DESPERATE: I need a transmission EXPERT

Yup I still have the other trans. I was hoping to rebuild it so my nephew had a spare.

 

When I was 16 (1975), I had a 1969 396 Camaro with a TH400. One day, it had only low and reverse. I had it towed to Aamco. They tried to sell me a complete rebuild. I couldn't afford it. I went to pick it up and the guy told me that "while it still needed a rebuild", out of the goodness of their hearts (and 85.00 bucks), they had "repaired" the hose to the vacuum modulator.
I ran that trans another 3 years.

 

You're thinking of Fords, they use a pin. The GM transmissions have the modulator valve acted on directly by the modulator, there's no pin.

 

Excellent! Now you have a point of reference.

 

You're right. I'm thinking of my Cruiseomatic. Damn!

Am I recalling correctly that the TH400 modulator is coded by a paint stripe for ID?

 

There isn't really anything external to the trans that will make it not shift at all. Even if the kickdown wire is getting power, and there's no vacuum at all, it will shift when you get going fast enough.

As far as I know all the replacement modulators are the same red stripe one. The fords have differnt flavors.

 

How fast is "fast enough" would you say? He's going to work on it tomorrow.

 

Well there isn't anybody local that rebuilds transmissions anymore. So I thought I would take a stab at it. If nothing else so I could rebuild my own. Well I can say one thing it does shift very well into the gears I do have. LOL

 

last professional shop i had rebuild a trans they put the forward sprag in backwards, start there it was fucked up to have to pull it back out

 

4500 rpm, unless it's a vette trans, then 5500

But it is a good idea to spend some time doublechecking the vacuum supply to the modulator.

 

So the trans should shift around 4500 rpm no matter what, as long as the internals are put together right????

 

That, right there, is why my truck has three pedals

 

Have you tried to manually upshift it? When I replacedthe flexplate in my '36 it would not upshift. I had good reverse and first. It would go thru all forward gears if I manually upshifted it. I traced the problem to the hose at the modulator. It fit tight and looked good BUT had a slight crack. I replaced this hose and problem cured. Worth a try...

 

I assume it free wheels when you lift on the throttle if the shifter is in drive. If you put it in manual first, does it have engine braking when you lift on the throttle? Does it try to shift and go to neutral or does it stay in first gear as you go faster?

If you have no governor pressure or if the modulator valve is missing it won't shift. I haven't looked at one for a while but I think it is possible to put the modulator valve in backwards also. Maybe not.. but it is easy to check.

Sounds like base pressure is OK but honestly, you need to get a pressure gauge on it and see what happens to line pressure when it should be shifting. If it is truly not commanding the shift, it is governor/modulator related. Those are the only two things that control the upshift on a 400.

 

Had a trans that would hold the low gears too long many years ago. The vacuum hardline that was connected to the modulator was clogged - this has been mentioned in a post above, and would be the first thing I would be checking.
Good luck with it, it does sound like you have been very methodical.
Mart.

 

Did you change the torque converter? I had a 77 blazer 4x4 and lost foward gears,took it home 2miles in reverse.I had the trans rebuilt and put in no go.Pulled again and trans shop said change converter.Did so and it was fixed.Just a thought.

 

I am no trans guy but it sure sound like something related to the trans and not the trans itself (as others have mentioned). Too many trans with the same problem.

One thing did cross my mind though ... I remember that the steel vac line on a TH350 has "funny" ends on it. The ends taper down so they can fit inside the fittings on the trans and the intake (basically a metal to metal joint that is not prone to collapsing like a rubber line might, although they did use a very short bit of rubber hose just to seal the point where the tapered steel line joined up/fit inside the hard fitting). I am wondering if (over the years) someone has made some changes to the steel vac line and it now includes a length of rubber line that is collapsing and preventing a vac signal from reaching the trans (someone else here has mentioned that the vac modulator is necc for 2nd and 3rd but rev and 1st will still work).

I personally don't know if the convertor would allow first and rev but not 2nd and 3rd but I'll bet that convertor is the same one used in all three installs. Could very well be your issue (maybe).

 

Check your transfer case selector lever. Are you sure you are in 2-hi.

 

This is not possible. The converter hooks the engine to the input shaft .....period. The vehicle would either move or not. It is impossible for a torque converter to operate a transmission in one direction and not the other. It is bolted to the flexplate and the engine only spins one way. The gearbox changes the output shaft direction inside the transmission itself behind the torque converter. The input shaft always turns the same direction as the engine crankshaft. A torque converter is a fluid coupler between the crank and the input shaft. Just like a manual clutch. I don't care what transmission you are talking about.

This would be the same as telling someone with a manual trans that the clutch is bad and that is why you only have reverse and no forward gears.

I don't see how any of this is relavent to the original poster's questions. His vehicle moves in both directions so the torque converter cannot be his issue.

 

Yes, actually I did change torque converters. I wanted to make sure I had a fresh one for the rebuild.

 

Yes it has engine braking when you have it manually shifted into first. It stays in first as you go faster, but as soon as you let off the throttle it tries to shift and then doesn't. I am assuming this is when I would get a vacuum spike and would try to activate the modulator?

 

I checked the fluid pressure from the port on the side of the trans with a regular oil pressure guage with the plastic tube. It pegged the guage at 100 psi. in neutral, but I think my book says it should be around 55-80 psi. Is this to much pressure and could that be part of my problem. Could there be blockage somewhere that is bringing the pressure up?

Going back to the garage to try a vacumm guage and check a few more things. Will try to up date later today. I thank everyone for there ideas and the support and I know my nephew thanks you too.

 

"tries to shift and then doesn't" - does it feel like it drops out of gear/slips when you let off the throttle expecting it to shift to second?

Those are the symptoms when I was having vacuum line problems.

 

Did you do anything with the torque coverter or your shit linkage?

 

yeah,i had a th 350 lost all but first gear,the hose to the mod valve had broke,pretty nice since i was a broke teenager.

 

Come, now the linkage aint that bad!

 

I installed a TH400 in my DD truck after the TH350 went out, for the second time. I'm completely sold on the TH400 now and even bought another for future use. The vacuum line I used was a rubber/plastic medical grade tubing, an d it worked great for almost 2 years. Then one day, no upshift to 2nd or 3rd. My vacuum hose was totally collapsed on itself; I replaced it and the trans works great again. My first vote goes for the vacuum modulator line is collapsing, plugged, or leaking. Did you disassemble the one way clutch/sprag at all? That would be my next guess. I'm not a transmission specialist, but I do build Powerglides for the local bracket racers, and have done TH350 and TH400 trans in the past. Watching closely for wear or broken parts, cleanliness, and proper sub-component assembly are the most important things to watch for and do. Rubber lip seals can be damaged easily and go unnoticed; too much, or too little, clearance in the clutch packs is possible; and improper end clearances, will prevent a trans from working properly, or kill it quickly. Butch/56sedandelivery.

 

Take the governor out and hold it by the gear with the weights facing down . now with your other hand push the weights in and out .while you are doig this look through the slots in the round shaft there is a spool inside and it should move in and out . if that spool is stuck it will not upshift . Make shure you have vacume at the modulator .check it right at the modulator also make shure it is manifold vacum not a carb port double check your linkage and make shure kick down solenoid is tight .If it will only move forward and backwards then the rear band ,forward clutch and direct clutch are working . the only one left is the intermediate clutch and center support . Did you tourqe the center support bolt ? also the center support has 4 seal rings .I hope this will help you .Remember check the easy things first.

 

Go straight to the modulator/line. If you changed modulators with the rebuild, its probably a problem with the vacuum source or line. If you re-used the same modulator, thats probably the problem.

 

+1 for vacuum issue. I'm no expert but I did stop a friend with credit card in hand about to purchase a new trans and it turned out to be 30 cents of vacuum line.... exact same trans and symptoms