1971 F100 with C4 Shifting Help

[Skkzhg]

1971 Ford F100
Original 302 and C4 transmission

Truck is supposed to have 41xxx original miles. I believe this is probably correct. When I bought it a few months ago the transmission shifting very rough and down shift very quickly when accelerating.

I changed the fluid and replaced the modulator valve. I believe the fluid was the original and modulator valve too. The fluid was a little dark but did not smell in any way. The filter screen was clean and so was the pan.

now the issue. New fluid and valve and it shifts very smooth. It just shifts very soon. It will shift into 3rd gear around 20 mph. And it won’t drop back to second when I step on the accelerator. I can manually shift it into second and it downshifts very smoothly. The kick down linkage at the carb all appears to be in good condition.

I could not not find a stripe on the original modulator valve. I ordered one from Rock Auto that didn’t have a stripe listed but it came with a WHITE stripe. Will the wrong modulator valve cause the condition I’m seeing? If so, which valve do I need to order? I don’t think the one I have is adjustable.

Thanks!

[Busboy]

There should be an adjustable screw on the throttle linkage that sets the point at which the kickdown rod downshifts the trans. I think it will need to be screwed in to do this. In other words the carb linkage should contact the kickdown rod sooner in the travel of the throttle rather than later.

[Skkzhg]

The screw is ran in (tightened) as much as possible...not sure why it would work before I changed the modulator valve. Thanks for the idea.

[jzjames]

FWIW my C4 shifts very early, 1-2, 2-3.

The downshift from 2-1 is very harsh causing a bad ‘clunk’. The modulator valve I put in had a white stripe.
It has just always shifted too early, 1 to 2nd. I’ve lived with it.

You can look into shortening the pin inside the modulator valve.

It might be normal if you have 3.70 gears?

[390FE1972]

To quote someone here on the website "Check to see if your vacuum modulator has 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". Napa shows a white stripped vacuum modulator for a 1971 F100 that is adjustable. Napa part ATP 13671. Hope this helps.

[Skkzhg]

Yes, thanks for the help. I’ll double check to see if mine has the adjusting screw inside the nipple. It wasn’t listed on the website as being adjustable but the picture on the site does show the screw. Thank you all!

[DuckRyder]

On the kick down on the Carburetor. There is a specific air gap that it is supposed to be set at. It shouldn’t change the shift points though.

Make sure the vacuum lines are good (not collapsing, leaking, rotted ...) and that you are only running short pieces of rubber hose to connect the hard line.

[Skkzhg]

I haven’t adjusted any linkage since it worked before. I did replace both small pieces of vacuum line at the transmission side and engine side.

How much adjustment should be made to the modulator valve?

[390FE1972]

There are some video's on youtube that suggest doing 1/4 of a turn at a time, then do a test drive. There is one youtube that is called Ford C6 transmission adjusting the shift points that covers this.

Regards

[Skkzhg]

I’ll check it out. Thank you!

[cep62]

I recall an incident a looong time ago about a ball and spring that will pop out when you remove the filter.
I think it was a C4 , it made it shift harsh.

[Skkzhg]

I recall an incident a looong time ago about a ball and spring that will pop out when you remove the filter.
I think it was a C4 , it made it shift harsh.

Evidently this is part of my problem. I found the spring today in the bottom of my drain pan but no ball or valve. I cannot find one listed anywhere to buy it. Any ideas?

[BobbyFord]

I recall an incident a looong time ago about a ball and spring that will pop out when you remove the filter.
I think it was a C4 , it made it shift harsh.

Evidently this is part of my problem. I found the spring today in the bottom of my drain pan but no ball or valve. I cannot find one listed anywhere to buy it. Any ideas?

https://www.rearcounter.com/D5AZ-7E368- ... 91904.html

[Skkzhg]

I recall an incident a looong time ago about a ball and spring that will pop out when you remove the filter.
I think it was a C4 , it made it shift harsh.

Evidently this is part of my problem. I found the spring today in the bottom of my drain pan but no ball or valve. I cannot find one listed anywhere to buy it. Any ideas?

https://www.rearcounter.com/D5AZ-7E368- ... 91904.html

This was the problem. I ordered the spring and valve linked above and finally installed it about 2 weeks ago. Shifts smooth now and the kick down works correctly. Thanks for the help!!!

[Busboy]

My valve and spring fell out also when I removed the filter. I read somewhere that it is a torque convertor anti-drainback valve. I bought a new one from Rock Auto for like 2 bucks.