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TH400 shifting late and harsh

ToWmaterK5

ToWmaterK5

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I just converted my rig to TH400auto & doubled cases. It shifts on the road 1 thru 3 late and then shifting suddenly/harsh. My hallucination: "I did get it to shift into 4th gear on the road only after getting my speed maxed out and backing off the pedal. But, can't get it back into 4th again", so I'm doing 45-50mph in 3rd at 4000 RPMs.
Its shifting and works, but I'm going too far between shifts at high RPMs.
The transfer cases was in high range(203) front axle neutral, rear axle High(205). The vacuum line is connected to the Quadra-Junk carb. Could it be I'm not generating enough vacuum?
I do have a RV cam...its nothing/not a huge racing cam or anything.

Any ideas where to start?
 
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A TH400 doesn't have a 4th gear. What RPM does it shift at? If it's way up there i would suspect a bad vacuum modulator valve.
 
X2
TH400 doesnt have 4th gear.
TH700R4 doesnt have a vacuum modulator.
Post pics of your trans.
 
as stated before you should only have 3 gears if you have a rare lockup style 400 you may mistake lockup for 4th gear but you would only see a drop of about 200rpm with lockup. delayed shifts are going to be from your choice of vacume source
 
BowtieBlazer said:
as stated before you should only have 3 gears if you have a rare lockup style 400 you may mistake lockup for 4th gear but you would only see a drop of about 200rpm with lockup. delayed shifts are going to be from your choice of vacume source
Click to expand...

TH400 never had lockup.
 
ToWmater, put down the crack pipe! Then start over on this thread with info that makes sense.:D
 
:( I deserve it :haha:
I tried to make it back and edit that before someone got it and ran! Yep 3 speed...But, I could have sworn it went into a higher "overdrive" like gear when it was pinned.....blame it on an exaust leak :wink1:

:crazy: With the late and hard shifts...other than bolting it up and running the vacuum line is there anything else with the TH400 that I'm not doing.
The modulator is new and the trans had been rebuilt. I have a tube going from the carb vacuum to the modulator on the trans. It up shifts 1-3 just a little late and hard.
I wonder if the rubber hose that was put in is collapsing or what would be another good vacuum source (I'm currently using the vacuum on the Quadra-junk carburetor.

:crazy: Another problem: If I'm trying to crawl a steep bank, the trans doesn't engauge even at 3000 rpm. The motors taxed and trans is straining but not going anywhere.
The stall converter is a stock GM converter (?2,500 stall)(used big block motor).

this is the best trans pic that i have now:
tm02.jpg
 
You mean it won't downshift on the steep bank? The late/hard upshifts could be adjustment on modulator, if it's adjustable. That adj would be by small screwdriver through the vacuum port. Sorry about the flame, couldn't help myself.:D
 
Oh, also forgot, there's probably a spring behind the modulator, assuming the same as th350. Bear with me, it's been awhile since I worked on these. Stronger spring= lower shifts. No spring = high shifts.:doah:
 
Ok, first you probably bought an adjustable modulator valve and as someone already said you adjust it with a small screw driver through the vacuum fitting.

Next, we needs pics of your set-up with all crossmembers installed. :D
 
TH400 has an electic kickdown the attaches to the throttle. Without it the trans wont downshift at hard throttle on its own, try manually downshifting it...thats what I do.
Make sure you have good vaccuum to the modulator. If not it wont upshift at a normal rpm and may damage the trans.
 
throttleissues said:
You mean it won't downshift on the steep bank? The late/hard upshifts could be adjustment on modulator, if it's adjustable. That adj would be by small screwdriver through the vacuum port. Sorry about the flame, couldn't help myself.:D
Click to expand...

I deserved it...
I got it working and its sweeeet!
I'm only admitting to this because I'm so happy it fixed my problems :D
The whole triple shifters are new to me and we'll :doah: what I thought was low on the 205 was high. so, that fixed the whole sitting on the hill at 3000rpms and not going anywhere. Found the lows and crept up the steep bank at 1000rpm.
Got it out on the highway and now ran it in the high range and she ran great/shifted great and rolled down the road at highway speeds!

Thanks :bow: I'm going to Rehab now.
 
4X4HIGH said:
Ok, first you probably bought an adjustable modulator valve and as someone already said you adjust it with a small screw driver through the vacuum fitting.

Next, we needs pics of your set-up with all crossmembers installed. :D
Click to expand...

IMG_1976.jpg


IMG_1977.jpg
 
Do you have rubber mounts between the frame and crossmember or is the crossmember solid mounted and rubber between it and the cases? The reason i ask is, if the crossmember doesn't have rubber between it and the frame i would be worried that you'll either break one or both of the adapters or something on the 205 case with the way you have that support off the rear passenger side of the case.
 
There are two poly bushings between the 203/205 adapter and the support. All other mounting points are metal/metal. The 205 is bolted directly to the support with a side bracket.
 
IMHO you need to have some kind of rubber between that rearmost 205 bracket or somethings going to break.
 
Thanks for pointing that out!

The 205 mount will be getting redone & some bushings added as well.
 
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