Maybe a stupid question, but could the vent be clogged?
Transmission woes
- Thread starter PotvinGuy
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- shifting th400 transmission
Maybe a stupid question, but could the vent be clogged?
Neshkoro
Well-Known Member
Potvin Guy, I don’t profess to know anything about automatic transmissions, but there must be some connection between the lever hitting the pan while trying to get it into park and your other problems. You have to know that the pan didn’t get shallower and the lever didn’t get any longer. I didn’t read back on some of your posts but I seem to remember you saying something about it didn’t seem to shift into the correct gear. Could it be that the lever has come loose from its shaft and causing it not to shift into the correct gear completely? There was some mention about the fluid getting foamy maybe causing the level appear to fluctuate.
Just thinking. It’s strange that it used to work and now it doesn’t.
Just thinking. It’s strange that it used to work and now it doesn’t.
fletcherson
Well-Known Member
Is the modulator valve ok? If the diaphragm was leaking it could introduce negative pressure, aka vacuum, to the case. Easy to test.
No, the vent is actually spurting fluid into a catch can I set up.
No, the glass has a small hole at the top so it is open to the atmosphere.
An interesting idea. I have a new one I can swap in and see if that makes a difference. But usually when the diaphragm is leaking, fluid is sucked from the tranny and appears in the vacuum line. Mine is transparent and no fluid is seen.
Again, I'm unqualified to judge, but if the device that is supposed to vent to atmosphere and equalize pressure is full of fluid, it's not venting. (Don't get mad at me; I'm trying to understand.
)
The transmission is vented from the factory-the small tube in the upper case that turns down.
The sight glass has to be taller to account for the drain back from the pump and T/C or it will puke fluid until it find its level in the case.
I think he is asking is the CASE vent stopped up--dirt mud dauber.
Take the parking rod off the front actuator and remove from tranny. Then shift through gears.see if it goes from park to low. With pan off, can you see the manual valve in the valve body move back and forth? Valve could be broken. If it does look towards rear of tranny to inspect where the rod with the spring end goes to see if the pawl is out of place or broken.
If that fails, rebuild/replace
The man who invented auto trans wet crazy trying to figure out all that could go wrong with it.
Beat it with a big ass hammer and replace before you go crazy .
If that fails, rebuild/replace
The man who invented auto trans wet crazy trying to figure out all that could go wrong with it.
Beat it with a big ass hammer and replace before you go crazy .
Gerry
Well-Known Member
Do everything above and let me know. If you could sent you T over here we would be more than happy to fix it, but unlike most professional auto shops we will not commit to running the hell out of it on the 12 to 15 road tests it will need to ENSURE its set up and performing as it should.
Come on you can solve this. Its going to be something silly or it needs a rebuild. Hard to tell form 3000 miles away.
Come on you can solve this. Its going to be something silly or it needs a rebuild. Hard to tell form 3000 miles away.
Some new info to bewilder us.
1) The Parking issue has resolved itself. The tranny shifts easily into P and I checked to make sure the tranny output shaft is indeed locked. I have no explanation for this. Perhaps something mechanical is loose or worn and the problem will recur, in which case I will drop the pan and check it out.
2) Here's the way the level behaves:
Hot and running, the level is a good inch below the pan lip, but I have the deep Derale pan and pickup, so this should be OK. The tranny goes into all gears and drives smooth; what more can one ask? When I shut it off, the level in the glass soars to the top and pukes a little out. I need a taller glass, or maybe it will stop eventually. Then when off and sitting (still hot) the level in the glass is about 5-1/4" above the bottom of the pan, or 2-1/4" above the top of the pan. I'm guessing this is normal, as all the excess fluid in the TC and tranny has drained back. Does that sound right?
Oh, this was done with the dipstick out, to eliminate any possibility of air pressure in the tranny. I still need to repeat the tests with the dipstick in place. But where would air pressure come from anyway?? Just maybe...aeration caused by too much fluid?!
And the puking from the vent has dropped to almost nothing. I still have the tube hooked to it and will monitor the situation.
So...what have I learned? Maybe all this madness was simply due to too much fluid! And the dipstick seems useless; it reads at the ADD point regardless of the actual fluid level. I can't explain that. The motor and tranny are perfectly level. The dipstick is a Lokar unit. Go figger.
I'll continue to test and report. Thanks to all for the thinking and suggestions.
1) The Parking issue has resolved itself. The tranny shifts easily into P and I checked to make sure the tranny output shaft is indeed locked. I have no explanation for this. Perhaps something mechanical is loose or worn and the problem will recur, in which case I will drop the pan and check it out.
2) Here's the way the level behaves:
Hot and running, the level is a good inch below the pan lip, but I have the deep Derale pan and pickup, so this should be OK. The tranny goes into all gears and drives smooth; what more can one ask? When I shut it off, the level in the glass soars to the top and pukes a little out. I need a taller glass, or maybe it will stop eventually. Then when off and sitting (still hot) the level in the glass is about 5-1/4" above the bottom of the pan, or 2-1/4" above the top of the pan. I'm guessing this is normal, as all the excess fluid in the TC and tranny has drained back. Does that sound right?
Oh, this was done with the dipstick out, to eliminate any possibility of air pressure in the tranny. I still need to repeat the tests with the dipstick in place. But where would air pressure come from anyway?? Just maybe...aeration caused by too much fluid?!
And the puking from the vent has dropped to almost nothing. I still have the tube hooked to it and will monitor the situation.
So...what have I learned? Maybe all this madness was simply due to too much fluid! And the dipstick seems useless; it reads at the ADD point regardless of the actual fluid level. I can't explain that. The motor and tranny are perfectly level. The dipstick is a Lokar unit. Go figger.
I'll continue to test and report. Thanks to all for the thinking and suggestions.
fletcherson
Well-Known Member
They usually produce white smoke too, just a thought with your pressure phenomenon.
fletcherson
Well-Known Member
I was reading comments on another site and a poster had a similar issue, fluid blowing out of their th400. Of all of the comments, there were several from intelligent, experienced sounding posters and the consensus was the problem could be caused by a plugged vent, overheating, obstructed cooler or line, or it is related to a leaking pump gasket or failing pump, allowing cavitation. Just thought of your issue and wondered if you had any luck.
Hi Fletch, thanks for the ideas. I've had no luck. It must be the pump. I've called 3 tranny shops, but when they hear "hot rod" they hang up. I just want someone to swap in a newly rebuilt 400. I'd do it myself, but one really needs a lift, jack, etc. People today, even mechanics, can't seem to think or function without a computer. Or maybe it's fear of liability. They think we're all crazy lead-foots.
fletcherson
Well-Known Member
You are right. It is frustrating trying to get stuff done these days. It is worse when we have knowledge but are dismissed by the new generation of mechanics for whatever reason. I have done most everything for myself, even for most friends, family, etc over the years. It’s not so easy now with the baggage of age and injuries. Very hard to find quality, reliable help. That is true for various other forms of work, not just mechanical. Too bad that we, as a group, aren’t closer geographically to help each other through these hoops! Harbor Freight sells a decent, budget friendly transmission jack if you can perform the swap with the beast on jack stands and are capable of wallowing around under it. I used to crawl under and swap em all the time in my youth... I have done it in the past several years, but it is hard on me physically.
JetMech727
Active Member
I here ya fletcherson. It's the self proclaimed experts that think they know it all after watching a 6 or 8 min long YouTube video that irritate the crap out of me.
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