th 400 cold shifting problem

[la-zdunck]

hey, I'm hoping someone has some insight to the problem i'm having with my trans.

when cold, the 400 doesn't want to upshift. I have to rev it pretty high to get into 2nd, and even higher into third. sometimes, if i put it into nuetral, and then back into drive while rolling, it will then go into the higher gear, but that doesn't always work. when fully warm, and after running through all 3 gears, it will shift fine, a little late, but nothing like when its cold. anyone have any ideas???

truck is a m1009 6.2 diesel.

oh, and i recently replaced the trans filter and fluid, the level is good, and doesn't leak at all.

 

[Mikey von]

I do not have the answer, but the same problem with my 6.2L -> Th400 on my sub. Would like to know the answer also.

 

[toomany]

What did the fluid look like that you took out? Did it smell burned? How many miles are on the trans? Did you put the correct fluid in?

I've seen a few cases where the fluid was changed then the trans started acting up. But that was more of a slipping issue. Maybe a problem with the governor or modulator?

 

[la-zdunck]

fluid was fine, i just serviced it thinking it would help, which it did, but the problem still exists. I can only guess the trans has about 100k on it, but does just fine when warm.

what does the governor or modulator entail?

 

[toomany]

Governor controls when the trans shifts. Has some weights and springs, like the advance mechinism in a distributor. Maybe the springs are getting worn out. Or the weights are getting gummed up and sticking. But I think the new fluid would have flushed anything out of there.

Modulator controls the firmness of the shift. Works of engine vacuum/load. High load, like when you mash the gas, the firmer the shift and the higher rpm it happens at. Low loads, like cruising around town, easy low rpm shifts.

 

[bradshawtech]

I say vacuum leak. I have seen it before. Steel line to the modulator

 

[la-zdunck]

thanks

I'll look into it, but there's no vacuum on diesels..

 

[bradshawtech]

There is a vacuum "generator" on your engine that builds vaccum for the various needs on the truck. You are correct though, the diesel engine does not need vacuum.

 

[4X4HIGH]

My vote is for modulator or governor. The modulator is the more probable of the two. Make sure to check the vacuum hose thoroughly first though.

 

[joedirt37]

on a 400 can you plug the modulator line,and shift it manually,just a offroad truck

 

[bradshawtech]

not sure, but why would you want to?

 

[pseudomike]

I'll second the vacuum pump/lines. If that checks out replace the modulator about $15. After that it is likely that your problem is fluid pressure problem. This means something internal is leaking that is either causing your pump to suck "air", or the center support or something is leaking causing you to loose line pressure on the pressure side. Your transmission pump could be getting old? If the vacuum stuff checks out I check the line pressure on your trans cooler. It won't tell you whats wrong other than it's probably time for a tranny guy. Good news is Th400 rebuilds are quite affordable.

 

[Greg72]

How "Cold" is cold when the transmission won't shift?

I've had shift problems when the temps are below freezing...the ATF just doesn't want to flow. The factory cooling system circulates transmission fluid through the radiator tanks to help warm it. If that system is no longer present, or you've got an auxillary transmission cooler hooked up maybe you're just trying to push thick, syrupy ATF around???

 

[la-zdunck]

the climate really doesn't seem to matter so much, it's just when the truck has been sitting a few hours or more. once it gets to operating temperature, and i run through the gears once or twice, it shifts pretty good. I haven't had a chance to pick at it yet, but i'm hoping to this week. Where would you guys recommend buying the parts? I like to shop online, any suggestions?

I really appriciate all the replys and input from everybody, I will be sure to update the thread. Thanks

 

[4X4HIGH]

You said you just replaced the trans filter and pan gasket. I would suggest dropping the trans pan and make sure the filter has not fallen off. This is a common problem if the wrong filter was installed as there are two different depth pans and each one has a corresponding filter and using the filter for a shallow pan but having the deep pan the filter can fall off. I've seen this several times in the past.