Transmission help

Discussion in 'The "Juice Box"' started by 71SkylarkAZ, Oct 20, 2015.

  1. 71SkylarkAZ

    71SkylarkAZ Well-Known Member

    Hey guys. When I drove in to work this morning (5 mile commute) I noticed my tranny wasn't down shifting like it should be. Any ideas? Its a 71 Skylark base model with 350 and AT. I don't know what type of transmission I have. I've only owned the car since last Dec. and am a novice to Buicks so I'm not familiar with the transmissions they have and if they even have a filter or not.

    Also, what type of transmission fluid should I be running? If I decide to do a fluid change what type and how many quarts do I need?

    I have been working on the car myself for easy change stuff but I may have to take it to my mechanic for this one as transmissions are way out of my league for working on them myself unless its a simple throttle cable or something.

    Thanks!
     
  2. 71SkylarkAZ

    71SkylarkAZ Well-Known Member

    I think I may have kind of diagnosed it, or narrowed down my problem. I believe I might have a vacuum hose issue. I drove it home for lunch and it sounded almost like a whizzing or hissing sound when it should be down shifting. It shifts fine when I get it up to about 55 MPH on the bypass but when I'm driving in the parking lot to around 25/30 MPH when it should do a low down shift it just doesn't.

    I saw this Edlebrock vid and might check my vacuum lines. I made some minor adjustments to the choke and idle screws so maybe that threw it off or maybe the vacuum hose is pinched or cracked or something :Do No:

    https://www.youtube.com/watch?v=vC3JbNCaOG4
     
  3. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    The transmission is a THM 350. It uses a vacuum modulator located at the rear of the transmission. There is a metal vacuum line with rubber on both ends. That line goes to the rear of the intake manifold close to the PCV valve, see below.

    [​IMG]

    Make sure the line isn't leaking at either end, manifold or modulator.
     
  4. Stagedcoach71

    Stagedcoach71 Well-Known Member

    It doesn't downshift in all conditions or just when you want to downshift into passing gear?
     
  5. 71SkylarkAZ

    71SkylarkAZ Well-Known Member

    Larry thanks! I'll try that tonight. I'm pretty sure I have a vacuum leak somewhere as I do hear a loud hissing type sound when the transmission should be down shifting.

    I subscribe to a few YouTube channels and think I'm going to go with Eric the Car Guy's way to diagnose it tonight. The engine will be stone cold by the time I get around to it so I shouldn't be at risk of a fire.

    https://www.youtube.com/watch?v=9CPqbaSgcok
     
  6. 71SkylarkAZ

    71SkylarkAZ Well-Known Member

    It doesn't downshift in any condition. I made a couple adjustments to the carb to make it run richer but I may have overdone it. I'll drive it between about 15 MPH and 55MPH before it will downshift finally. I don't have a tach so I don't know how many RPMs I'm running before it does finally shift but in between those speeds it makes a whizzing or hissing sound and just doesn't downshift like it should.
     
  7. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    You don't need to get that sophisticated. If you hear hissing and the transmission isn't shifting, I would remove the air cleaner, locate the vacuum line I referenced before, and simply see if it is connected or not. I think you may find that it simply worked it's way off the nipple, or the rubber deteriorated enough to leak. Either way, it should be easy to spot.
     
  8. 71SkylarkAZ

    71SkylarkAZ Well-Known Member

    I'm going to check around the manifold and such. Would a heater vacuum hose cause that issue as well? I changed out my heater core a couple weekends ago and have run the heater a few times since so I dont know if those hoses are intact or not, the heater works fine though.
     
  9. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    No.
     
  10. mrolds69

    mrolds69 "The Cure"

    I agree with Larry, if you hear hissing, it sounds like a hose problem. Doubtful the heater core has anything to do with it, but you might have knocked it loose working on that. If you don't see anything up top, look under the car by the vacuum modulator. That end may have been knocked loose where the short 90 degree hose is located.
     
  11. 71SkylarkAZ

    71SkylarkAZ Well-Known Member

    Ok. I'll definitely check out the hoses. They may either be off or may have just cracked with age. I'm hoping either is the case and its just a simple fix :pray:
     
  12. Stagedcoach71

    Stagedcoach71 Well-Known Member

    You may want to replace them in any event. I think you'll only need 5 or so inches of new vac hose.
     
  13. 71SkylarkAZ

    71SkylarkAZ Well-Known Member

    What if I'm super cheap and don't want to pay $2 for a new hose? :puzzled:

    I agree, I'll check to see what type of hose and length I need. I just replaced my power steering hoses last week as some preventative maintenance since the old ones were hardened with dry rot cracks in them.
     
  14. 71SkylarkAZ

    71SkylarkAZ Well-Known Member

    So I've checked the vacuum modulator line and it appears to be ok. It doesn't have a clamp on either end but if it didn't have one before why would it need one now right? I sprayed every line with the car running with carb cleaner and didn't notice any change in idle so I'm confused at this point :Do No:

    I might try to put a clamp on at least the top of the hose that goes into the manifold and see if that helps any. I noticed when I put the air cleaner back on it kind of hits that line a little. If I need to replace that rubber part how does it go back in? It looks like the metal line just goes into the hose which somehow goes into the manifold, there's a threaded fitting that threads into the manifold but I dont know how that rubber hose piece goes into that fitting.

    The end that goes into the transmission looks fine, its connected and I didn't notice any cracks or anything.

    The hose that goes into the lower driver side of the carb goes into a round black canister on the front driver side by the fan shroud. Another hose comes out of that canister and goes into something by the steering column. I don't know what the canister and those hoses are for but they dont appear to be cracked either, the hose from the canister to the column looks old and dry rot but I didn't see any major issue with it.
     
  15. 71SkylarkAZ

    71SkylarkAZ Well-Known Member

    Hold up. Larry was right, the hose at the transmission was disconnected. It doesn't have a clamp, not sure if there's a reason for that but after I re connected it and test drove it for 1/2 a block it came off again! It shifted awesome like it should have when I got it to about 20/25 MPH in the neighborhood. I plan on putting a clamp on it tonight :3gears:
     
  16. yachtsmanbill

    yachtsmanbill Well-Known Member

    First off, that empty tube under the steering column runs along the frame to the fuel tank vent/return off the carbon canister filter next to the radiator.

    Bear with me here as mine is a 1972 with a 455/TH400...

    The vacuum modulator hose was ported to the base of the carb by the PO (previous owner), and the manifold fitting was capped and plugged; note torn hose! The two ends of the modulator line are about 3/16" OD and slip fit into the fitting on the manifold and on the modulator. Its actually a pretty snug fit and the hose renders it "sealed" and prevents the two from separating. The barb on the tubing retains the hose if its "supple". Notice the bulge in the pic... NO CLAMP IS NECESSARY! The PO had the OEM tube so mangled it needed renewing with a brake line and compression fitting. Shifts swell now too!! ws

    [​IMG]

    [​IMG]

    [​IMG]
     
  17. 71SkylarkAZ

    71SkylarkAZ Well-Known Member

    So do you have that canister on yours?

    the hose on my manifold has a good seal. The hose to the transmission was pretty loose and where I needed the clamp. I'll have to put a smaller clamp on it some other time as the one I had didn't cinch it down as well as I'd like plus it was a pain to get to with a screw driver. I plan on getting the type with the key you turn by hand.

    I'm attaching a pic (bad pic sorry) showing where the hose was disconnected at the transmission.

    Vacuum hose.jpg
     
  18. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    The hose at the transmission can get soft with age and slip off. Easy enough to replace it.
     
  19. 71SkylarkAZ

    71SkylarkAZ Well-Known Member

    What type of hose is it? Mine seemed pretty pliable but not cracked or hardened. I was planning on getting one of those clamps with the key you turn to tighten since I had a clamp that was slightly too large and I couldn't tighten it with a screwdriver because of the angle.

    It drove great this morning with the fix though, no shifting problems at all :TU:
     
  20. yachtsmanbill

    yachtsmanbill Well-Known Member

    Use a nut driver or 1/4" drive U-joint socket probably 1/4" socket size. The hose may be supple, but oil soaked is gonna make is mushy. It still needs to do its job. When you pull the lower hose off, any trans fluid come out? You may have a bad modulator diaphragm... ws
     

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