why me the transmission saga continues!

[randymc1]

I cound not get my th400 to shift with a vacuum modulator so I put a one way valve on the vacuum feed line. So the problem is it will go from a stop straight to second gear when boosting . I mean it will skip to second right on the line while trying to take off. A one way valve is not going to fix this is it! What is going on and what do I need to do?

 

[WE4]

put a manual valve body in it.....problem solved.

However, I bet I could make it work.... LOL

Bruce
We4

 

[Eric Stage I]

I use a TH350 with a vacuum modulator. You cannot use a single check valve to the modulator. The problem is the vacuum is not being "released" from the modulator (because of the check valve). My car uses no special valving, the modulator vacuum line is tee'd into by boost gauge line. The only quirk I have is the shift points are high at about half throttle (because the turbo cars get to zero vac faster). Under normal driving conditions it shifts fine and at WOT it shifts fine.
I would take out the check valve. If it doesn't shift at WOT, I'm not certain that it has anything to do with the modulator, maybe the governor setup (that is a Bruce question ).

Eric

 

[WE4]

Eric, not to doubt you but if you continue , you will pressurize the can ( modulator) under boost and it will fail shortly. Also it can pressure the trans and vent big time. YOU DO NOT WANT positive pressure at a vaccuum modulator. They are not designed for it.

Bruce
We4

 

[MeanBuicks]

Bruce, Do you know if B&M still makes the Modulink cable-actuated modulator? That'd be the hot ticket for an auto-shifting trans.

 

[Eric Stage I]

Bruce, so your saying the modulator can go past 0" vacuum? Doesn't it bottom out on something? I assumed (bad word) once you got to 0", the valve could not go any further. Or are you saying there's a diaphram that could break and pressurize the tranny with boost. It seems like if it can take vacuum, it should take some boost. However, if you've seen 'em break, thats good enough for me and I'll switch to a manual vb. I wonder if a small pressure reg could be put inline to hold psi to zero....
Anyway, I appreciate the info. You may have saved me from doing something very bad to my tranny .

Eric

 

[Eric Stage I]

I looked around for the Modulink also, but couldn't find it anymore. Maybe someone could make one up for us...

 

[HOTTOGO]

Eric, 15" (inches) of vaccum does not compare to 15# (lbs) boost and will most definately blow the diaphram, and possibly vent the tranny as BT said.

 

[Eric Stage I]

Yeah, I guess 15" is about 7.5psi.
Maybe we could get someone to sacrifice a modulator and hook up an air source to see when it fails .

Eric

 

[HOTTOGO]

PS. also vaccum is not a pos. pressure, like trying to blow up a balloon by inhaling instead of blowing?

 

[Eric Stage I]

I should have said 15" is about negative 7.5psi.
There!

Eric

 

[WE4]

Trust me.... IT will not work... I am telling you.

Now as for the modulator cable thing I do have access to one. Let me ck out and I will report.
Bruce
WE4

 

[Eric Stage I]

I decided to test my modulator to make sure I haven't blown it. So I fired up the air compressor and set the regulator to 25psi (where my boost is set). I ran 25 psi into it and no leaking. So I thought "I will not be defeated!", and turned the psi up to 40 to see if I could blow it. Not a peep. Then I decided "I will destroy this thing!" and turned the pressure up to 120psi and repeatedly applied it to the modulator. The thing won't die!

So then, I placed it behind my rear tire and backed over it 10 times. My tire blew and the modulator was unscathed. I have decided that I have a freakishly strong vacuum modulator and the diaphram must be made of kryptonite . Its definitely a keeper.

Seriously though, it seems to be ok. I would still rather have the cable operated dohicky.

Eric