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T-400 Issues - Hunting gears and slipping?

7K views 20 replies 3 participants last post by  Torque454 
#1 ·
My TH400 that I swapped into my 79 C-10 is giving me problems. There appears to be a loud whining noise, like a low power steering pump level but its not the power steering pump. The trans was low on fluid (it leaks out more than i realized when I had the speedo housing out changing the gear and housing) and it was run that way for a while. It wasnt low enough to cause issues but it was indeed low.

Now ive noticed that the transmission is either jumping in and out of 1st gear or jumping between 1st and 2nd. It does it 3 or 4 times and then finally stays in 2nd. I dont know if its slipping or shifting into neutral or what, maybe a little of both.

This is an old transmission, its originally from a 78 suburban and it sat outside of a truck for about 3 years before finally getting installed in this truck. It worked great for a year or so tho its not hardly ever driven. I have never changed the transmission filter or fluid and i've had possession of the transmission (and the original truck it came in) since 1999 or 2000.

Is it possible that the new fluid has cleaned up the gunk in the transmission and that it just needs a filter change (and changed again several more times after that to clean it all up) or does it sound like there is another problem?

I dont know if i could rebuild the transmission myself or not, i've never done transmissions or rearends before but i've done some engines. I am hoping its a clogged filter and not a clogged valve body or something. The fact that there is a whining sound bothers me tho. Either the transmission (pump?) isnt getting good flow of fluid due to a clogged filter or its going out. Hoping its just not getting good flow, lol. I dont have the money for a transmission right now.

Btw transmission has around 120,000 miles on it, most of which are all highway miles. Fluid smells fine and is red, not burned smelling or black.

Any ideas?
 
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#2 ·
It doesn't sound good. You could change the filter to see if it helps and do a pressure check on it. That should tell you if it's going out. Normally once they start whining they don't stop.
Pressure Port is about middle of Pan, a few inches up on driver side.
Brake applied @1000rpm.
N @1000 = 55-70psi
D @Idle = 60-85
D @1000 = 60-90
2 @1000 = 135-160
R @1000 = 95-150
D @1000 w/Down Shift Solenoid On= 90-110
 
#3 ·
I though that the whining was from the pump working hard to pump fluid and that it would quieten down once fluid flow was restored. It would make alot of sense in this case - with the old transmission and new fluid put in it. That would clean it up and plug the filter, causing the problems I am having.

I am just trying to talk myself out of it being the transmission itself. lol.
 
#5 ·
check for plugged filter first , that is a deep pan , there is a filter extension , umbrella seal at filter - o-ring in the case end if the o-ring is missing ,broke or just plain old and hard the trans will pull air and make the wining noise . old time trick was to double o-ring the case end.
 
#8 ·
I wanted a wix but the parts store didnt have it. They gave me a "Power Torque" brand filter. Looks like a nice filter but who knows. Ill find out how to get that extension tube off and check that o-ring. Ill probably need a new one for sure (the o-ring) if I mess with it. lol.
 
#15 ·
Yes its there and was tight. Filter still flopped around tho. Is that right? The only thing holding it up and in place at all I guess is the pan. Filter element is black but doesn't look plugged up really. Still going to check that o-ring but i think I may let the trans drip all night and then tomorrow i'll wipe things off so i dont get drips all over me and then ill pull that tube and check it out. The fluid wasn't clear but it was still red. Some evidence of water in the pan at the very bottom, but not much. Didn't look like it was being pumped through the system. Some grey gritty stuff in the very bottom, some metal shaving it feels like but just some not much, i think its pretty normal especially for an old transmission. No chunks tho and the fluid smells fine.
 
#19 ·
Id like too because alot of it is new fluid (that was poured into a dirty transmission) but when I drained it i drained it into a pan that had little bit of oil in the bottom of it and some dirt and leaves.I didnt get a chance to clean up the pan before I used it, i got a little ahead of myself. I had planned on keeping it so i could reuse it. oops.
 
#21 ·
It appears that changing the fluid and filter fixed it. I am thinking that the filter had come loose from the pickup tube. It has been installed and removed while attached to the engine from 3 different trucks on several different occasions and then moved around several times while outside of a vehicle.

The lesson here I guess would be to drop the pan, clean it up and change the filter after its been out of the vehicle.

The filter element was black - but didnt have anything stuck to the filter that you could scrape off with your finger so I dont know that it was plugged. I think the filter was just barely hanging on to the pickup tube and was sucking air around it rather than sucking fluid through it.
 
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