TH400 modulator installation help

[wallew]

OK, I seem to be doing something incorrect.

According to the -34 to replace the modulator, you basically loosen the bolt, remove the retainer plate and swap in the new modulator.


That went just fine. But I'm apparently INSTALLING the modulator, because now I can shift into reverse just fine, but not into forward gears.

So, help an old man out. What am I doing incorrectly?

Yes, it worked OK, but given it had not had ANY service since I purchased it four years ago and god only know when it got serviced before this. Then it started making whining noises.

Everything was working ok. Then it started making noise. So I changed the filter and the fluid (5.5 qts) and it runs WAY better, but the shifts were beating me to death.

So I chased all the vacuum lines and did not see any issues. But now that I installed the new modulator, I've got no forward gears.

It's the red striped one from Napa.

I installed it with the red stripe up. There appeared to be no other marks on the outside of the modulator, so I figured I installed it incorrectly.

So, is there a specific spot the modulator is supposed to be working properly?

 

[wallew]

OK, I'm going to pull and clean ALL the vacuum lines. And the vacuum electrical piece on the front of the manifold. What is it's name?

 

[Bobert]

The modulator plugs right in. There isn't really a wrong way to do it.

It would not stop you from having forward gears.

Did a little rod fall out when you took the modulator off? there should be one inside the hole where it plugs in.

 

[Crash_AF]

Dude, you should have called me when you ran into trouble.... I see how it is...

Anyway, as Bobert said, did the op-rod fall out when you pulled the mod? Can you put the old one back in so you can see if it's the modulator or something else?

Give me a call when you get a chance.

Later,
Joe

 

[wallew]

It's still there. But it seems that I do have a rotten piece of vacuum hose up top. And I have heard that the steel part of the lines can get 'cruddy'. So I'll let them soak 24/48 hours and then reinstall.

Might have been a bum vacuum line all along. I'll let you know tomorrow when I have light to work by. I have no garage. So I work by light of day.

JH, I'll call tomorrow after I crawl back under there. Once I have replaced the upper hose, I'll start it up again and see what's what. It might have been as as simple as a bad vacuum hose. But we will see. Once I've gotten going tomorrow, I'll call. Geez, I wanted to solve ONE of these stupid things by myself. Guess that's moot, eh.

 

[rnd-motorsports]

The modulator can be checked just bye simply pulling the vac hose off. If trany fluid drips or is present replace it if not its good! the stripe on the modulator tells the stiffness of the spring in the modulor dif colors means dif shift points takes more or less vac to pull on modulor and the shift points will be a little dif. I like the green stripe but the red would be fine! I would get a small round mag and pull the modulator back off then pull the small rod make sure no dirt got into it to make it stick and that it is put in correctly I think one end is smaller than the other the small end goes out to the modulator! let us know good luck!!

 

[wallew]

OK, I thought I might have installed the tranny fluid pickup in backwards when I put the filter in. So I pulled the pan again. I found that I had put it in backwards. But I resolved that. Then put the filter back in. But before I buttoned it back up, I pulled the modulator and then pulled the rod out. It was kind of stuck, so I cleaned it off as well. It slipped back in with no problems.

STILL NO FORWARD GEARS.

I just don't get what I did by changing the modulator. I changed vacuum hose into and out of the TSM. I have what feels like strong vacuum at the pump but I don't have a gauge to measure it. I have equal vacuum at the TSM. BUT if I by pass the TSM and connect the good vacuum to the vacuum line that goes down to the modulator, I get very little down at the modulator. So I probably will have to pull the steel vacuum line to clean it.

But still, I feel that I should at least HAVE forward gears. Harsh shifting forward gears maybe. But I've got nothin...

So if anyone has any idea's I'm all ears.

 

[Barrman]

You are correct. The vacuum just has to do with shifting. The transmission should still move the truck in forward and reverse. If I am reading this correctly. You drove the truck, pulled the modulator, put a new one in and now the truck won't go forward. Nothing else was done at the same time?

Is the shift linkage going all the way to drive? Does it have fluid? The usual simple stuff because it shouldn't go from working to not working with what you did. Sorry I can't offer any more advice than to pull and rebuild.

 

[rnd-motorsports]

The modulator will go in any way. There is no up or down I would say that you had a problem growing in the trans and just so happened what ever is wrong with it just quit not that you did anything wrong. You said it started wineing I will bet the forward clutch pack is toast or something internal best bet is pull it out and get it rebuilt if you dont feel like doing it yourself or have the know how! but sounds like just bad luck !!

 

[niferous]

I know it seems simple but check your trans fluid again. I had the same problem so look at my old posts. I'm on my iPhone so it's hard to look up. Basically I put in the 5.5 qts but it turs out my torque convertor was dry so it took a lot more.

 

[wallew]

You are the SECOND person who has made that suggestion to me. I'll do it. What's a few extra quarts in comparison to a rebuild?

Thx. I'll let you know what happened.

 

[Barrman]

If you are looking for short term fixes because a rebuild is probably in the cards. Try adding Trans X if you need to add some fluid. I had that stuff get me 150 miles home one time years ago when I picked up a truck with a TH350. Reverse worked and drive got the truck to rocking forward, but not moving. I was young, dumb and needed to get that vehicle home. Trans x got me home. I drove slow, but at least I could drive.