TH350 Hitting 3rd at 70MPH!

[texascowboy593]

The truck: 77 K10, 400 powerplant backed by a TH350 with an estimated 60K miles.

I've had this truck (2nd owner) for 3 months now. A couple weeks ago while driving at highway speed, I felt the truck kick down from 3-2 although my speed didn't change. I accelerated and it dropped back to 3. Very strange.

Before this incident, it would shift perfectly in city driving (40-45MPH) thru all the gears and at highway speeds. Now I can't get it to hit 3 unless I race the RPM's up like crazy and hit about 70. The 1-2 shift still hits at about the right point. I do not have a tach yet (budget).

I started with the simple stuff: Checked fluid level/quality. Level good and slightly burnt smell although still crisp red.

I checked vacuum line from direct port on manifold down to modulator. Was worn and flimsy so I changed it out. Checked other vacuum lines throughout system. All good. Changed out modulator for a new redline one.

I pulled the governor to check all moving components, weights, springs and gear. All moved properly including the valve inside and gear was clean with no wear.

My next step will be to change filter/fluid but that will be a couple weeks off. I've done it before but that was like 15 years ago and I'm not a transmission expert by any means.

The only other suspect item that may be a clue for y'all is that ever since I got it, reverse always takes 2-3 seconds to engage after shifting it into reverse.

Any ideas on other things to check? Any clues from this account that smells of impending transmission failure?

Thanks!

[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

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!

[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!

[Dead Parrot]

If you have vacuum power brakes, might double check for leaks in the booster. I had one partially fail. It leaked enough air to cause a lean run condition but it still had enough boost to mostly mask the failure. Long shot to be the cause of your problem but easier to check then opening up the transmission. Less of a long shot if a PO tied the booster to the multi port vacuum source instead of the carb base.