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Recently rebuilt th350 doesn't always shift into 3rd gear

76k5blazerr

76k5blazerr

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Alright yall so I had my th350 and np203 rebuilt like a month ago. It was a quality rebuild, 700 bucks for both. Just a couple days ago it started to intermittently not go into third gear automatically, I could shift it manually though. It seems now that it will only go into 3rd auto or manually when it wants to... About half the time. First and second shift great. It has a 1 year warranty so I'm getting in touch with the guy but until I get up with him I wanted to see if anybody knows what's going on. Or has an idea. Somebody told me it could be the 2/3 shift valve sticking or something? Yall got any ideas?
 
The modulator can make it not want to go into 3rd,if its not getting enough vacuum or it is defective...it'll also feel like it freewheels when you let off the throttle if the modulator is not getting enough vacuum when it is in second,but wont upshift to 3rd..

There should be a steel tube going from the modulator to the fitting on the intake manifold,with only a short rubber hose at each end ,to the modulator and the fitting..some try to cheat and use all rubber hose,that's not correct and can cause this type of trouble,so can hooking the vaccum source up to some other fitting on the carb or other spot,the correct place is the fitting right behind the carb,that is manifold vaccum..

It could be in the valve body too,but that isnt too common...unless it had been taken apart and fooled with maybe,or was high mileage..

Another possibility is the detent cable (for passing gear) isn't adjusted right,so its staying in second,and not letting it upshift..

I'd go right back to the rebuilder and have him look at it..if you try fixing it yourself,he may not want to stand behind his work...

Greg Ducato is the man here who knows automatics best ,I'm just a backyarder who's had a few trany's that acted up,and I managed to figure out what was wrong with help from him,and some books at the library,online googling too...
 
Alright, let's hope Greg sees this post. Your not gonna like hearing this but... My truck didn't come with the steel vacuum hose when I bought it, I've been running a full rubber line for a year now, also it's hooked to the carb... I have had no problem with it until now. The port on the intake that you talked about is where my brake booster vacuum hose comes from. Is that not right? I'm beginning to think I have a leak in that vacuum line. I sure hope so. I'm BEYOND sick and tired and sick and did I mention tired of F ing transmission problems.
 
Using all rubber hose might get you by temporarily,but after awhile,the hose can collapse internally and choke off the vacuum to the modulator..

I tried looking for a decent picture of the OEM fitting and location,but there wasn't any good ones on google images...you can probably get away with using any manifold vacuum source,but having the modulator "alone" is how GM designed it..tying it in with the brake booster might not be good,since it stores vacuum and might cause the modulator not to respond right...also GM had a "restrictor" made into the end of the steel tube where it goes into the manifold fitting,some purists claim its a must to have that--I have run regular 3/16 brake line tubing with no issues though..
 
So should I try a different vacuum port on the intake? Or will I get by on the carb. Like I said it worked fine the past year and I think if I do have a vacuum problem it's in the hose and not being caused by the carb.
 
I'd try to stick with the stock fitting location if possible..but if you can't,like if your intake is aftermarket,you should use a vacuum gauge on whatever port you use to make sure its getting full vacuum at idle,and it drops off to zero when you floor it...if you hook a modulator to ported vacuum (which means it'll have vacuum under acceleration instead of at idle only),it wont work right and will lead to slippage and poor shifts..
 
So if the stock fitting is my brake booster vacuum how should I go about putting tranny vac there too?
 
The older carbed small blocks I had ,the brake booster got its vacuum from a fitting screwed into the back of the carb,under the throttle plates..

The modulator had it's own fitting in the intake right behind the booster fitting.

If the way it was hooked up before used to work OK,first thing I'd do is check for good vacuum at the modulator end of the hose with the engine idling...if it seems weak or none,then the hose is probably split,worn thru from rubbing on something or collapsing inside...if that's the case I'd buy a 3 foot piece of brake line tubing (I think 1/4" is the size,might be 3/16"),and hook it up with new short rubber hoses at each end and see if it shifts OK then..

There is always a chance the modulator failed too,the diaphram inside it can rupture..you'll see ATF in the hose nipple usually if that happens,also you may get ATF sucked into the engine and burned,that will make a white smoke,but it might not be enough to be noticed easily..
 
Alright cool man, I really appreciate your input. I'm gonna take a look tomorrow and see what I can do.
 
Well,hopefully it will just be lack of good vacuum to the modulator,and nothing internally wrong with it...good luck,let us know how it pans out..
 
I agree with Robert, a good, full manifold vacuum source is the best way to go for this trans. Take a little time and use a steel line from the intake to the trans and use good rubber at both ends. Even if the trans has internal problems, you will want to have all of the bases covered from an installation standpoint for the trans to function as it should. On the kickdown cable, make sure it is adjusted correctly, should be pulled out all the way at wide open throttle only, be sure it isn't binding. A worn governor bore or loose governor shaft can cause erratic shifts, especially in high gear.
There is a 2-3 accumulator piston in the trans that could be damaged or leaking and as has been mentioned a shift valve could be sticking. There are other valve body issues that could be there as well.
The 350 is a dirt simple trans but we see a lot of builders who just blow off a valve body and pick the valves to be sure they stroke and call it good. A builder should completely remove all valves from the valve body, clean the housing and restore and install each valve one at a time. It takes more time but it is the only way to be sure.
Good luck with it and post results!!
 
Can I use brake line for the steel line? What size? 1/4 inch? Gonna try and pick some up today. Then I would just hose clamp rubber ends on it right? What kind of rubber hose just regular vacuum hose?
 
The brake line will work great, zip tie it to the dipstick tube to keep it stable, good quality vacuum line at either end is fine as long as it is a snug fit, you can use a zip tie at these locations as well for extra confidence.
 
Well I finally had time today to work on the truck. I bought the fitting for the back of the carb for the brake booster vacuum to go to, freeing up the tranny vacuum port on the intake. Got the tranny vac line hooked there now and put a plug on the carb where it was hooked... Also bought a new vacuum hose but did not put it on yet. Figured id try it with the old one first and see if I even needed to replace it. Well it seems to be working, I geuss what had me thinking it wasn't going into third was that I never really can feel it shift 2-3. But I took the truck out on the highway today and was running 55-60 at like 2200 rpm so I was thinking there's no way it's still in second gear. So I geuss it is shifting I just can't feel it... Is this normal? Anything else you think I should check?
 
If you have an adjustable vacuum modulator there is a screw inside the spout where the line attaches. Using a small flat blade screwdriver adjust the screw clockwise three full turns and see if that doesn't give you a little more shift feel. You can also manually upshift through the gears to confirm shifting.
 
And if I don't have an adjustable modulator? I'll check on that later today if I have time. And it seems that even when I manually shift the tranny I feel 1/2 shift but I can't really feel 2/3. Think the modulator could be bad?
 
If you don't have an adjustable modulator then get one. Your builder should have installed a new modulator at the time of rebuild. By turning the modulator screw clockwise you will create higher tension on the diaphragm which will increase line pressure and make the shifts later and firmer.
 
remove bracket, remove modulator , (have bucket underneath to catch whatever fluid comes out), install new modulator and 0-ring, install bracket and hose, check fluid, test drive.
 
I'll check and see if mine is adjustable and look at adjusting it or changing the modulator, also gonna go and put on the new vacuum line just to knock the off the list as a problem.
 
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