View Full Version : TH400 transmission shifting problem
01-16-2011, 07:06 AM
Hello. New to the forum and I've found lots of good info on here.
I just bought a 90 z71 which has an awesome 400 small block with modified 454 throttle body and boy does she get down the road. Was taking my wife out last night in it (she was riding in the middle seat-got to love bench seats) and right at 50 mph it started shifting back to 2nd gear. It had not been doing this since I had purchased the truck two weeks ago. I did notice the shift points are very low for this transmission but this might be normal for a 400. I'm not a mechanic but can do some basic things. The truck shifted fine until last night. Any idea what would all of a sudden cause the tranny not to stay in 3rd gear past 50mph? If I goose it, it always drops back into 3rd but if I ease up in acceleration it kicks back to 2nd right a 50. I hope this is not serious tranny trouble. I do not feel any slipping whatsoever.
01-16-2011, 07:39 AM
sound like you may have a problem in the governor pressure vs throttle presure....check the vacumm source to the modulator make sure its got a dedicated vacumm source and is manifold vacumm (behind the throttle plate) also try adjusting the modulator there is a small screw inside the vacumm port of the MOD try turning it a half turn counter clockwise and see if that helps with the 3rd issue. but if its speed related then you may have a problem in the governor side of the trans.
01-16-2011, 07:45 AM
thanks for the tip. I'll give that a try and post back if that fixes!
01-16-2011, 09:05 AM
come to think of it, the throttle body gasket has a leak and the truck doesn't idle consistently. At idle the rpm's bounce up and down around 2-300 rpm's.
I was told the throttle body gasket had a bit of a vacuum leak. Do you think if the leak has gotten worse, that could throwing off the throttle pressure to the transmission?
01-16-2011, 10:11 AM
yes vacumm issues will cause trans problems like that. remmeber at 50 mph yo are at light throttle and pulling vacumm if you lose vacumm the trans thinks you are giving it gas by the vacumm drop and downshifts. although the modulator is probably adjusted to high this issue is probably making that issue show up more.
01-16-2011, 01:08 PM
Well, I went out and took a water bottle and sprayed a fine mist all around the throttle body and didn't hear anything idle down.
In fact, i didn't even hear the lobing/surges that I was hearing before. It seems as though whatever was causing the idle surging that it's not as apparent now. Funny how that coincides with the tranny not staying in 3rd too.
I took it out just now and at first acceleration through all three gears it shifted fine and ran great. as soon as I backed off throttle and eased back into it, it started down shifting. Will start looking into the vacuum mod adjustment now.
01-16-2011, 02:26 PM
Ok, I adjusted the modulator clockwise almost a half turn and the shift points are actually much better now. Might turn it back just a hair.
I assume that means the MOD is working properly.
What about kick down switch adjustment? I guess I'm not 100% sure I checked for vacuum leak properly on the throttle body but if it's not a vacuum leak how could I check to see the kick down switch is functioning normally?
Man. thank God for the internet!
01-16-2011, 04:46 PM
well you would need t osee if its grounding out intermittently that could also cause a 3-2 down shift as thats what it supposed to do...you could alsways disconnect it and see what happens
01-16-2011, 06:04 PM
That's exactly what I did. I unhooked it and found that the downshift problem went away. I am going to go buy a new switch tommorow and replace it.
Thanks so much for your insight. It was very helpfull and I really appreciate it.
01-16-2011, 07:02 PM
not a problem why i am here.
06-18-2012, 06:46 PM
I have a 1989 Chevy 2500 3 spd auto 2wd. A few weeks ago, after suspecting that the transmission was crap due to the previous owners having messed around with other parts on it, and the tranny shifting rough and too fast, we had it replaced. Turned out that the flywheel was sheared off, and it may not have been the transmission. Now, the transmission is shifting really late, and the rpm's get way too high befoe it shifts, especially from 1st to 2nd. I am suspecting either a vaccuum line, throttle body gasket leak, or the TPS sensor. Any other suggestions as to what else it could be? It's a TH400, with a mild shift kit.
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