87 Silverado 1/2 ton. 3 speed auto will not shift. - The other day I started into town and the transmission in my 87 1/2 ton 4x4 wouldn't shift. The ...

The other day I started into town and the transmission in my 87 1/2 ton 4x4 wouldn't shift. The day before I had taken the breather off to look at something. Before this it had always shifted out. There never was and still isn't any type of "noise" from anything. No pops, bangs, snaps, grinding etc. Reverse and low is all I have though. It s till goes into four wheel drive like it should. I am thinking/hoping I knocked a vacuum line off, or just have a leak somewhere. I have looked and looked though.
This was an old farm truck when I traded for it, and was just kept up in good enough running condition to use on the farm. So, there are several vacuum lines and electrical connections hanging loose. I'm hoping someone can point me at a specific place to look. Since I've seen vacuum leaks cause this problem in other vehicles, and it also started right after I had the breather off, I'm hoping this is my problem.
Any advice is appreciated.
I actually love tinkering with the older models, but this one has puzzled me long enough. lol

I'm assuming it has a TH350 transmission. The TH350 does have a vacuum modulatorwhich tailors the shift point. The truck should shift up even if the vacuum line is disconnected, althought it will hold the gear a long time. The actual shift is made by the governor. It sounds like you may have a loose vacuum line to the modulator. They are usually attached to the port on the intake right behing the carburator. If it is knocked off it should be pretty easy to see. There is also the possibality that it was knocked off the actual modulator, which is on the right rear of the transmission.

Man I sure appreciate the info! I did a quick glance at the port behind the throttle body and the hoses seemed attached. I know from a previous post of yours how to tell if that's a 350 or a 400 trans. so, I'll check that next when the sun goes behind the hill and there's some shade. I'm still hoping it's a leak.
After reading your reply I remember the transmission seemed slow to shift out. Now I'm wondering if maybe the modulator valve itself might be bad.

It could be. Luckily thet are easy to replace. Most of them are adjustible by slightly turning a small screw that is down in the hole where the vacuum line attaches to it.

Well, Jordan, I didn't have any luck! I crawled under that thing it has a big square pan, and no modulator valve. It's a mighty big old transmission! lol I checked the fluid again and it doesn't look that great, and did smell burnt. I have no idea of the maintenance history of this old truck, or even if the fluid and screen have ever been changed. Since it was an old farm truck when I got it I doubt much time was spent on upkeep. All I know to do is drop the pan and see what I find. I really hope it's not full of filings!

Sounds like you have the 700R4 transmission. Does the shift indicator show an overdrive? Unfortunatly it may be time for a rebuild. Good luck.

No, no overdrive. 1st 2nd and drive. I really do wish it had an old 4spd. manual trans in it. What I learned to drive on and always had.

It's probably a TH350 then. If you were close I could build it for you pretty cheap. I think you can get a kit for about $75.

I sure hate to bother you with questions, but evidently I'm going by the wrong things to tell the differences between the 2 transmissions. Doesn't the 350 have what I call a "dog eared" corner on the transmission pan. And the bigger transmission a square pan?
I'll be the very first to admit I know nothing about transmissions. I sure am willing to learn what I can though. I've got a cousin that rebuilds and if it isn't that expensive I'll sure go that route.

A TH350 has a square pan with the one corner lopped off. It should have a vacuum modilator on the right rear of the transmission (near the tail shaft). The TH350 has a detent cable on the passengers side near the front. A TH400 has an odd shaped pan with a vacuum modulator on the passengers side near the front. It will also have a wire attached going into the side of the transmission for the electronic down shift.