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my 3rd gear is going out?

68MUDSTUD

68MUDSTUD

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Problem: for a couple years now this has been happening... when cold, get in after engine warms up and pop her into drive and head out, about a mile down the road after it shifts into third, it starts lugging, like im pulling the slack out of a tow rope and bouncing it down the road, then, if you floor it so it shifts down, it shifts, but seems like its slipping, rpms go way up, then down. then let off, and it drives fine.

The other night it wouldn't shift into third coming home from town. wtf is going on? Thanks in advance!
 
T-350 ?

T-400 ?


could be a valve body piston sticking or broken spring.....if a piston is stuck or floating around in the valve body due to a broken spring.... that's not good....

trying to engage two clutch packs at the same time is not good...

I would drop the pan and have a look ...quick.
 
its a th350. I'm not a tranny man so what would I be looking at? Or, for?
 
replacing the whole trans would fix it but the tailshaft on those is WAY too long.

you would end up having to swap a different tailshaft depending on what T-case you have.

This is for that blue K5 right? is it a 205 Tcase?
 
What about having someone rebuild yours? or maybe find a core? The Th350 is the same for trucks with either a np203 or np205 so a core wouldnt be too hard to find.
 
I'd have to find a way to get it to billings... and since I have no wheels, it would be a pain in my ass. I'd rather just get a brand new one. I could just reuse my old tailshaft, right?
 
yes, but to change the tailshaft, everything has to come completly apart.

The tailshaft is one of the first parts to go back into the case during rebuilding if I remember correctly.

I know Ive seen 4x4 versions for sale somewhere maybe summit or some other mail order place. its been a long time since ive looked for them.
 
You might just need a new modulator valve,which is located at the rear near the driveshaft--also if the vacuum hose going to it has come off or has a leak,it will cause it to shift weird...

When a TH350 gets no vacuum to the modulator or it fails,it will still upshift into second gear,though a bit later than usual and more harshly--you may not notice it or chalk it up to a shift kit--but when it goes to shift into third,it usually wont unless you get it up to 50 mph,or coax it in by pulling the shifter back into second gear,then shift into drive...then it'll upshift to 3rd,but will also free-wheel and feel like it slipped into neutral when you back off the throttle in 3rd hear....the modulator also controls line pressure and can cause slipping if its failed or aint getting any vacuum..

So--before ripping it out,I'd check the vacuum at the modulator ,the hose and any tubing for leaks ,and see if any ATF is present inside the modulator,which would indicate a failed diaphram in it--a peice of thick string shoved in the hose nipple on it will come out soaked with ATF if its junk...so far I've bought 3 trucks with "blown trannys" that were TH350's and needed only the vacuum hose fixed,or the modulator replced--one had a stripped govenor gear and it only had 1st gear & reverse...after I popped a used one in it,I drove the van to TN and back without a bit of trouble..and 40+K since!...

If the detent cable is way out of whack it could cause shifting woes going into or staying in 3rd too...
 
diesel4me said:
When a TH350 gets no vacuum to the modulator or it fails,it will still upshift into second gear,though a bit later than usual and more harshly--you may not notice it or chalk it up to a shift kit--but when it goes to shift into third,it usually wont unless you get it up to 50 mph,or coax it in by pulling the shifter back into second gear,then shift into drive...then it'll upshift to 3rd,but will also free-wheel and feel like it slipped into neutral when you back off the throttle in 3rd hear....the modulator also controls line pressure and can cause slipping if its failed or aint getting any vacuum..
Click to expand...
This sounds exactly like what it's doing, thanks man! Where's the best price on the net for a new modulator?
 
First, check the vacuum line and see if its hooked up. If it got knocked off, just put it back on and you're in business.
If it fell off because the rubber swelled, then the modulator is bad.

Common problem.

The diaphragm gets a pinhole in it, and the engine starts sucking small amounts of transmission fluid out.
That causes the little piece of rubber hose between the modulator and the steel line to swell and fall off.
Most people just either cut off the swelled end, or replace it and wonder why it keeps swelling.
Right up until the transmission starts slipping because the engine has finally burned enough of the fluid to make it low.
 
Well, changed the modulator valve out with the one in my 78, which is a different style, after losing about 2 quarts of tranny fluid.... :haha::doah:

the one that was in my blazer was a straight through design, pretty simple, looks kind of like a fuel filter, and the one in the 78 was longer, bigger around, and the hose attaches to the top, unlike the end on the previous valve. it also has a nut and screw on the end. Am I going to have to mess with this?

I would tell you if it worked or not but I need more tranny fluid. :doah:
 
That adjustment will change the speed ir upshifts at and the firmness of the shift,but its not that much difference really,just a few MPH and a bit harder shift...

They had numerous part numbers for modulators from the factory--some brands wanted to know the color codes or gear ratios to get the correct one--but tranny shops use one "universal" style that fits TH350 & TH400 trannys,and I haven't noticed any real difference..the main thing is to have a working modulator and have it get a good source of vacuum!....if yours has any fluis coming out of the hose nipple the diaphram has ruptured in it...I'm surprised you lost 2 qts of fluid,you must have had the front end jacked up or parked facing uphill??--usually less than a pont escapes when I yank a modulator out....
 
Had the same problem in my 80 camaro, never figured it out. Seeing as I could do 90-100 in second gear, I drove the car for 2 years like that. Fun times with 2.56's and 28's.
 
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