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TH-350 slipping in first...mostly......

vandelay industries

vandelay industries

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1) 2WD

2) Trans is new (reman). Only a yr old with no more than 10,000 miles

3) "Regular" TH-350, not TH-350C

4) The trans at first felt like shudder-slipping, that is more of a fast pulsating slip in First, rather than a constant slip.
a) This "pulsating slip" only started last sunday after i had done some, let's say, "acceleration tests." Full throttle, pedal flat to the floor, which i never do because the 4.3 doesn't have any power.

5) So, today, i ASSumed it was low on fluid and added about 1/2 bottle/2 pints.

a) It then and now slips a lot in first, and a little in second and shifts real soft (not complaining about soft shifts, just saying). While cruising at highway speeds seems ok, no slip.

b) Makes no difference if doing manual 1, 2, D etc.

c) i then checked the fluid the right way: Fluid level is now good----i had to add another 1/2 bottle.

d) If i only use 1/4 throttle or less, it seems ok, no slip. But anything more = slipping.

6) Reverse is just fine.

7) During the "acceleration testing," the throttle actually stuck open a little. This may be a cable or carb issue, i'm not sure yet. But returned to normal after a few miles.

BUT, i noticed that the kickdown cable was unconnected/ (became unconnected?) from the throttle arm, possibly from these "acceleration tests."

Could the kickdown system have anything to do with the slipping, since this all started happening rather suddenly?
 
Have you checked the vacuum modulator?--if its diaphragm failed,it can cause slippage ,soft shifts and other issues..make sure its getting good manifold vacuum too,if its hooked to a ported vacuum source it will cause issues like this..

The kickdown cable only engages "passing gear",if it isn't connected it will not affect anything as long as the cable isn't stuck in the "pulled out" position,in that case it would refuse to upshift..

I had more than one TH350 that didn't have the detent cable hooked up and it didn't seem to affect them other than you not being able to mash the gas pedal and hit passing gear,instead I had to manually downshift ..
 
i haven't checked the modulator yet, because i refused to believe it went bad after only a year (this is a top trans company), but i will in a few mins.

i'm basically hosed at this point as this is my daily driver. Oh well, i had my eyes on that 2012 colorado crew cab with the 5.3 LS on carvana...........
 
Start engine,remove the vacuum hose,put your finger over the end of it--strong vacuum should be present..if you have a vacuum gauge ,it should get at least 15" at idle,and drop when you open the throttle rapidly..
Look inside the hose for ATF--if any drips out of it,or the modulator has any in its "can",then its junk..some transmission shops use "generic" modulators that fit "all" TH350's and TH400's,and they aren't the best quality..

Modulators can fail in other ways too,like the internal spring breaking..
Usually when one takes a dump,you'll have to either get up to 40+ mph to get it to upshift into second gear,or shift it manually from first to second--then it'll delay going into third until its winding out pretty good in second--then once its in third gear,when you let off the gas,it will feel like it free wheels,instead of having engine braking ..
 
But if some ATF comes out of the modulator's side that plugs into the trans, then that's ok?

Just not the "can" side or the line running to the intake manifold, obviously?
 
P.S.
1) It now slips in all gears. When accelerating from a stop/going up hills and/or if more than 3/4 throttle when cruising. Not terribly slipping, but like you have a hi-stall 3000rpm converter or something.

2) It also engages reverse just fine, but engages pretty hard, which just starting happening 1-2 days after the above, original symptoms. But this would indicate that the converter is not the problem?
 
vandelay industries said:
1)
BUT, i noticed that the kickdown cable was unconnected/ (became unconnected?) from the throttle arm, possibly from these "acceleration tests."

Could the kickdown system have anything to do with the slipping, since this all started happening rather suddenly?
Click to expand...
Supposedly, the kickdown cable also boosts pressure near WOT, so having it disconnected could let you damage the direct or forward clutch. If there's a problem in plain sight, fix it first before making guesses at other causes.

vandelay industries said:
Actually, how would you test?
Click to expand...
If you have a mighty vac or other vacuum source you can hook it up and make sure the modulator holds vacuum. You can also replace it. As stated before, unplug the vacuum line from the modulator and put a vacuum gauge in it's place to verify you're getting good engine vacuum there. Sooner or later every rubber line cracks. You can also verify that the modulator is properly seated and the O-ring that seals it is good.

The other thing you can do is drop the pan and make sure the filter is properly installed. This will let you look for clutch material in the pan or other clues.
 
It's normal for some fluid to escape if you remove the modulator..not much,but have a catch pan handy..
There is a valve behind the modulator that can be removed and cleaned if its suspected of being dirty & sticking..

I'd talk to Greg Ducato,he is the man when it comes to automatics..
He may suggest you get a pressure check done using the plugged ports on the side of the case,that will give a better idea of what may be faulty..
 
1) Linkage has been checked and was/is adjusted ok.

2) The modulator checked out ok. i vac tested it and it was dry and yes you were right---they put a generic one on. In fact, i put the modulator on the 350 that was sitting in the garage (the trans that was replaced with this new one.) this modulator tested good too. Both modulators were the red stripe generics. And no difference.

Still slipping. Not that bad---the truck still accelerates and drives ok, just feels like i have a 4000 stall converter now. but i dont' want to drive it anymore if it will risk damage.

i think maybe the test port is the next move.

i'm basically hosed. The trans does need to come out irregardless as the converter is wrong for my application---i think 4.3 v-6 needs a looser converter than for v-8's, but i don't know if i have time for all this tinkering around.

This all just started saturday or sunday.
 
Is it normal to have to/ want to change fluid and filter on a relatively new unit----like with engines you should change oil and filter after break-in?

Because that seems to be the next thing to look at, but i don't see why it would be plugged so soon?
 
Greg will have better answers--I'm not a transmission expert by any means,I can only relate problems and symptoms my vehicles have had over the years..

So many things can cause one to act up,it could be practically anything,ranging from low fluid level,to low pressure from the front pump,valve body issues,even a bad valve body gasket or loose bolts can make an automatic seem "blown"..pressure regulator valves sticking,cracks in the case between the valve body passages,these are only a handful of things it could be.

I rather doubt there is much of a difference between stall speed on a V6 vs a V8,as I've seen used transmissions get swapped around off different engines like V6's to a V8 or a Diesel to gas engine,and they had no real noticeable difference --though the specs may differ,the seat of your pants dyno cant really tell the difference..

The converter could be faulty though,or its some other internal defect like clutches slipping..at any rate driving it any further will likely just do more damage..if you take the pan down,I'd look for evidence of wear in the form of brass & steel filings and black clutch material..
I would think the new fluid should be good for at least several thousand miles,but it is not a bad idea to drain & refill it after it "breaks in" if your really worried about any debris from the break in causing issues..
 
i can't get my 4.3 to idle in gear without setting the idle speed to 1400 rpm or so. And it still doesn't idle well at 700 in gear. That's why i think i need a looser converter. But that's a topic for another thread----seemingly?

2) But if reverse is fine with no issues then that would prove the converter is not the problem?
 
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If i could add one more thing (not sure how to summon brother greg.....), that just a few minutes ago i am jockey cars around because i'm planning on deactivating the truck and re-activating a car (which has it's own issues......).

But i noticed that if i put the truck in gear and just let it move on idle alone----no throttle (although, it's on hi-step idle), it will move no problem-----even uphill with seemingly no slip. This seems like a converter issue vs. a trans issue? Not saying it IS the converter, but this seems strange.......

i will say that in the past year i have had the converter up against higher than normal rpm (see my comments regarding hi-idle speed/ possible converter mismatch above) at traffic lights frequently. Could this possibly damaged the converter? The converter pulled the engine rpm down to 700 when in gear though.
 
I'd suspect a vacuum leak or other issue like an EGR valve not closing fully that is affecting the engine's idle quality..this can also affect the modulator,it needs a good steady vacuum supply..

I have seen a TH350 that someone hooked the modulator up to a ported vacuum source instead of manifold vacuum,and it caused it to slip past 1/2 throttle ,because a ported vacuum source delivers just the opposite of what it needs--it'll get zero vacuum at idle and at open throttle, the vacuum increases..
The modulator regulates line pressure by vacuum,it should get maximum vacuum at idle,and taper off as more throttle is applied..the less vacuum it gets,the higher it lets the line pressure go up..thus if its "backwards" it may reduce the line pressure enough to let the clutches slip under load as more throttle is applied ..

The fact the transmission seems normal in reverse does kind of suggest its not the torque converter,I would think it would do the same thing in all gears if it was...but again,I'm no expert..
 
I would make sure the vacuum line is hooked to direct manifold vacuum and that the line is clear of any restrictions. Using ported vacuum will cause low line pressure on acceleration and can cause slipping. A restricted vacuum line or one that collapses will act as a vacuum reservoir and produce low line pressure. Check the vacuum connection and line first before you do anything else. The next step would be pan removal to check the filter bolts....
 
Yes, the modulator is on manifold vacuum and has always had manifold vacuum. The line, as well as the rubber ends are clear and leak free.

Also, both modulator's plungers (you'll have to excuse my terminology as i don't know jack about transes) moved down at about 12hg and were fully retracted at about 20hg. And they both held for 30 secs at 20hg.
 
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