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700r4 not downshifting 4-3

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by marcus, Oct 7, 2002.

  1. marcus

    marcus Registered Member

    Dec 21, 2001
    Likes Received:
    England UK

    My fairly newly rebuilt 700 tranny is working great. It's been in the truck for about 1 month and seemed fine. Yesterday, after the forst long drive in it since the rebuild, I've noticed that the 4-3 downshift isn't happening at 'high' speeds. Basically, at anything over 60mph, the tranny shifts out of 4th into what seems to be neutral and the engine revs up high. When I ease of the accelerator, it drops back into 4th fairly quietly. This means that it's hard to keep a good speed up steep inclines. I normally cruise at 75mph.

    Also, when the tranny was rebuilt, it developed a leak from an oil pressure switch on the side of the tranny case. My mechanic couldn't work out what this switch was for, so he blocked the hole up. Could this be a related problem.

    Before anyone mentions it,the tranny is under guarantee, and the work will be free. I just get on well with my rebuilder, and living in the UK, he hasn't had that much experience with 700r4s and any guidance would be great...


  2. imiceman44

    imiceman44 1 ton status Premium Member

    Aug 5, 2002
    Likes Received:
    Sacramento, CA
    I am sure that eliminating that switch isn't going to help but I think your problem is in the lockup converter.
    I would wait for some more info though, there are more experienced people on this forum.
  3. Blue85

    Blue85 Troll Premium Member

    Jul 26, 2000
    Likes Received:
    Grand Rapids area
    Are you sure that it going into neutral and not downshifting in 3rd? The diesel trannies have really heavy governors, so they upshift at low rpms and tend to stay in the higher gears. Maybe you could try pulling the shifter from D to 3 and see if you get the same result.

    The oil pressure switch on the outside of the transmission case is not a factory item. Normally they are installed in the 4th gear pressure port to make control the torque converter lockup. This is done because the vehicle lacks the standard lockup controls or the owner doesn't know how to make them work right. MAKE SURE THAT THE LOCKUP IS OPERATIONAL. If this switch was used to supply voltage to the lockup clutch, you may have disabled it. If you don't have lockup, you will cook the tranny fluid in overdrive. If your transmission really is screwed up right now, this is likely the reason: overheating from non-operating TCC lockup. Here's how you can test it: Drive down the highway in overdrive at a steady speed. Keeping your foot on the gas, just barely move the brake pedal, like you want to turn on the brake lights but don't want the drag from the brakes. If the lockup is working right, this will bring the engine rpm up by a couple hundred rpm.

    If your transmission is in fact just dropping back to 3rd and not going into nuetral, it could be as simple as a misadjusted TV cable. If it is really going into nuetral...man...that thing is screwed up. Maybe a blown sprag clutch. Who knows.

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