Has anyone ever used a TH400 with the vacuum modulator and boost? I figure that under boost the trans goes to full pressure, just like it would with no vacuum. I applied 15 psi to a vacuum modulator and the diaphram held up. I figure the worst thing that could happen is if the diaphram bursts it will pressurize the trans and fluid will go everywhere.
I switched to a full manual valve body so i would have complete control and have nothing to worry about, and just unhooked the modulator, but if your trans if fully automatic and needs the modulator just use an inline check valve so that you wont have trans fluid flying everywhere.
When you have boost, the modulator can be replaced with a mechanical linkage that operates the modulator valve. There is a picture of it in the Ron Sessions THM400 book. It's called the B&M modulink
I think that's been out of production for a while now, but other companies may have something similar. Full manual may be the way to go, but I want to keep the column shift, so I need to come up with a rachet mechanism. I'm running only 5 psi right now, so I'll try it and see what it does.
Bookmarked! I just tried an experiment using a regulator that's intended for use with air compressors. I hooked a vacuum pump to the IN, and pressure/vacuum gages to the OUT. With the pressure adjustment all the way out, I applied 20 inches of vacuum to the regulator, and turned the knob until the gauge on the output side showed vacuum. I turned it a couple more clicks for good measure. Then I applied air pressure to the input side (about 20 psi), and the pressure gauge showed 0 psi on the output side. I switched between vacuum and pressure a few times and got the same results. I'd like to try this again using two regulators so I can "sneak up" on it and see if it still blocks all the pressure to the output side. If this works, it will be a simple fix.
Using a second regulator and turning the pressure up slowly, no pressure makes it through to the output side, but the vacuum goes through freely in both directions. If the weather permits it's going to the track on the 1st, and I'll see how it works in the real world.
i am using the adjustable vacuum modulator on my turbo 400 transmissions. using the cable set up did not work. i do not use any check valve or anything else inline. car shifts perfectly at 6000 rpm off of the governor. the trans was built by mark deconti and has no issues other than rolling over the sprag under extreme tire spin into second gear. numerous nine second passes with a best of 9.45 at 145 at a weight of 3970. also only on 10 pounds of boost. car also shorted a 1.40 off the footbrake. no trans brake in this trans and no manual valve body. mark builds a great trans and it always does the job. john jr
Can you post a picture and a part number. I am currently using a power brake check valve. Only problem I have is that it will not allow vacuum through right away. It sounds like your solution is a better solution. without the check valve my trans would not shift under boost.
Sounds like I need to talk to him when I need a trans built! I'll post the info when I go back to the shop.
This thing has been laying around so long it says "made in USA" on the box!!!! The only thing I know about the style is that it is a relieving regulator, which lets the output side bleed off pressure as the input pressure is reduced/removed.
Alan, The way I've ran mine the last 5 or so years is as follows. 1. A one way check valve as mentioned above so the module doesn't see any boost. 2. Place a hose "T" inline between the check valve and the module. Fill one of the line puts of the T with epoxy. Once it's dried, drill a VERY small hole in it. Hook your vac lines to the part of the T that doesn't have the epoxy in it. The very small hole will allow trans the needed vacuum to bleed off so the trans functions normal while under boost......
I've tried the check valve/bleed hole thing in the past, and it seemed to work, but I fried two transmissions in the process. I don't know if they would have burned up anyway, but I thought I'd try something different and see if the results were better. The local track didn't run on the 1st, so I still don't know how it all will work. It's about 20 outside right now, so it may be a bit longer before I know! :spank:
I would have to say they probably would have cooked anyway.... It never hurts to try something new anyway you look at it!
So, has anyone figured out a solution that will work for a turbo 350?? I want this tranny to shift just as it does now. I don't think pressuerizing the modulator valve will hurt it, but it surely won't shift right..
did you just try using the mechanical modulator billmah posted? thiswould sove the issues with a th400 under boost correct?
