My son's '68 F100 (390 + C6 Automatic Tranny) has developed a leak. It looks like it might be leaking a little from the pan but looks more likely forward of that near a verticle plate near where the tranny connects to the motor.

The fluid got a little low (maybe a quart) and started having trouble shifting into 3rd gear. It will shift from 1st to 2nd like butter but then will stay in 2nd or too long. Sometimes, if you let off the gas, it will shift.

The truck only has 24,000 original miles. Might there be some adjustment that can be made without dropping the tranny? I thought I heard from somewhere that there was some kind of thing that could be adjusted from the outside to help things like this.

What do you guys think?

Please let me know if you need more information.

Keith

 

I would first check the vacuum modulator valve and it's vacuum source. If you have a hand vac pump, disconnect the modulator line at the carb/manifold and see if the line down to the valve holds vacuum. Also, pull the rubber hose off of the mod valve and see if the valve is leaking trans fluid.
There is an adjustment screw inside the nipple of the modulator valve. Turning the screw counter-clockwise will result in an earlier shift pattern, clockwise will result in a later shift pattern.

 

Also are you for certain that it actually has 24,000 original miles? Sounds iffy for something as old as '68. I don't know the story of the truck, but it's highly likely that the odometer has rolled over once or twice. I wouldn't be surprised if it actually had 224,000 miles on it

 

Leak might be front seal or pump o ring. Fix would require trans removal.
Next: post #2.

 

I was skeptical too. I am the third owner. The first and second owners were neighbors in Italy Texas. The first owner owned the truck on a farm from 1968 to 1997 when he passed away. He willed that truck to his neighbor who owned it for 7 year. I spoke to the second owner at length about the truck including the mileage. He confirmed the original mileage
at 24K. He said the truck lived it's entire life on the farm in a garage. He also said that he didn't think he put 50 miles on the truck the time he had it.

Even still, it's hard to believe. However, if you saw the truck, you would believe it. It's remarkably clean. Wires behind the dash look brand new. The seat looks like it has never been sat on. It's a 45 year old time capsule. We are extremely lucky to have it.

Keith

 

There is a band adjustment screw that you might want to leave to the experts. The leak symptoms indicate a front seal problem which may occur due to age and not necessarily to mileage. I have used "rebuilt-tranny-in-a-can" when I know the transmission needs way more than that, often with good results.
Eric

 

See, that's awesome. I knew there had to be a great story behind that truck. I'm glad to hear you scored a clean ride like that, it makes me happy to see something like that pop up. Anyways, onwards with the C6! I'm tuned in too, as I have one.

 

Tuning in, my C4 is shifting late into third. Never used to.

 

Has your's been ever since you put the 4V on? Might just have to adjust the lever on the carb for the kick down rod. The throttle linkage on the 4v might be a liitle more forward than the 2V was.

 

The sealing area on the converter will rust up from sitting. This will cause the front seal to leak. Drop the trans, replace the seal and speedy sleeve the sealing area. BUT if the trans slipped much at all when you ran it low
the chances are great that the clutches are now worn. I do not think this would cause it to shift slow into high. But a plugged filter and or a dirty valve body from the sludge build up from the now worn clutch packs would. The diaphragm in the modulator can dry out from sitting as well. This Can cause hard slow shifts to high with a lot of rpm. Pop the rubber
vacuum line off the modulator and check for any ATF in it or the modulator. If any at all it is bad.

If the modulator checks out or you replace it and it was mine. I would
drop the pan, Clean it out and replace the filter and fluid. Than use some
trans seal stop leak and a bottle of Lucas trans treatment. It's the only
snake oil I have found that works and works well. Other than that you
need to overhaul the tranny. Really though the Lucas treatment has got
me a few more years out of a worn slipping tranny. But it is only a
band-aid at best.

 

Nah, it ain't that. It just started a couple weeks ago. I might have got it hot when I was pulling, but it couldn't have been THAT hot.

As far as the kickdown rod, I ain't ever had one.

 

Really? Interesting. A C4 knows when to shift by 3 inputs. Kick down rod,vacuum modulator,and an internal governor. 7c063 if I could paste pics like Bill can. You are missing one in the equation then.

 

Yea it doesn't downshift with throttle. But that's ok because with the floor shifter I can do that. I like it because I can usually just power through the gear instead of having it downshift and rev up.

 

C6 front pump has a seal, o-ring, gasket and bushing. All 4 of these parts (1966/97 same) should be replaced.

Inside the modulator valve is the 'throttle control valve rod' .. when the valve is removed, the rod will usually fall out. If installed without the rod, the trans won't shift.

 

Well, I replaced the Vacuum Modulator and it seemed to help at first. Here's what I experienced right after the replacement:

I drove the truck and it shifted from first to second like it's supposed to. It didn't seem to want to go into third. Once in second, the truck had lots of power and pulled well. However, when it hit about 35 - 45 mph, it seemed to slip. The engine had power but it did not pull like before. After a couple of minutes, it started shifting from 2nd to 3rd at around 30 - 40 mph and it was awesome. I drove it for around half an hour and it was perfect. I thought it was fixed.

However, I drove it again today and it started doing the same thing again. I'm sure when it gets warmed up, it will shift correctly but I have no idea what is causing the problem.

Any thoughts from you guys?

Here is the old modulator. Somebody had botched the hose before. I replace both hoses (above and below) so everything is air tight now.