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TH400 leaking, also how do I replace vaccume modulator?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by dawson444, Mar 28, 2002.

  1. dawson444

    dawson444 1/2 ton status

    Jul 5, 2000
    Likes Received:
    Raleigh, NC
    My tranny is leaking fluid somewhere on the pass side of the tranny. It only does it when it gets hot. I did have a small fire on the exhaust pipe from this the other day so I would like to get this fixed. I think it is either leaking from the vaccume modulator or the tranny cooler lines. I am going to replace the modulator, and cooler lines and see if that fixes it.
    Is there anything else on that side/area anyone can think of that could be leaking. I would like to get this fixed on the first try so I won't have to deal with anymore fires.

    When I do replace the modulator is there anything I need to do? Or just pull the clip off, slide old one out and slide new one in?

    Also does anyone know the size of cooler lines on the side of a TH400 to save me a little guessing when I buy new lines.

    BTW- it is on a TH400/203 in an 88K5 if it matters.
  2. Ryan B.

    Ryan B. 3/4 ton status GMOTM Winner

    Jul 18, 2001
    Likes Received:
    Dude. Why guess and do the shotgun approach trying to replace everyting trying to fix it.

    First wipe it all down clean! Drive it for a day and then crawl under there and look really good where its leaking from.
    Tighten all the pan bolts.
    Check the fluid level and make sure it isn't over full.
    If its leaking from the lines... the lines will most likely have fluid on them and dripping off of them. IF the lines right off the tranny are still dry after wiping it down and then driving it for a while then you know it's probally not coming from the lines.
    If you pull out the dipstick and the dipstick tube there is an o ring and if it is broken it will be leaking sometimes from the location you are talking about.

    The modulator is pretty easy like you said just takin it out and popping a new one in... but you might have to adjust the new one to get it close to where the old one was adjusted...
    You can just stick a really small flat screwdriver inside the vacuum hole in the modulator and turn the little screw inside there clockwise to make the shifts harder and later. /forums/images/icons/smile.gif

    Hope this helps ya some dude. /forums/images/icons/smile.gif
  3. Leadfoot

    Leadfoot 1/2 ton status Premium Member

    Nov 28, 2000
    Likes Received:
    Western Massachusetts
    While I agree the throw $$$ at it and start replacing parts until it's fixed approach is not the best in most cases, sometimes it can be the prudent thing to do. If the tranny lines and modulator are old, replacing them anyway (regardless if they are the cause of the leak or not) is probably a good idea and also A.) eliminates a source(s) of the leak and B.) gives piece of mind knowing the components are new. It's not worth losing the rig to a fire over a cooler line or modulator leak (if that is what is causing it now, or could cause it in the future). Neither the lines or the modulator are high $$$ items so I would say do it. It's just a matter of good preventive maintenance (especially on older rigs). There is nothing like being far away from home and have a rotten cooler line blow on you on the interstate. Yes it can be fixed, but what a PITA on the side of the road in good clothes. I would say do it regardless, but as Ryan B. stated ALSO clean the area good with something like Brake Kleen and let it idle in gear in the driveway and watch for leaks. If it doesn't leak, take it for a test drive (going through all the gears F and R) and then bring it back and inspect it again. There is no mechanical kickdown on that unit so you can rule that out as a source of the leak and the tranny selector is on the other side. If those don't cure it I would say it is probably the pan gasket or the front pump seal trickling down to the passenger's side (although it usually wets the front of the pan when it leaks as it drips down).

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