Rebuilt t-375 shifting very hard when engaging Drive but not when fluid low?
Last year I purchased a rebuilt THM375 tranny off the net. It was listed as rebuilt with a stage 2 shift kit and came with a 2500 stall.
I hooked it up to my rebuilt 350 with a tall cam (450 lift, 224 duration, 206 lobe centre). When first driving it I didn't like the stall so replaced it with a brand new 1500 stall so I wouldnt scare the neighbours too much (unsconstricted exhaust + high stall = loud!).
Everything works great except the tranny shifts really hard when you go into drive or reverse. It is not so bad if you let the entire package warm up good (i.e. 5 minutes in idle).
Now, I assumed it was normal and the truck has been running great for the past year. Tranny fluid is still pretty pink and tranny temp always runs normal but....
rear seal has started to leak a little and as the fluid went down the shifts have gotten smoother and smoother. Add more fluid back to normal zone and shifts are hard again.
1. I got the transmission without a dipstick and tube so I sourced one from a wrecker but he got it out of truck so he may have mistaken the 375 for a 375B (which is a totally different beast) so is it possible my hard shifts are due to too much fluid and when it went down it was more in the 'normal range'?
2. I like the smoother shifts at low tranny fluid (i.e. it's just below the 'add one quart' on the dipstick), is that hard on the tranny? (assuming the problem is not as noted in quest 1)
3. Is it possible that something is sticking (governor valve)?
4. Am I using the wrong stall for that cam and causing that hard shifting (I read that you should use a high stall with a tall cam)?
5. Am I just paranoid and the shifts should be this hard so just fix the rear seal, put fluid to normal zone and live with it :)
Thank You very much for any assistance.
That Cam needs at least 2500rpm stall. Operating range is 2500-6400rpm.
With the Engine running the fluid level should be even with the Trans Pan mounting surface. You should drop the Pan & check the level marks on the Dipstick.
The Bang Shift is caused by the Shift Kit, High Idle speed & Low Stall TC.
Higher stall TC should calm it down and help creeping in gear.
Thanks for the reply, the information on the dipstick is invaluable, could not find anything on the net on figuring out how long the dipstick should. Thanks!
I will work on putting the 2500 back in - bit more work as I have to drop the tranny.
One thing still does not makes sense to me, the big bang happens when I put the transmission in gear, forward or reverse. I would not think that the TC is in play yet at that point? No?
The idle is currently set at 750.
The torque converter is spinning as the engine runs.
As mentioned a higher stall rated converter will slightly soften engagement depending on the stall ratings. A 4500 stall converter would soften up shifts and engagement more than a 2500 stall converter would.
I think your shift kit is causing the promlem,the`builder` did not know what he was doing and removed or installed the checballs incorrectly to the valvebody and the line pressure is increased and does the hard engagement.
What is the point to engage the gear with full line pressure? :rolleyes: :rolleyes:
Yes I do know, basically, how a torque converter/transmission works but, to clarify, if it is a 1500 stall and your idling at 500 then when you engage there should be no transfer of power yet to the rear axle I would think, at least not to with such a hard slam....
Let me rephrase, is it normal with a shift kit like this to have this loud bang when putting it into gear?
I think TimoFinn explained it best by indicating line pressure is too high at gear engagement. That would then also explain why low fluid would not have as high a line pressure. At least that's what makes sense to me.
This weekend I'm going to replace the rear main seal on the tranny - leaking a bit. Replace the governor and valve and drop the pan and verify the dipstick. Then, once I get the correct markings on the dipstick I will ensure it is filled to the correct amount.
If, after that, it still slams so hard into gear then I will replace the torque converter with a 2500 and see where I'm at. If that still doesn't do it then I will take it to a qualified shop to see what they can do.
Thanks for all your continued help people.
Ok, I tackled all the potential problem areas except the TC, which is a bit more work. that will be done later on this summer.
On the dipstick. As noted below, the full mark should be where the mating surface of the pan to transmission is. Well, mine is definitely not there so before I go and remark the dipstick I want to make sure that i got it right.
The fluid level of a transmission at full-hot should be level with mating surface of the pan to tranmission - right?
Currently that is equivalent to 1/2 pint over hot-full on my dipstick.
I have included a couple of pics.
Pretty sure> Cold level/Engine running is even with Pan mounting surface, so yours looks good. Hot fluid will read higher.
Thanks SSedan64, I've sofar only measured it hot so will try it with cold engine and after a couple of shifts back and forth between R and D,2,1 to see what that gives me.
Not having any luck with this and I'm finding or getting conflicting information on how high the fluid should be. My old tranny shop even told me that it should be 1/4" ABOVE pan rail when a number of websites told me 1/4" below rail level (and of course here and others sites tell me pan rail level).
The 1/4" below rail level throws me for a loop as it just happens to be the exact full mark of my aftermarket; which my suppliers swears is what everyone uses who has a 400. jeez.
I'm going to repost and reword the title hoping to get some more people chiming in.
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