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transmission pressure question

Discussion in 'Chevy Trucks' started by Detroitdan, Nov 20, 2010.

  1. Detroitdan

    Detroitdan PlowSite.com Addict
    Messages: 1,937

    I had to replace the seal between the transmission and t-case, the t-case was filled to overflowing with atf that was pumped in from the tranny. No way of knowing if it was because the seal was bad, or if the seal was bad because the pressure got too high. So Idrive it today and after about 4 miles it apparently burped some fluid out of the dipstick tube, it appears that the pressure is still a problem. What could cause this? I recently changed the filter and fluid, but I have only put about 10 miles on the truck since I got it a year ago, so I know it was like that before I got it.

    Could a bad vacuum modulator or leaking vacuum line cause high pressure? What about a plugged up cooling line? Maybe the filter is in wrong, the 400s are kind of weird. But it was like that before I touched the filter too.
    I need to figure this out quick because I have to drive it again, and this time I need to go a few more miles than todays drive, and I'm afraid the fluid will catch fire when it burps out and hits the exhaust.

    This is in my 82 K30. 454/th400/np205.
  2. tac48

    tac48 Member
    Messages: 41

    The seal between the t-case and the trans does not seal pressurized fluid. It just keeps the fluid from flowing between the two.However, it sounds like your tranny vent may be plugged up allowing the entire trans to pressurize. There should be no pressure in the trans pan, which is were the end of the dipsick is. If the vent is plugged, the only other place for the trans to vent is out the dipstick or external seals like the one between the trans and t-case. To start with, you need to check the transmission vent on the top of the bell housing. There should be a rubber hose attacked to it, but it may be missing. The vent almost has to be plugged up or restricted somewhere there.
  3. Detroitdan

    Detroitdan PlowSite.com Addict
    Messages: 1,937

    well the trans and t-case are pretty dirty, there is a lot of crud in that area because the t-case vent was working and spewing the overfill coming out of the trans. So all the dust and dirt stuck in it and caked on. The truck was mostly driven on dirt roads, so it is mostly spotless except where there was some leakage, and that's all caked-on crud. So I got my fingertips on the vent hose and all I could do was snap it against the bellhousing a few times, hopefully enough to free up any junk in it.
    Trans turned out to not be overfull. Lot of places I found online said the th400 will spit out any fluid that you overfill. I ended up getting it warmed up and finding nothing on the stick, had to put 4 1/2 quarts in it. Here's what I think happened: when we changed the seal after the tranny pumped it's fluid into the t-case, I didn't realize it had pumped out the torque converter too. So I filled the pan up with 7 qts, but after running it and moving it around a little it got drawn into the converter, leaving the pan low. I don't know if a low condition could cause pressure, or a plugged vent is still at fault, but I rattled the vent and topped pan off and it is working, Also possible the dipstick wasn't seated in the tube. Drove it a few miles with no problems, no burping, spewing smoke or slipping,

    I checked the t-case, it was not overfull so it seems like the seal stayed intact. In fact, there was nothing in it. Apparently after we put it back up some nitwit forgot to refill it. It's not leaking. Really lucky I didn't drive it much, and this spurt actually saved me by making me look into it. It was last winter when I worked on it last, so it sat for many months and I'd forgetten what else we had left undone I guess. With a half empty tranny and an empty t-case, I would have done some serious damage once I put it on the road. But all I did was move it to the new house, it will probably sit there for the winter.

    Once again my poor memory and attention span almost bit me. Oh well, I guess I'd rather be lucky than good!

    Thanks for the help.