This is an issue with my 1986, 2wd f150, with the original straight 6 engine and C6 transmission.
I was 200 miles into a trip up the coast, pulled into a fast food drive-thru, put it in park while waiting. When it was time to move, I put it in drive and, nothing. It acted like it had an insufficient level of fluid, though that was not the case. After revving several times, it 'caught', and was back to normal. After that incident, all remained fine, I drove it every day as always.
Last weekend, I drove 55 miles out to some remote property where I worked for a couple of days. When I left, it went into reverse fine, I backed out a couple hundred yards to the paved road, put it in drive, and it did it again. Several revs later it caught, drove me back, and has since been trouble free.

I checked/replaced the hard & soft vacuum line, and swapped in a new modulator between the incidents. I don't think they're part of the story.

Each time it was after a longer than average drive, no idea if that means anything.

Does this occasional pressure-loss scenario ring a bell with anyone who knows more than I about the c6?

Thanks for any help.

 

Since D and N are neighbors I'm wondering if you linkage needs adjustment? Does it do it when shifting to L?

 

That's a common problem with older C6s. The clutch seals get old and hard and don't seal well. Unfortunately, the only way to fix this is to take the trans out, tear it down, and replace the seals. You may as well replace the clutch plates while it is apart.

You MIGHT find some help with Lucas Oil's transmission product. It may help soften the seals enough to work properly again.

 

Pull the dipstick and check the fluid on the stick itself. If it's burgundy or lighter you are ok on that front, maybe the Lucas will help it. If the fluid is very dirty, very dark or a brown color, I have brought a c6 back to life by changing the fluid. The fluid is not cheap, but it's way cheaper than a rebuilt transmission.

Pull the transmission pan and drain that out and change the filter and clean the pan out. If you have large chunks in the pan, it's probably done. If you have what looks like some gold dust in it, that is normal for a high mileage transmission. Then go up to the flywheel area and take the little plate off. Then turn the engine by hand till you see a little plug on the torque converter. It's hard to see sometimes, it's inset into a hole in the flexplate. Turn the engine till that plug is at the bottom, then pull the plug. This will drain the converter also.

Then go to the store and buy about 13 quarts of transmission fluid. They usually take the Dexron type of fluid. Put it all back together, and get a long decent funnel and put some oil in it, and then start the engine. Put about 10 quarts in it and then pull the funnel out and put the dipstick in place. You will probably need more but put it in carefully till you get the oil between the marks. Always check the fluid with the engine running and in park.

The c6 is a tough transmission. If the fluid is really bad, new fluid may bring it back.

 

I haven't noticed that, but then again the notion wasn't in my mind to look for it. Thanks, I will take note next time.

It did shift into reverse normally just before the last incident. Could that also point to linkage?

 

I will give that a shot, thanks.

 

Thank you for taking the time to spell out the procedure.
I carry a spare water pump, starter, battery, set of timing gears, distributor etc, a sort of never get stranded strategy. It's easy enough to work on, and the i6 will, I believe, outlive me, even though it's 48 years old. The transmission seems a weak spot in that strategy however. I travel to some remote'ish property I have, 50+ miles away, where there's no cell service, and no local cab/Uber or nearby neighbor for that matter. I have a second 86 f150 (103k) which I bought after a storm dropped an oak tree on it. The woman I bought it from said she just used it for occasional trips to the dump. She said it overheated sometimes but was always good to go again after she let it sit.
it turned out to have no cooling system (radiator was disconnected) and she drove it like that for 6 years. That's another story, but relative to this, I was thinking

a) swap the trans from that into mine
b) convert to a manual trans
c) buy a rebuilt c6, if one can be had & trusted

Do you have any recommendations?

The oil on the dipstick looks clean, and what I've seen at the cooling lines does as well. It's all been red, no brown or burnt that I can see. I will follow your advice and change oil in it before trying anything expensive. Thanks again,

 

If you are shifting from park to L and you still are not getting into gear then I would say linkage is not the issue. More likely as Mark suggested your seals are dried out. If your fluid looks good maybe try the Lucas transmission additive to soften the seals. If that doesn't work the easiest thing to do is swap in another rebuilt C6. Going to a manual is an option but more work required.

 

If the fluid looks good, then don't waste your money on new fluid. Go to plan B or C

 

Where plan B would be make room for a quart of Lucas and top back off with their transmission conditioner. Some say it's snake oil, but I have used Lucas to recondition a rear main seal with good results. Mark is the resident Ford transmission engineer so I would feel totally comfortable with the suggested treatment. If it works for you its saved you a ton of money and time. I'm not certain but it might be easiest to suck some trans fluid out the top to make room for the conditioner.

 

Had Lucas transmission fix cure a problem on my daughter's car. In certain instances it does work. If you put it in in cold weather, make sure you keep it in the house before you use it. It won't hardly come out of the bottle when the weather is cold.