I have a 1976 F150 4x4 with a 360 and C6. The truck is mainly used for hauling trash, etc. and often sits for a month or so without regular use. The transmission has always worked OK since I bought the truck two years ago, but when I went to drive is a few weeks ago it would no longer shift into 3rd gear. It had sat idle for about 6 weeks before this. The last time I drove it it didn't have any problems and I've never known it to slip or anything. I checked the fluid level and it seems fine. Not sure about the color since i'm not familiar with automatics, but it doesn't look real dark or smell burnt. I also checked the vacumn hose from the carb and it looks OK, although I didn't try clipping off a few inches to get a better seal. I asked a couple of shops and both said they thought I needed a rebuild and quoted ~$1200. Neither of them actually looked at the truck. I can't see spending a ton of money on this old work truck so I started looking for used C6 but no one in my area has one. The yards all said I was looking at around $250 for an exchange. Is there anything else I can check/try before I replace this thing? I can't believe the tranny just self destructed while sitting for a month or so. Any advice is welcomed! Thanks!

 

Check to see if the kickdown linkage has it stuck in passing mode. There is a plastic hickey down at the tranny kickdown lever that sometimes wears and binds it up.

Nathan

 

You might also try replacing the vacuum modulator and the entire piece of vacuum hose.
Jason Kendrick
1970 F100 Custom 390/C6
1978 F150 Ranger Lariat Supercab 460/C6

 

 

is there any answer to the problem? I seem to have run into the same problem. changed the filter and topped off the fluid. I am not too familiar with the transmission. it is a 79 F150 with factory 400. seems like it is stuck in third. I can shift to 1st and 2nd manually but it is an automatic. I also put a new Transfer Case Shifter in a few weeks ago. the pickup also sat for a while as it was used for garbage hauls. Quite curious of your results.

 

Check the vacuum modulator and the line that goes to it. Check the line for kinks or other problems, make sure it hasn't come off. If the line checks out, take the line off where it connects to the modulator. If trans fluid leaks out, its bad. If no fluid, it still could be bad. They're simple to replace, disconnect the vacuum line, remove one bolt that holds the clamp in place and the modulator comes out. It has an o-ring seal.

Modulators are identified by the color of the stripe. Pretty much the "go to" one is the black stripe. If they don't show that one in the catalog, go with the green stripe. Avoid the pink stripe. That one is a dual diaphragm version that has two vacuum line going to it. If by chance you have one of these, replace it with one of the other two and use the vacuum line that goes to direct manifold vacuum, cap the other line. When you pull the modulator out, be careful. There is a pin that goes in the modulator. Sometimes it comes out with the modulator, sometimes it stays in the tranny. If it comes out, don't lose it.

 

Is it normal for the hose to be spliced to a metal line between trans and intake manifold? As I follow it from the modulator, it’s hose for a foot, then a 3mm (approx) metal line all the way up to engine bay. It transitions back to hose before manifold. My c6 won’t shift up to 3rd.

 

Yes, it works off of manifold vacuum. If a shop will not look at it and quotes, look for another shop. Yes, if kickdown is depressed it probably will not upshift. Seals will harden with time, $1200 in today's dollars seams to inexpensive to believe. If modulator or vacuum hose problem in my experience it will not shift to 2nd unless manually selected.

 

I’ve got the same problem- (‘72 F250 w460/C6). The truck sat for a long time and won’t shift up to 3rd. I dropped the pan and the fluid didn’t look all that bad considering, and there was no chunky evidence of a failed trans. I’ve been internet stalking this issue for some time and came across someone that had a busted spring in the valve body. Since I had the pan off and still no clue what is going on, I took out the valve body armed with YouTube knowledge. Everything appeared to be fine. No scoring or busted springs, but there was dark spot that was unexplained. Was there a fluid flow problem? Could there have been a blockage somehow? I won’t know for some time because I chewed up the paper gasket and have to wait a week until the replacement is delivered.

Weird dark spot