TH350 Hitting 3rd at 70MPH!

[Palf70Step]

If the vacuum modulator on the back side of the transmission is bad or maladjusted it could give the indications you are seeing. It has been ages since I had to adjust one, so I forget. I had the same issue, but mine was just the hose slipping/vibrating off the modulator nipple. New hose and I used a clamp to hold it on and it solved my issues.

[TrailerTrash]

It sounds like it is shifting into 3rd by means of the governor only. Which shouldnt be the case. I am mostly an amateur with transmissions so I can only speculate and offer subpar advice lol it sounds like a modulator problem but since you replaced it that has me leaning toward another issue

Make sure the shifter is actually shifting fully into drive. Go down to 1st then up to 2nd then 3rd and make sure you can feel the shifter working properly.

Try shifting it manually while driving and see if that works.

[TrailerTrash]

I have actually had this problem before. My 79 K10 kicked down into 2nd gear 2 times in one short trip. It turned out to be a cracked vacuum line at the modulator. I replaced the unit and the line and never had another issue like that. Again your issue could be something else. But it is not a fluid change issue.

Did you do the "gravity" test with the governor? Where you flip it over and make sure gravity moves it. I had a separate issue with my K10 where it wouldnt shift properly and I had checked the governor and it looked good but I didnt do the gravity test. Pulled it back out to do the test and it failed. Cleaned it up really well so it passed the test and didnt have that issue again.

[JesseA422]

i just rebuilt my th350 over the past month because i was having the same issues, only more severe. it would fall back to second after going into 3rd, and only go into third at high speeds and would also not go into reverse for a while like u mentioned.

once i got in it i found out the direct drum/pump bushing got so worn the drum had wobbled on the stator, scarred it up and took out the sealing rings causing low pressure throughout.

[SunSoaked]

I had the same problem with my th400. Replaced the vac modulator and no change.

My vac line from the modulator was T'd into the vac advance cannister on the distributor.

The vac advance cannister developed a leak and killed the vac to both. Replaced it and all was good.

[texascowboy593]

Originally Posted by TrailerTrash

I have actually had this problem before. My 79 K10 kicked down into 2nd gear 2 times in one short trip. It turned out to be a cracked vacuum line at the modulator. I replaced the unit and the line and never had another issue like that. Again your issue could be something else. But it is not a fluid change issue.

Did you do the "gravity" test with the governor? Where you flip it over and make sure gravity moves it. I had a separate issue with my K10 where it wouldnt shift properly and I had checked the governor and it looked good but I didnt do the gravity test. Pulled it back out to do the test and it failed. Cleaned it up really well so it passed the test and didnt have that issue again.

Yup, did the gravity test and it was operational. Also, vacuum lines throughout the system are clean and not too old. From what I can tell thru inspection, they are all in decent shape. Thanks for the reply!

[texascowboy593]

Originally Posted by TrailerTrash

It sounds like it is shifting into 3rd by means of the governor only. Which shouldnt be the case. I am mostly an amateur with transmissions so I can only speculate and offer subpar advice lol it sounds like a modulator problem but since you replaced it that has me leaning toward another issue

Make sure the shifter is actually shifting fully into drive. Go down to 1st then up to 2nd then 3rd and make sure you can feel the shifter working properly.

Try shifting it manually while driving and see if that works.

Yup, did the manual 1/2/3 with no positive result. I appreciate you bring it up though!