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Rebuilt TH400 sluggish 1-2 shift.

Posted: Sun Oct 29, 2017 11:45 pm
by Roadknee
I recently rebuilt a TH400 for my '79 BBC truck. The core trans came out of a running '81 6.2 diesel motorhome. I bought a complete kit with all the bushings, seals, torringtons, Alto high energy frictions and regular steels. I bought new roller clutches and the kit to rollerize the rear thrust. I installed the TransGo 400 1+2 kit with the heavy clutch springs but retained the stock pump relief spring. I initially drilled the 1-2 hole to 0.0.095" and the 2-3 hole to 0.082" I assembled the trans using the ATSG manual. I installed it in the truck and got the following pressures:

Neutral 80 psi (spec 55-70)
Drive 80 psi (spec 60-90)
Drive "2" 165 psi (spec 135-160)
Reverse 140 psi (spec 95-150)

The trans seems to function well, upshifting and downshifting as it should. The WOT 1-2 shift is pretty soft. The WOT 2-3 shift is perfect with a good firm bump on the shift. I drilled the 1-2 hole to 0.113. It firmed up the 1-2 shift some, but it's still pretty soft. The BBC is a worn out 7.5:1 468 that makes 350 HP on a good day. I don't think the 1-2 feed hole needs to be any larger. Is there anything else I can check?

Re: Rebuilt TH400 sluggish 1-2 shift.

Posted: Mon Oct 30, 2017 9:21 pm
by 77cruiser
Did you air check it before you put it in?

Re: Rebuilt TH400 sluggish 1-2 shift.

Posted: Mon Oct 30, 2017 9:24 pm
by Roadknee
77cruiser wrote:
Mon Oct 30, 2017 9:21 pm
Did you air check it before you put it in?
No

Posted: Thu Nov 02, 2017 6:35 pm
by dwilliams
Did you write down the clearance of the 1-2 clutch pack? Too much can cause a soft shift.

Re: Rebuilt TH400 sluggish 1-2 shift.

Posted: Mon Nov 06, 2017 11:14 pm
by kirkwoodken
It's out of a diesel and you are putting it in a gas engined truck? Should be a completely different valve body than a gas car. Maybe a smart person will chime in. Never worked on diesel TH400.

Re: Rebuilt TH400 sluggish 1-2 shift.

Posted: Wed Nov 08, 2017 10:04 am
by 77cruiser
What did you do for the modulator? Diesels have a mechanical one I think, hooked to a throttle cable.

Re:

Posted: Fri Nov 10, 2017 4:20 pm
by Roadknee
dwilliams wrote:
Thu Nov 02, 2017 6:35 pm
Did you write down the clearance of the 1-2 clutch pack? Too much can cause a soft shift.
Clearance for all three packs was around 0.050"

Re: Rebuilt TH400 sluggish 1-2 shift.

Posted: Fri Nov 10, 2017 4:22 pm
by Roadknee
77cruiser wrote:
Wed Nov 08, 2017 10:04 am
What did you do for the modulator? Diesels have a mechanical one I think, hooked to a throttle cable.
This one has a modulator. I installed the new one from the kit.

Re: Rebuilt TH400 sluggish 1-2 shift.

Posted: Fri Nov 10, 2017 8:57 pm
by 77cruiser
Is the modulator adjustable?

Re: Rebuilt TH400 sluggish 1-2 shift.

Posted: Fri Nov 10, 2017 9:46 pm
by Roadknee
77cruiser wrote:
Fri Nov 10, 2017 8:57 pm
Is the modulator adjustable?
Yes

Re: Rebuilt TH400 sluggish 1-2 shift.

Posted: Sat Nov 11, 2017 2:25 am
by Geoff2
Modulator does not control shift quality, it controls shift timing.

Re: Rebuilt TH400 sluggish 1-2 shift.

Posted: Sat Nov 11, 2017 6:10 pm
by amcenthusiast
Ah, I'm almost certain to know what is causing your problem:

Apparently, the kit's recalibrated 1-2 and 2-3 shift springs are made to work with the kit's re-calibrated higher pressure pump spring?.. therefore the shift valves are not operating as intended by the maker of the kit?..

If I were you, I would install the shift kit's proper pressure pump spring ASAP to reduce slip with the new frictions and most likely cure the driveability problem as described.

IF you think most of the running clearances are 'good' and there are comparatively few 'loose' worn parts (like bearings or bushings), a thinner viscosity ATF may help to reduce parasitic drain on the pump caused by the stronger kit pump spring... hence a 'high peformance' ATF might not be a bad idea (study viscosity pros and cons and use your own best judgement)

Re: Rebuilt TH400 sluggish 1-2 shift.

Posted: Sun Nov 12, 2017 9:18 am
by 77cruiser
Geoff2 wrote:
Sat Nov 11, 2017 2:25 am
Modulator does not control shift quality, it controls shift timing.
Thought the modulator controlled line pressure too.

Re: Rebuilt TH400 sluggish 1-2 shift.

Posted: Sun Nov 12, 2017 12:24 pm
by Roadknee
amcenthusiast wrote:
Sat Nov 11, 2017 6:10 pm
Ah, I'm almost certain to know what is causing your problem:

Apparently, the kit's recalibrated 1-2 and 2-3 shift springs are made to work with the kit's re-calibrated higher pressure pump spring?.. therefore the shift valves are not operating as intended by the maker of the kit?..

If I were you, I would install the shift kit's proper pressure pump spring ASAP to reduce slip with the new frictions and most likely cure the driveability problem as described.

IF you think most of the running clearances are 'good' and there are comparatively few 'loose' worn parts (like bearings or bushings), a thinner viscosity ATF may help to reduce parasitic drain on the pump caused by the stronger kit pump spring... hence a 'high peformance' ATF might not be a bad idea (study viscosity pros and cons and use your own best judgement)
You might be thinking of a different kit than I installed. The 400 1+2 kit recommends the high pressure pump spring only for over 450 HP AND race applications. My truck is probably 350 HP and street only. Springs are included to tune the 1-2 PART throttle shift feel. WOT shift firmness is adjusted by the size of the hole in the separator plate. There are no spring provided for tuning the 2-3 shift.