TH-400 Trans won't hold back in 2nd gear

[dieselcrawler]

I know, this is The Diesel Page, not The Transmission Page, but maybe someone will know...

I have a TH-400 in my K-30 that will not hold back in 2nd gear if you down shift to hold the truck back on a hill. It holds fine in 1st and 3rd, just not 2nd. It is like you put the trans in neutral when it's shifted to 2nd, but pulls fine if you hit the 'go' pedel.

What can cause this, how can it be repaired, and is it doing any damage to the trans if it is let go? I'm not really ready to go tearing the trans out of my truck, and it's been this way for as long as I've had it, about 10 months, with no ill effects yet. It would be real nice to have the compression breaking use of 2nd gear...

Greg

[britannic]

The NetGearHead TH400 forum (http://bbs.netgearhead.net/postlist.php?Cat=&Board=th400) is a great help: TH400 Forum (http://bbs.netgearhead.net/postlist.php?Cat=&Board=th400)

[dieselcrawler]

Thanks for the info... it seems like I'm gonna have to tear the trans apart to find the prob. Still seems to work fine, hope it will last another month or so... have to wait till I get the other truck back together! Thanks again...

Greg

[ropinfool]

I had a 79 Chev w/454 that had this same problem. I took it to my college auto shop instructor and found a "solenoid"? was unplugged down at the tranny. It was a T400. Plugged it in and if I remember correctly it fixed it. If yours is still plugged in maybe the "solenoid"? is bad? I'm trying to remember more about it but that was a loooong time ago in a place far far away. Good luck. John

[gmctd]

Sounds like a broken sprag, a type of one-way clutch - slips when driving forward, holds to decel. May notice large quantity of 'offroad-machined' metal in pan\filter.
The only solenoid in a T400 is for downshifting - can be tested by using the accel pedal to downshift 3 - 2 or 2 - 1.

[dieselcrawler]

Hey all... thanks for the replys... the trans down shifts fine when you hit the throttle, no prob there. I've even adjusted it to suit my driving style by turning the screw inside the vac line in the side of the case.
The only thing I notice it that when it upshifts, it's really snappy. Not that this is a bad thing, just that even when accelerating slowly and easlily, it's a snappy shift. Maybe it's just the tork of the diesel! *grin*
Otherwise, seems to work fine... clean fluid, doesn't seem to leak, haven't had to add any as long as I've had the truck, about 10 months, and maybe 3-4K miles.
Does anyone know if this '84 model K-30 CUCV would have a lock up tork converter? I don't know, and don't even know what to look for... all my other trucks have been older th350's, no electronics at all...
Have a great new years!!!
Greg

[88 K30]

Originally posted by DieselCrawler:
...The only thing I notice it that when it upshifts, it's really snappy. Not that this is a bad thing, just that even when accelerating slowly and easlily, it's a snappy shift.... My civilian truck does not shift very hard, unless I really wind it out, but my CUCV shifts hard even on lazy acceleration, too. However, I have driven CUCV's that do not shift hard. I think it has to do with the adjustment of the vaccuum valve on the IP. If mis-adjusted, the transmission shifts so early, that you almost don't feel the shift. I think my civilian truck has this problem, as it usually shifts into Drive by the time I reach 15mph on a lazy start, whereas my CUCV doesn't. Holding the shift off longer would help build up more internal pump pressure and, probably, make the shift feel "crisper".

[dieselcrawler]

Ok, let me get this right... there's an adjustment on the Injector Pump that will affect the transmission shifting? I am aware of the vacuum adjustment on the trans it's self...
If this is the case, should I adjust it? Will it hurt anything if I do/don't? How would adjusting it affect how the truck runs, if at all?
Thanks for the info... seasons greetings!
Greg

[britannic]

Originally posted by DieselCrawler:
Ok, let me get this right... there's an adjustment on the Injector Pump that will affect the transmission shifting? I am aware of the vacuum adjustment on the trans it's self...
If this is the case, should I adjust it? Will it hurt anything if I do/don't? How would adjusting it affect how the truck runs, if at all?
Thanks for the info... seasons greetings!
Greg Yes, the TH400's with vacuum modulation use the injector pump vacuum throttle position valve to set the shift points.