Thanks for posting this excellent series of videos on the TH400. I have a classic GMC motorhome from the mid 70's with the Olds Toronado FWD drivetrain (455 + TH425). The TH425 was based on the TH400, but in a front wheel drive configuration. After having the engine rebuilt in 2009, I opted to have a rebuilt transmission installed as well. That was a mistake as the rebuilt transmission would randomly not shift from 1st to 2nd, but I could pull the shifter into 2nd and it would shift. Then putting back into Drive it would shift to 3rd. I've searched around the internet looking for info on how the valve body knew when to shift and the fluid flow causing the shifts to happen, but no luck. In the spring of 2020 it decided to not to shift most of the time. I figured it had to be a valve body issue. The modulator and governor had been check and replaced several times with no change. I had the transmission pulled and sent to rebuilder. They found and replaced a seal contact point where they though it would lose line pressure since it would go too low. Replaced that and rebuilt it, but still no change once it was installed. So they found a replacement TH425 case, moved over the new internals from my old tranny and its been working fine. If you can point me to a good instructional on how the TH400 shifting works that would be great! Thanks again!
Hi Bruce, thank you for the kind words! The TH400 bible is “How to Work on and Modify the Turbo Hydramatic 400 Transmission” by Ron Sessions. Has everything you ever wanted to know about the Th400 including build procedures, functionality, design, parts interchangeably and more. Highly recommend to anyone working on these transmissions. Please subscribe if you haven’t already as ill be doing more vids on the TH400 soon, including a complete tear down and inspection.
Also, did the shop replace the valve body? That’s likely the problem assuming your vehicle has sufficient vacuum to the modulator and the modulator is working. Was the modulator replaced with a new one and did they go over how to adjust it with you?
@@nickstransmissions The modulator was replaced twice and the modulator tube was replaced along with the rubber hoses on each end a couple of times. I even had a vacuum sensor on it so I could monitor the vacuum level and logged it on my EBL equipped ECM. Tested the governor several times and replaced it twice. Of course being random it never happened where you could do some tests. I'm an electronics guy so I like to see the issue causing the problem to make sure I've actually fixed it. I also like to understand how things work. I know how the transmission works, but what happens to cause it so shift and how that all happens is what I'm looking to understand. An electronically controlled transmission is easy to understand, but the old transmissions not so much.
@@brucehislop8860 the principals are the same in terms of shift command but instead of an electronic command and control, the th350/400 and like-kind units use governor pressure, modulator pressure (part throttle) and detent pressure (wot) to command, time and regulate shifts while the vehicle is on the road moving forward. All of these pressures are ultimately derived from main line pressure inside the unit (no line pressure, no movement). Once you begin moving in first gear the governor begins to spin and builds pressure and starts stroking the governor regulator valve inside the governor. Once enough pressure is built via rpms, the governor pressure overcomes modulator pressure otherwise holding back the 1-2 shift valve and that pressure strokes the 1-2 shift valve train enabling apply oil to flow to the intermediate apply piston. The piston compresses the intermediate clutch pack while the intermediate sprag goes from freewheeling to holding and the first-to-second-gear upshift takes place. Note that this is a very stripped down and simplified explanation but covers the gist of how this system works. All that said, did they replace the valve body? If not that’s most likely the problem.
@@nickstransmissions Thanks for that info on the TH400 operation. They replaced the entire transmission case and valve body with another core. They could not find an issue but something was causing the pressure to drop off. Ran it for two years now with no issues.
when assembling the 2-3 value, which way is up on the last sleeve next to the roll pin? One side has an extra notch in it (the center slot). Thanks for your time.
Hi Brokenwing...I don't think it matters, at least I've never paid attention to it in all the TH400s I've done. You can re-watch the video to see what side was facing towards the worm tracks when I reinstall the 3-2 throttle valve sleeve and mirror that if you'd like...That particular transmission has been on the road since the last rebuild video was published.
@@nickstransmissions Thank you! It looked like you had it facing up or to the case, wasn't sure or if it mattered. All the diagrams online don't show enough detail and no one made a distinction between one side or the other, didn't notice the difference until assembly. I didn't want to screw this up after all the work I put into my trans. I appreciate your help, THANKS again!
Is the 1-2 throttle valve ALUMINUM SLEEVE interchangeable between valve bodies? I have a different style one than in the video that is gouged up that I would like to replace with the style in the video.
The sleeves are matched to the valvetrains they come with and the TH400s have a few diff configurations, including different 1-2 shift valves that are paired w/their respective 1-2 TV valves, sleeve and VB castings. I don't know for 100% certainty but leaning toward not being interchangeable. You can always test fit to see what it looks like (i.e. if everything still fits like it should, etc) and try it but plan on having drivability issues and needing to take it back out....
