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1985 GMC S-15 Sierra Classic 2.8 shifting problem

Discussion in 'General Chevy & GM Tech Questions' started by b717doc, Apr 19, 2013.

  1. b717doc

    b717doc New Member

    1985 GMC Sierra classic, 2.8 liter V6, 3 speed overdrive transmission (700-R4 I think). The engine has been rebuilt by Jasper engines here in Georgia, as well as the transmission overhauled at the same time, approximately 8 years ago..The vehicle has been sitting, and or driven very little since...The little truck starts, and drives fine, except on the interstate. It seems to be revving really high rpms at 55 mph, but may be normal for that truck, however, it will NOT shift into the overdrive, or 4th gear under 57 mph, and unloaded, or on even roadways. At 57 mph, it will shift into the 4th or overdrive gear, but will immediately downshift into 3rd if any grade, no matter how slight, is encountered, or if I increase the gas pedal even a tiny amount. I feel like, from my "internal tach gauge" in my mind, that the thing should go into overdrive around 50 mph, but refuses....If you Google the TV cable adjustment for that truck, it tells you how to do it, which I've done, and also says it should shift into overdrive at about 45 mph...NOT HAPPENING... I took the truck to AAMCO transmission after a long talk to the proprietor, and he seemed experienced, and assured me he could find the problem...He called later after I left him the truck, and said that the transmission was working properly, and they couldn't find any fault with it, and that it did appear to be freshly overhauled, and everything working as it should.. The 30 year tech that did the work, felt like the carb could be the problem, as it may not be working as it should, and that a new carb would do it ALOT of good....Bought a new carburetor, no help...shifting problem still there....TV cable adjusted and re-adjusted, no help. I called the carburetor people, says that the vacuum hose routing has to be correct in order for everything to work right....The Chiltons manual shows an "air pump" on that engine, which simply isn't there on mine....All vacuum hoses connected to best of my ability, with what I have, and engine runs perfect, but still takes ALOT of RPM's to get her to shift into overdrive....Tech also suspected that the rear end may have been swapped out for better "low end" performance....no idea, but previous owner swears he hasn't touched the rear end, and he bought the truck new in 1985. Im SURE someone here has had this problem, or is familiar enough to recognize what's going on with the truck, or to even tell me that this is normal for the transmission to take such high RPM to shift....Just doesn't seem right for it to take such high RPM to shift the darn thing...Does this sound like a weak engine output, or transmission, or rear end? Frustrated in Atlanta.....ANYONE HAVE A GOOD 4.3 ENGINE FOR SALE????:grrrrrr:

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    Oh yeah, the electrical connector coming off the tranny goes up to the firewall, then is not connected to anything....Is this connector important? A knowlegable friend at work says it's for the torque converter "lock up" circuit or something, but says it has nothing to do with the shifting, just "locks everything together" he says, so there's no loss of motion, or something like that.....Thinking of running 12 volts to the connector to see if it makes any difference...thanks in advance for any help guys.......doc
     
  2. b717doc

    b717doc New Member

    Attached vacuum hose to TSS sensor on left fender, a hose that actually had suction on it, drives like a new truck, all shifting normal, cruises at 70 mph!
     

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