranny, and that they suspect the rear end has been swapped out..."No sir, I've owned that truck since it was brand new, and I guarantee you, that's the original rear end...That's good news to me, if it's true, as now I won't have to go scrounging thru the junk yard for a rear end.... IF he's telling the truth....The tech also said that the carb has a flat spot in it upon acceleration, which I have encountered myself, but said it shouldn't have anything to do with the transmission shifting into overdrive.....Thanks for letting me vent here, as I have been up most of the night worrying about what Im gonna do....I have no idea what to do with the truck, and am contemplating purchasing a new vehicle, as bad as I hate to do that, but I HAVE to have a reliable way to work, and the little truck isn't it.....and Im out $3900, as he refuses to give my money back....No local laws to help me either, just the good of his heart....Stuck in this messed up and expensive situation.....All I do is pray and hope things work out....Again, sorry for the long thread, but all is bundled inside, and it helps to vent.....any ideas will be greatly appreciated....

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