1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

TH400 acting oddly

Discussion in 'Automatic Transmission' started by DoubleDingo, Mar 22, 2019.

  1. DoubleDingo

    DoubleDingo Vacuum Leaks, The Root Of All Evil

    Posts:
    2,779
    Likes Received:
    761
    Joined:
    Jul 3, 2012
    Location:
    A few hundred feet above sea level
    First Name:
    Shane
    Truck Year:
    1981, 1965
    Truck Model:
    81-C20 Silverado Camper Special-TH400-4.10s; 65-C20 with 4:57 gears and Borg Warner Overdrive
    Engine Size:
    Carb'ed Vortec 350; 1972 L48 350
    Rebuild? Or clean what can be cleaned and replace the old gaskets/o-rings? @HotRodPC @chengny @da_raabi or any other th400 guru that has some good info on the subject.

    Again, this is a low mileage, number's matching truck that has sat most of its life.

    It's only done this three times now, and spaced out about a week apart each time over the last few weeks.

    Leaving for work, truck still cold from sitting overnight, the pump would make a high pitched noise at higher rpm's, shift into 2nd it'd go away until the rpm's got high again, shift into 3rd and it'd go away. Next stop, drop it into 1st, take off, pump would make the noise when the engine revved, 2nd it would go away then come back at higher rpm's, 3rd it'd go away. Next stop, no more noise and it would shift normal. Even with the noise, after the second stop, which is about a mile of driving, it would shift normal whether shifting manually or letting the tranny shift.

    This morning I was backing out, and it was doing its shudder thing, so I stopped, put it neutral, then drive, neutral, reverse, and back and forth a couple more times before backing out. No more shudder. Taking off in 1st, the pump was making that noise but not as loud as the other two times, shifted into 2nd and it went into gear but not the solid 1-2 shift that a th400 is known for, and going into 3rd it was slipping. I put it in neutral while rolling and back into D, slip, back into N and then D, slip, drop it into 2, slip, D, slip. So I pulled over, put it in P, R, N, D and each gear was a little mushy. Reverse didn't slam like it was trying to rip the driveshaft out, and D didn't engage strong either. I was just down the street from the house so I was going to take it home and grab the Forester. When I went to take off again it was still slipping. Turned a corner and drove about another 1/mile to start back home, and the tranny was shifting normal again. So I brought it to work. No more issues.

    I don't want to grenade my tranny, and besides it's weird acting up from time to time, which has become less and less, it works great. Shifts are good, passing gear works, etc. I've read there is stuff I can remove and clean without removing the unit from the truck. How do I do that, or should I just pull the unit and rebuild it? I wonder if the new filter got plugged after running the conditioner in the tranny?
     
  2. HotRodPC

    HotRodPC Administrator Staff Member

    Posts:
    44,118
    Likes Received:
    3,642
    Joined:
    Aug 29, 2010
    Location:
    OKC, OK
    First Name:
    HotRod
    Truck Year:
    85
    Truck Model:
    Silverado K10 to K20 Conversion
    Engine Size:
    6.2 to Gas???
    SO did these problems start after a fluid and filter change? Or did you try the fluid and filter change to correct the problem. I can only make guesses but the shuddering sounds like a problem picking up fluid, and high pitched noise might be a pump going dry too.

    When you put the filter in did you by any chance double O ring the tube where it goes into the transmission? For some reason, many Th400's I've pulled apart had 2 O rings and I've seen before on certain sites where that's a fix for some problems.
     
  3. DoubleDingo

    DoubleDingo Vacuum Leaks, The Root Of All Evil

    Posts:
    2,779
    Likes Received:
    761
    Joined:
    Jul 3, 2012
    Location:
    A few hundred feet above sea level
    First Name:
    Shane
    Truck Year:
    1981, 1965
    Truck Model:
    81-C20 Silverado Camper Special-TH400-4.10s; 65-C20 with 4:57 gears and Borg Warner Overdrive
    Engine Size:
    Carb'ed Vortec 350; 1972 L48 350
    Shortly after I got the truck, which was almost 6 years ago now, I first changed the modulator valve because the thing shifted so quickly it was in 3rd within 50-100 feet. That's actually when I first signed up here and I believe you were the first one I communicated with regarding the modulator and kick down switch. A couple months after that I did a fluid filter change, and it did shift better. The truck has sat for very long periods of time for the last 6 years, with most of it being since 2016-2017 because of it running so poorly and me not able to pinpoint the culprit. Last Summer I noticed the shuddering after getting it smogged when I backed it in the driveway and noticed it acting weird. It sat again until the Fall when I started getting material to make garden soil, but during the sitting periods we got some decent precipitation here. From 2016-2017, actually it was probably sooner than that, it had massive vacuum leaks that I was unaware of until just about a month ago.

    Fast forward to the beginning of this thread, and that's when I discovered a lot of things that were in need of replacement or worked over, and fixing the vacuum leaks was part of it. Before discovering and fixing the vacuum leaks I added some Motor-Purr Tune-Up and it seemed to help very little if at all. Then I found the vacuum leaks and fixed them, which helped tremendously. Then after talking with Pop I added the sea foam trans tune. The motor purr supposedly evaporates after 48 hours, so I wasn't concerned about mixing since at least a week had passed. I ran the sea foam in it for a few days, than payday arrived and I got the filter kit and some fluid, along with another bottle of the sea foam trans tune. Reading the label and what the trans tune is supposedly designed to do, and with the fluid being mostly old, but not burnt, brown or stinky, just having condensation or the like, I thought the trans tune would help get that out. The filter kit didn't have o-rings on the filter, but a thick rubber grommet like you'd see on valve covers for a pcv valve. It fit on snuggly, and held in place.
     
    Last edited: May 10, 2019
  4. HotRodPC

    HotRodPC Administrator Staff Member

    Posts:
    44,118
    Likes Received:
    3,642
    Joined:
    Aug 29, 2010
    Location:
    OKC, OK
    First Name:
    HotRod
    Truck Year:
    85
    Truck Model:
    Silverado K10 to K20 Conversion
    Engine Size:
    6.2 to Gas???
    I"m talking about where the tube that the filter grommet goes onto, but at the transmission end of the tube, not the filter end. Most guys recommend putting 2 O rings on that tube where it goes into the transmission. The pump is sucking trans fluid through the filter via that tube. If that tube isn't sealed off good up there, then it's sucking air and less fluid. Same principle as a fuel line having a crack in it. Sometimes too if you don't have your filter attached right, the weight of the fluid filled filter will pull the tube down some. I mean, it's supposed to fit very snug and it usually but sometimes there are those occasions where it's not. It sux that you'd have to drop the pan to inspect it but it might be worthy to do since the shuddering very well could be a lack of fluid problem and the noise you're talking about believed to be coming from the pump.
     
  5. DoubleDingo

    DoubleDingo Vacuum Leaks, The Root Of All Evil

    Posts:
    2,779
    Likes Received:
    761
    Joined:
    Jul 3, 2012
    Location:
    A few hundred feet above sea level
    First Name:
    Shane
    Truck Year:
    1981, 1965
    Truck Model:
    81-C20 Silverado Camper Special-TH400-4.10s; 65-C20 with 4:57 gears and Borg Warner Overdrive
    Engine Size:
    Carb'ed Vortec 350; 1972 L48 350
    Well said sir. I think that may be it. I didn't know that tube had o-rings up top, thought it was built in so to speak. It did the slip/noise thing this morning for the first mile and was fine after that, except the shudder in reverse.

    If I shift between R-N-D enough times the shudders stops. The pump noise is only when it's cold and that stops within the first mile after first-startup-of-the-day.

    I plan to park it this week so I don't do any more damage to it. Hopefully I can get to it after work this week but I've been putting in extra hours to get a project ready for bidding/construction.

    I will be getting more fluid and some o-rings and another filter kit and will report back with photos of what I find.
     
  6. HotRodPC

    HotRodPC Administrator Staff Member

    Posts:
    44,118
    Likes Received:
    3,642
    Joined:
    Aug 29, 2010
    Location:
    OKC, OK
    First Name:
    HotRod
    Truck Year:
    85
    Truck Model:
    Silverado K10 to K20 Conversion
    Engine Size:
    6.2 to Gas???
    Hopefully that's it. I won't swear to anything, but that's certainly worth a try. Sounds like the transmission is fine being that the problems are intermittent or when cold, but seems fine otherwise. You'd think it had broken hard parts or other failures it would all the time. It's almost like the after it sits and the fluid goes back to the pan, it takes quite and effort to get it back up there and going again but once it's there then it does OK. Almost like it's just having a problem getting the pump primed but once it finally does then it's OK. Lack of getting the fluid up there quick enough I'd think could certainly cause that issue. It'll be interesting to find out. Good Luck with it. Fingers Crossed.
     
  7. DoubleDingo

    DoubleDingo Vacuum Leaks, The Root Of All Evil

    Posts:
    2,779
    Likes Received:
    761
    Joined:
    Jul 3, 2012
    Location:
    A few hundred feet above sea level
    First Name:
    Shane
    Truck Year:
    1981, 1965
    Truck Model:
    81-C20 Silverado Camper Special-TH400-4.10s; 65-C20 with 4:57 gears and Borg Warner Overdrive
    Engine Size:
    Carb'ed Vortec 350; 1972 L48 350
    Yesterday while visiting the parents for Mother's Day, I spoke to Pop about this. I mentioned the o-rings on the pick-up tube possibly being cracked and sucking air, and he agreed that is very possible. He also agreed that this being a transmission with only 76k miles on it, and that it mostly operates like it's supposed to, that it doesn't need a full rebuild. I asked about the steels and such, and he said to just get a gasket/o-ring kit and do a soft rebuild to just replace those items, and then drain out the torque converter while it's being tore down and put back together. So I'll be ordering a gasket kit. I saw one a couple weeks ago on ebay or amazon for $25. Then I can pull the unit, freshen up the gaskets, and get fresh fluid in it. When we were driving home I was talking about this with my wife and mentioned that I've gone through everything else, so now it's time to re-seal the transmission. I've never seen the inside of an automatic transmission, so this will be fun!
     
  8. DoubleDingo

    DoubleDingo Vacuum Leaks, The Root Of All Evil

    Posts:
    2,779
    Likes Received:
    761
    Joined:
    Jul 3, 2012
    Location:
    A few hundred feet above sea level
    First Name:
    Shane
    Truck Year:
    1981, 1965
    Truck Model:
    81-C20 Silverado Camper Special-TH400-4.10s; 65-C20 with 4:57 gears and Borg Warner Overdrive
    Engine Size:
    Carb'ed Vortec 350; 1972 L48 350
    I've heard mention of Trans-Go Shift Kits. Looked them up today and saw a few options available. Which one would be best? This is mainly a stock truck that will not be raced, but will haul and/or tow stuff at times. Thinking it would be an ideal time to install it while replacing the gaskets and such.
     
  9. DoubleDingo

    DoubleDingo Vacuum Leaks, The Root Of All Evil

    Posts:
    2,779
    Likes Received:
    761
    Joined:
    Jul 3, 2012
    Location:
    A few hundred feet above sea level
    First Name:
    Shane
    Truck Year:
    1981, 1965
    Truck Model:
    81-C20 Silverado Camper Special-TH400-4.10s; 65-C20 with 4:57 gears and Borg Warner Overdrive
    Engine Size:
    Carb'ed Vortec 350; 1972 L48 350
    I inquired here, and also from a transmission parts supplier, and the three responses I got were to go with the Transgo 1-2 Kit. In a few weeks I'll get the parts and refresh the transmission.
     

Share This Page