One problem after another. TH400 shift issues

[Orion88]

Ever since I swapped out my original carb/intake on my 86 K10 Silverado I've had nothing but headaches. Took me forever to get the idle settings right, my automatic window got stuck in the down position, and I never could get the electric choke to work for a damn, so last weekend I pulled the 1407 Edelbrock and replaced it with a 1405. I knew the 1407 was too big to begin with but I had it sitting around and I wanted to learn how to rebuild a carb. Since it was too much carb I eventually swapped it out for the more appropriate 1405. Drove it from Indy to Bloomington the other night (45 miles, mostly highway cruise control set at 60mph), she drove smooth as silk and got way better fuel mileage. I had to kick it into 4wd to get into my snowy gravel driveway. I finally felt like everything was working right.

So I fire it up the next morning, pull out of my drive, turn 4wd off, and start driving to school. It doesn't take me long to realize something isn't right. The tranny doesn't want to shift out of 1st, and when it does it's at 35mph and it shifts HARD. I had this issue once and it was as simple as hooking up a loose vacuum line. I check all of the lines and everything seems fine. So I go to Advance and buy 12 feet of vacuum line, vacuum connectors and T's, vacuum plugs, etc. I replaced the line going from my manifold to the transmission vacuum modulator, and the line going from the manifold vacuum port on the carb to the distributor, and I replaced the plug on the timed vacuum port on the other side of the carb. That didn't do anything, so I replaced my transmission vacuum modulator (covered myself in tranny fluid in the process in the Advance parking lot). That didn't do anything either, so I spliced the modulator vacuum line into the line going from the manifold vacuum on the carb to the distributor. And still, the problem is not fixed. The only vacuum lines I haven't touched on the carb or manifold is the one going to the cruise control, and the PCV valve.

My guess is there has to be a vacuum leak somewhere affecting the whole system. All of the smog equipment is removed. I'm not sure where else to look. This problem literally popped up overnight and for the life of me I can't figure it out. Is my transmission ruined? I feel like it has to be something simple...

[rs74]

So is it still shifting then? I would say if not then check the governor gear in the transmission. I have had a couple of transmissions that the nylon gear went bad preventing it to shift out of first.

Have you tried to shift it manually to see if there is any difference? Pull the vacuum line off the intake while running and see if there is any vacuum there.

[Orion88]

Yes, it still shifts through all three gears, it's just super late and it shifts very hard. I've tried dropping it out of drive and into 2nd manually but it stays in 1st when I do that. I will try checking the vacuum line when I leave campus later today.

[Jaysonb]

I would pull the gov out and take a look at it.

[GRADYS Performance]

Fluid Level, Shifter adjustment, Vacuum modulator, Governor, Kick down electric hookup are the external things that make a TH400 shift. Check ALL of them.

[Orion88]

Fluid level is good. Shifter hasn't been touched and it's always been perfectly fine. Just replaced the vacuum modulator. kickdown worked the other day, although it needs adjusted a little. I assume I have to remove the tranny to get to the governor? If that's the case I might replace it with a 700r4 Depending on cost

[GRADYS Performance]

Originally Posted by Orion88

Fluid level is good. Shifter hasn't been touched and it's always been perfectly fine. Just replaced the vacuum modulator. kickdown worked the other day, although it needs adjusted a little. I assume I have to remove the tranny to get to the governor? If that's the case I might replace it with a 700r4 Depending on cost

The Governor on a TH400 is at the back of the main case. 4 Bolt square cover, Take out the 4 bolts, should not leak fluid, pull out governor and look at gear. Clean the cover and get a new gasket for when you put it back together. If the gear looks bad then replace the gear it is held in place with a roll pin. Torque cover bolts to 10 ft lbs.

[Orion88]

That's right, had I been outside looking at my transmission I would have known that. Sorry. So... the real question is where do I find the replacement part? I can't find it on any of the local auto parts store websites, or Summit, or Jegs. I found it on eBay and it's only about $6 to replace the plastic gear and gasket. Is there anywhere local I can buy the part from, like a tranny shop?

[rs74]

I have never replaced one on a 400. The 350 and 700r4 just have a round cover that is held down by a clip. Pull the governor out and look at plastic gear. I bought a new one for mine at the transmission shop for $14.

[Orion88]

Local transmission shop wasn't able to help me out. I went ahead and ordered one off ebay. Should be here friday if I'm lucky. I'll be doing the gear swap in a random dry parking lot somewhere since my garage has my GTO restoration project in it and my driveway is gravel, and covered in snow. Will I need anything other than basic hand tools and the governor gear repair kit? Looks like a simple enough swap but I need to make sure I have everything I need when I do it cuz I won't be near my tool box or a parts store when I do it.

[GRADYS Performance]

Originally Posted by Orion88

Local transmission shop wasn't able to help me out. I went ahead and ordered one off ebay. Should be here friday if I'm lucky. I'll be doing the gear swap in a random dry parking lot somewhere since my garage has my GTO restoration project in it and my driveway is gravel, and covered in snow. Will I need anything other than basic hand tools and the governor gear repair kit? Looks like a simple enough swap but I need to make sure I have everything I need when I do it cuz I won't be near my tool box or a parts store when I do it.

Tools required, 1/2" socket and ratchet, torque wrench, 1/8" pin punch, hammer, something to support governor while driving roll pin, gasket scraper, maybe a pair of pliers to pull gear off main body.

Have you looked at the gear yet? It may or may not be your problem. Only 1 of many possible causes. But pretty easy to check.

[Orion88]

I haven't looked at the gear yet because I don't have a spare gasket for the cover. Once I get the part in the mail I will pull it out. If the gear looks fine I will just replace to gasket and keep the extra gear for a rainy day. If it's worn, I will replace it.

I certainly hope it's the problem because I've checked everything else...

[bnoon]

The vacuum modulator you mentioned you replaced it. Adjustable most likely... have you tried adjusting it???

[Orion88]

Didn't look adjustable. Had a red stripe on it. No adjusting screws or anything like that

[donut]

red stripe are adjustable with an allen wrench through the air nipple.

[Orion88]

Considering putting on a new modulator changed absolutely nothing, I don't think it's the adjustment that's the issue. Since I already ordered a new governor gear I will check that first. If that's not the problem I will try adjusting my new modulator.

[GRADYS Performance]

Modulator adjustment should not be needed with a new modulator. It is more for fine tuning shift points in unusual combinations.

[donut]

Usually they don't go bad per se. Only time I had to adjust one was on a fresh rebuild. Even then, it was because it shifted way too quick. 3rd gear by 25mph...

You replaced the vacuum line to the modulator, did you verify it had vacuum? In my mind I'm thinking big vacuum leak somewhere. On my old truck, i did chase couple vacuum leaks that didn't show up with the "spray and pray" method. 1 was carburator base gasket. internal leak on a q-jet. 2nd was a brake booster gone bad.

Just thought of something else to check, make sure the vacuum nipple on the manifold hasn't plugged up or broken.

[Orion88]

Well... I finally got the part in the mail yesterday, so I laid down some cardboard and crawled under my truck in my muddy gravel driveway while it was raining to pull my governor only to find out it was in perfectly good working order and showed minimal wear. So it's back in and now I'm back to hunting for vacuum leaks.... Hooray....

[GRADYS Performance]

Did you check the valve inside the governor to make sure it moved freely?

[Orion88]

Yes. Everything moved freely.

[bnoon]

I've had to adjust every adjustable vacuum modulator to not shift either way too early or way too late. Not one has been set "right" out of the box. Stock or not.

[Orion88]

You have a point. I just think it would be extremely unlikely that it would be shifting at exactly the same rpms after I changed the modulator. But it's worth a try I suppose.

Counterclockwise to back it off? What size allen wrench?

[Orion88]

Well, looks like she's off to the transmission shop this weekend. I've checked everything that's within my capabilities of fixing. Modulator, vacuum, governor, fluid level, etc. It's looking like the valve body may need replaced or rebuilt.

[Orion88]

Dropped my truck off at the transmission shop this morning. Driving from bloomington to Indy was not fun. It wouldn't shift into 3rd until 65mph... And it would downshift if I dropped below 60. luckily my motor didn't explode, but she sounded like she might. This truck was used for snow removal a couple times but the transmission would overheat and we never knew why. I looked at the build sheet in the glove box, turns out this truck was ordered from the factory with 3.08 gears, and they have since ripped the 305/700r4 and swapped in a 350/th400. And since the 400 doesn't have as low of a first gear as the 700 the transmission was overworking itself trying to plow snow. So because of the one or two overheats I figured a full rebuild was safest. Once I save up some money I will be hunting for a matching front and rear end, probly 3.42s, maybe 3.73s. Because this truck is an absolute turd off the line with 3.08s.