1985 GMC S-15 pickup shifts into overdrive early

Discussion in 'GM Powertrain' started by b717doc, Apr 9, 2013.

  1. b717doc

    b717doc New Member

    I've just purchased a 1985 GMC S-15 "Sierra Classic" pick-up, and the engine and transmission has supposedly been rebuilt about 7 years ago, but the vehicle has been sitting pretty much since then. The truck runs good, and shifts just fine from 1st through 3rd gears, but will not shift into the overdrive gear until about 57 mph on secondary roads, and is horrible on the interstate...If Im doing 65 mph on the interstate, and encounter even a mild grade, the transmission shifts down into 3rd gear, and the engine, I assume is spinning around 5500 to 6000 rpm...NOT NORMAL! I've already adjusted the TVI cable per the manual (Haynes, and internet instructions), and am convinced that it's NOT the TVI cable... There are a couple of connectors hanging from the firewall that I have no idea where they go, as well as a few open vacuum ports (two from the "can"). As you probably know, the Haynes manual is not much help as far as wiring and vacuum diagrams, so Im basically stumped here....I HAVE to have a way to work, which is about 72 miles of all interstate, and I simply can't keep running the truck at 6000+ RPM whenever I encounter a slight grade....She seems to run in the overdrive gear just fine as long as Im on level ground, at 65+ mph, but once the vacuum increases from the incline, she kicks down into 3rd. If I let off the accelerator, she will then shift back into overdrive around 57 mph....If ANYONE can help me with this problem, I would deeply appreciate it...I just feel its something to do with the vacuum input to the transmission...This is URGENT, so please call me with any ideas...1985 GMC Sierra Classic 2.8 liter V-6...automatic transmission....Please call "Mark" at (404) 444-6077...again, I appreciate any help before I blow this engine up!!!

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    Nevermind, taking truck to AAMCO....says he is familiar with the truck and transmission and guarantees a fix by repairing or replacing the valve body...maximum cost is $375.....Seller says he'll pay half.....
  2. b717doc

    b717doc New Member

    UPDATE: AAMCO shop called, says they can't find anything wrong with the transmission....valvebody working just fine, but agree that the truck has a problem...The 30 year experienced tech drove the truck, says it's definitely NOT shifting correctly into overdrive, or 4th gear properly...Re-assembled transmission, no charge....He also said the carb has a "flat spot" on acceleration, but shouldn't have anything to do with the tranny shifting...1st, 2nd, and 3rd gears are as smooth as butter, but will not go into overdrive until around 58 mph, or about 6000 rpm....The truck WILL go into overdrive (4th), but takes ALOT of RPM....Called a few transmission shops before the visit to AAMCO, and all verify that vacuum is not an input to the tranny, just the TV cable, which is properly set...There is an electrical connector going into the transmission just above the speedometer cable, but I have no idea what its for, or what it does...If I follow the harness coming off the transmission up to the firewall, there is a connector that is not hooked up to anything...but the rest of the harness goes into a larger harness...The $30 "HAYNES" manual I bought, says a lot of real technical stuff I don't really understand, something about a TCC or TAC switch that has something to do with the clutches in the tranny....not really clear to me...but the diagrams in that manual are not a lot of help, and down right confusing....Left the truck overnight, going to get it here in a couple of hours, and will ask them about the electrical connector going into the tranny, and what it's for, and if perhaps the unplugged connector could be a problem...Im putting ALOT of faith in this forum, as it is the only and best resource for me, and wait eagerly for any responses and ideas...This situation is URGENT, as it is the only way to work that I have, 72 miles of mostly interstate....Next option is to go to car dealership and try to get financed for a new vehicle, which I really HATE to do......please contact me with any ideas at area code four oh four, four 4 4, six oh seven seven.....Oh, and the technician at AAMCO also said he strongly suspects the rear end has been swapped out with a manual transmission rear end to give the truck a better "bottom end" around town....I asked the previous owner, and he swears that the rear end is the original, and he hasn't touched it....He is the original owner, as he as the receipt where he bought the truck brand new in 1985.........call or e-mail me at mark.alexander@wnco.com
  3. b717doc

    b717doc New Member

    Its all good......thanks for all the input....

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