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Wierd TH400 Problem

Discussion in 'The Garage' started by cbbr, May 19, 2005.

  1. cbbr

    cbbr 1 ton status GMOTM Winner

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    Driving down the interstate between 55 and 75 mph the engine will begin to howl. I'm not nailing the gas, just driving along. This happens quickly, but not abruptly. It's as if I hit the gas to pass someone and the tranny is shifting down into a passing gear.

    If I had OD, I would swear that the tranny had downshifted or was in a passing gear. Of course I do not have OD.

    I have slowed down (it sticks in the "passing gear") and shifted into 2nd, which works, so I don't think that the tranny is shifting down. There is a noticeable drag (almost like being in 2nd) when this happens.

    Nothing else seems wrong, although this did start after my last run on the rocks. I have driven the truck about 5,000 miles since then, but never noticed it before so it may or may not be because of something done on that run.

    '90 Burb. 350/TH400. 100,000 miles. All original drive train. No smoke/burning smell.

    WTH??????
     
  2. fatboy

    fatboy 1/2 ton status

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    could be your clutch fan engaging and causing the howl. I have heard that before. :)
     
  3. fatboy

    fatboy 1/2 ton status

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    also that will create some addition drag on the engine making you think it shifted down :D
     
  4. Eric M.

    Eric M. 1/2 ton status

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    Mine did the same thing last week and it only has 9K on a rebuild. I thought high gear was going. I took it to the trans shop that rebuilt it and he checked it out. He couldn't get it to happen when he test drove it. He pulled the valve body and cleaned it and put it back together. I haven't had the problem again, so he is thinking some dirt got stuck in one of the valves reducing the pressure in 3rd gear and the problem was cured when he cleaned the valve body. I'm guess with 100K on your trans, you're not going to be so lucky. The symptoms your describing is usually what happens when a trans reaches the end of it's like

    I doubt it would be the clutch fan. The clutch does not "engage" and "disengage" like an A/C pump. It has a viscus coupling (usually silicon) that will increase and decrease in viscosity with heat. The reaction is gradual, not something that happens all of a sudden. If the clutch is bad (usually the silicon leaks out), it will not engage much at all and the berings will start to wear as the silicon acts as the lubricant for the berings as well. Sometimes it can lock up when the berings are going. Best thing to do is grab the fan and see if there is any play in the berings. It should push in and out a little, but no side to side movement. Spin the fan and see if there is some resistance, that will tell you if the silicon is still in there.

    Good Luck,

    Eric M.
     
  5. nvrenuf

    nvrenuf NONE shall pass! Premium Member

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    Any chance you've got a tach where you can check for an actual rpm jump?


    FWIW, I have experienced a fan clutch failure where it locked up suddenly. I was on the interstate when it happened and it created a load roar that came on fairly quick. With mine the fan would not turn when the clutch was hot but would free up *some* after it cooled off.
     
  6. cbbr

    cbbr 1 ton status GMOTM Winner

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    No tach. Would be nice, but no. I just replaced the clutch on the fan - it went out and the engine was overheating.

    I really don't know much about AT's - how do you check the valve body (or is that something that I need to take it to a shop to get checked?)
     
  7. Eric M.

    Eric M. 1/2 ton status

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    I don't know enough about AT's either to tell you. I'd like to learn ... I'd say take it to a shop and let then test drive it.

    If you can pick up a cheap tach and just hook it up temporarily like someone above suggested. That might be a good start just to make sure the trans is slipping and it isn't some other sound your hearing.

    Eric M.
     

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