I had some things to haul so I decided to take the trusty old dentside to work this morning - or not so trusty... Barely left and realized the truck wasn't gonna shift so here's how I drove. Take off in drive (first gear), rev up, shift out of drive into second on the column, rev it up till I hit about 42 mph, hold that for anywhere for 5-15 seconds. Then shift into drive, and the truck would usually cluck into 3rd. Occasionally it would shift from 1st to 2nd on its own in drive, but would never shift to 3rd. It always downshifted when stopping, but it really clunked from 3rd to 2nd at 35mph which felt really early.

I have never had trouble before. This truck always shifted like butter...you wouldn't notice the shift unless you tried to. I just hauled a big load of scrap a few weeks ago - probably about a 40+ min. run. Never noticed anything the matter.

So I'm afraid my faithful old truck is becoming a project. I really don't know what would be causing this, so any input will be helpful. (Obviously I have an issue with the transmission but I don't know any more than that.) I was planning to use the truck this evening too but it looks like that ain't happening.

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The first thing to check would be your kickdown linkage. make sure it's coming all the way up .

 

Bad vacuum line/vacuum modulator?

 

Definitely check out the vacuum line to the modulator

 

Found this at the back of the block. What is this exactly?

And this on the front of the carburator.

I also think something is missing here because of what I found lying on the head below it. Is there supposed to be a second spring on here somewhere?

And this is what was lying on the head below where the throttle arm comes to the carburator

 

Fix all your vacuum leaks for starters.

You have no kickdown linkage you need to get it working. for the time being look underneath and be sure the arm it hooks to on the transmission is fully in the rest position.

 

Where do I look for to the kickdown linkage on the transmission? I know very little but I love to work on vehicles so I'm learning as I go.

 

Is this what I'm looking for? And how do I know it's in the rest position?

 

The inner lever should be connected to your carb linkage with a curved steel tube what pushes it down to downshift. the lever needs to be in the full upright position for normal driving which it looks like it is. push up on it and see, and it might be a good idea to tie it up to be sure it doesn't move. I don't mess with automatics much but I know this is important.

 

Yes it is the whole way up.

I hooked up that hose at the back of the block and took it for a spin. Got all the way up to 50mph an it felt totally natural, I didn't even feel where it shifted. So is it safe to run like this? And what purpose does the kickdown linkage serve anyway?

 

the modulator is what tells it to shift, so without vacuum it wouldn't know. if you really can't feel it shifting or hear the rpm drop it may need replaced .

The kick down is so when you go to pass it quickly shifts down into 2nd for faster acceleration. it's not as important as the modulator but if that lever isn't held up it will prevent proper shifting and can lead to damage. it would be worth fixing.

 

Yes it is shifting fine, rpms drop like they should. It is very smooth though. Does the smoothness mean I have a healthy transmission, or is it slipping through the gears?

The kickdown lever is fully up and I can feel a strong springy up pressure when I pull down on it.

 

The kickdowns purpose is that after around 30 degrees of throttle opening the vacuum level in the intake doesn't change much. The modulator needs a little extra help and that is the manual kick down linkage. The lever should remain up on it's own. There is a spring in the valve body that pushes on the interconnected valve and linkage. The carb to lever pushes lever down and moves a valve in valve body to raise shift points. Not to beat a dead horse but a set of Genuine Ford shop manuals explains a lot, and a good read.

 

I know that you have a different carburetor, but you should have a kick down lever and linkage that goes to the transmission, it doesn't travel as far as the truck's throttle so it's adjusted so that it reaches full travel at wide open throttle. Leave it off, you'll have to manually down shift which means it's just a non automatic automatic, ... or a manual with a torque converter.

You also have a vacuum modulator, those hoses need to be fixed. The modulator is replaceable externally too. I have never seen a need to adjust it. It is a needed item on a C-6.

In the pictures I labeled below, the ball chain is just a link to my aftermarket cruise control cable, ignore it.

If you fix those vacuum hoses and get that linkage deal fixed, if the transmission isn't toasted from running with the vacuum hose busted, you are gonna notice a good difference.

Just for reference, if the carb you have isn't OEM, if it's a replacement aftermarket, often the kick down linkage had to be purchased extra. It may be a prior owner just skipped that step. Without that linkage in place, you are going to have soft early upshifts. Mine is an Edelbrock, but this Holley video explains the need and adjustments.

You can see the extension I made, reason needed on a 4 barrel is the primaries are moved forward when compared to a 2 barrel.




Good luck.

 

Like I said, with the vacuum hose back on it is shifting like it always did before. I think I'll put a hose clamp on there so it won't come off again. I'm certain I've never had the kickdown linkage since I bought the truck a year and a half ago. Been driving this way. Anyone know where to get linkage? I have an Edelbrock carb so I know that could make it a little hard to match up.

The little rubber cap on the front of the carb that's broken... anyone know where to get those or any DIY suggestions? I like to fix my stuff right when I do it, so nothing too sloppy.