TH350 question (1980)

Discussion in 'Transmission & Driveline Topics' started by phat80, Dec 14, 2017.

  1. phat80

    phat80 Veteran Member

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    Jan 4, 2013
    Victoria BC
    My trans is giving me some issues. It worked fine when I got the car. When I was redoing the engine I had the trans bench serviced at a local shop. After I got everything back together I just moved the car in and out of the shop, 20 feet. So is about 2 years I've been doing this in and out of the shop.

    The other day I had to take it a few miles up the road to another shop. It was not a happy camper! Shifted up to 3rd okay then it was like it was in neutral. I eventually got it to the shop, a few days later I drove it back same issue if not worse.

    So I took the governor out, it seems to okay. Today I had the Vacuum Modulator out and tested, it holds vacuum, although there is oil on the manifold side of the diaphragm...but it holds vaccum. I used a magnet to remove the spool valve behind the modulator. It was kinda sticky and not too happy about coming out.

    But here's the thing..there was no spring in the bore behind the spool valve. Now some diagrams I see show a spring, some don't. So is there supposed to be a spring in the bore or no? Hope somebody can help.

    PS checked oil level several times, all good in the hood. Can the pump cavitate? Doesn't reasonable but ya gotta ask.
     
  2. GoldenOne7710

    GoldenOne7710 Veteran Member Lifetime Gold Member

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    Jun 23, 2004
    Athens, GA
    Can you explain what it's doing a little more?

    Does it pull in 3rd but 'free wheels' when you come off the throttle? Does it do the same in 2nd if you have the shifter in the 2nd gear position?
     
  3. phat80

    phat80 Veteran Member

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    Jan 4, 2013
    Victoria BC
    Oh Goldie that's a good question. I was so freaked between actually having the car moving and derivable ( wondering if the brakes and steering were all put together properly) that it was kind of sensory overload when the trans wouldn't shift.

    Let me try and play it back through my mind's eye. Let see I pretty much idled down the hill out of the shop. Then when I got out on the road I started out in what I guess the trans selected as low gear. I was surprised at the power of the new engine.

    I just gently throttled up, it shifted into the next gear ( drive I guess) and at that point I had a bit of a grade to climb. As I throttled up the rpm climbed but car didn't seem to accelerate, I pulled it into 2nd and that seem to give it some traction (in the trans). after I got up the grade I shifted back to Drive and the same thing happened, rpm increase bit no speed increase.

    I pulled the car over and checked the oil level, it seemed to be low by 1qt. ( not sure it actually was in hindsight) ( added 1qt.). I had to get it to the shop up the road so I just put in 2nd and drove slowly. When I picked the car up I had to back it out of the shop and up a pretty good grade (about 20%), in reverse it was fine, seemed to have no problem moving the car.

    On the way back to where I work on the car 3rd was pretty much a no go, lots of rpm no speed. So I used 2nd which seemed to work but I "think" it may have been "slipping" in 2nd also because I do remember shifting into 1st at some point for a very short time.

    You know how it is when the excreta is flying off the fan blades. Things all seem to happen at once! hope that offers some insight. thanks for your interest.
     
  4. camaro71/holland

    camaro71/holland Veteran Member

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    Aug 27, 2003
    The Netherlands
    Sounds like a slipping third gear. Do you have the possibility to check the line pressure while in gear?
     
  5. GoldenOne7710

    GoldenOne7710 Veteran Member Lifetime Gold Member

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    Jun 23, 2004
    Athens, GA
    When you say you had the trans bench serviced...what exactly was done to it? If the pan was removed and the strainer changed, it's possible the strainer wasn't put back correctly and fell off. It provides the 'pick up' the same as the oil pump in the engine. If it fell off, the fluid can't circulate. The only way to know is to remove the pan and inspect.
     
  6. phat80

    phat80 Veteran Member

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    Jan 4, 2013
    Victoria BC
    Thanks Goldie. I will check with the trans shop to see what they did. I now know they did not remove, inspect and clean the governor and they did not vacuum test the modulator...not too impressed.
    [
     
  7. phat80

    phat80 Veteran Member

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    Jan 4, 2013
    Victoria BC
    I don't think I can at the pressure port plug but I'll have a look. Thanks for the reply.
     
  8. Twisted_Metal

    Twisted_Metal Moderator Staff Member Lifetime Gold Member

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    Feb 26, 2004
    Bloomington, MN
    There shouldn't be trans fluid on the manifold side of the modulator diaphragm.
    (Shouldn't be engine oil from the manifold either.)

    The only way it can get there is if vacuum is pulling it through the modulator. (Bad diaphragm)
    I would replace the modulator and check the fluid level again before you tried to drive it any more.
     
  9. phat80

    phat80 Veteran Member

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    Jan 4, 2013
    Victoria BC
    Twisted, I agree...a friend of mine said the same thing. If there is oil on the manifold side of the diaphragm it can only come from one place regardless of what a vacuum gauge says.

    I ordered a new modulator today it will be in Thursday, so by next week end I should have something to report. Thanks to everyone.
     
  10. Lou6t4gto

    Lou6t4gto Veteran Member

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    Aug 18, 2010
    Ocala, Fla
    what exactly IS "Bench Serviced" ?
     

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