C6 transmission slipping

[F100 Dad]

I wondering if anyone had any diagnostic thoughts on this?

1970 F100, 2WD, C6 trans, unknown miles

Since I purchased the truck it, the transmission slips pretty bad. It does this just after starting out of the hole and when I accelerate in all gears. It does this a lot less when a) the engine is cold and b) when I keep my foot light on the gas pedal. It also tends to drop out of 3rd in to 2nd when it is warm.

The problem lessens when it goes below 32 degrees outside and is significantly cleared up when temps are down near 5 degrees.

I have checked the fluid level - full
I smelled the fluid and it doesn't smell "burnt"
I have checked the fluid for metal particles or other material - nothing significant
I have replaced the vacuum modulator with a stock black stripe one.
I have replaced the rubber vacuum hoses and leak tested the metal one.
I have adjusted the intermediate band per spec.

I have NOT drained the fluid and replaced the screen(filter)

[F100 Dad]

A couple more thoughts after some more research.

[*]Could someone tossed out the pickup tube with the last filter change?

[*]"1978 was the year that Ford changed their transmissions from Type F. A stock 1977 C6 will slip and destroy itself on DEXRON or MERCON. " Could the PO have used something other than Type F?

[Donnie]

Hello, unless someone installed a deep pan or a 4x4 pan, the filter screen has no pick up tube.. If you have the original flat bottom pan, the screen in close to the pan...The 2x4 filter should not have a rubber grommet in it, if it does, this is a 4WD filter..make sure that there is clearance between the filter & the pan.
A C6 will not destroy itself if you are using Dexron, your shift's will be noticeably softer, but if your trans is in good shape, Dex will not harm it.... Has the pan been off recently, has the fluid been changed? look for signs of the pan being removed. I know that you said that you did not change the fluid, but CHECK FOR SIGNS OF PAN REMOVAL.
Did you install the modulator to try & correct this problem? You didn't accidently drop the pin that connects the modulator to the valve?
If I am understanding you correctly, it is slipping on pulling out, not between shifts which is the most common complaint. But any time that you accelerate, correct? Changing the modulator made no difference?
If the fluid has been changed, the color & smell now are useless when trying to find your problem..
BTW, how is reverse, any slipping, try backing up a steep hill...If we can eliminate reverse..I will head into another direction
Make sure that the rod from your carb to the trans is returning & not rubbing on any thing..Check the spring, it should stroke freely & evenly.
The fact that it reacts differently in relation to the thickness of the fluid, could be an -0- ring failure in the foreward drum.
This is only a guess, Run thru all the things that I mentioned & post back....Donnie

[F100 Dad]

Hello, unless someone installed a deep pan or a 4x4 pan, the filter screen has no pick up tube.. If you have the original flat bottom pan, the screen in close to the pan...The 2x4 filter should not have a rubber grommet in it, if it does, this is a 4WD filter..make sure that there is clearance between the filter & the pan.
A C6 will not destroy itself if you are using Dexron, your shift's will be noticeably softer, but if your trans is in good shape, Dex will not harm it.... Has the pan been off recently, has the fluid been changed? look for signs of the pan being removed. I know that you said that you did not change the fluid, but CHECK FOR SIGNS OF PAN REMOVAL.
Did you install the modulator to try & correct this problem? You didn't accidently drop the pin that connects the modulator to the valve?
If I am understanding you correctly, it is slipping on pulling out, not between shifts which is the most common complaint. But any time that you accelerate, correct? Changing the modulator made no difference?
If the fluid has been changed, the color & smell now are useless when trying to find your problem..
BTW, how is reverse, any slipping, try backing up a steep hill...If we can eliminate reverse..I will head into another direction
Make sure that the rod from your carb to the trans is returning & not rubbing on any thing..Check the spring, it should stroke freely & evenly.
The fact that it reacts differently in relation to the thickness of the fluid, could be an -0- ring failure in the foreward drum.
This is only a guess, Run thru all the things that I mentioned & post back....Donnie

Donnie, the truck came with an aftermarket pan (with drain plug) installed. Not sure if it is standard depth or a deep pan.

The truck has had this problem since I purchased it. I installed the modulator to try to correct the problem. I don't recall seeing pin (I replace the modulator back in August). Would it have been real obvious?

The kick down rod is installed and not binding. I didn't see a spring in the carburetor and transmission linkage schematic, where should I look for a spring?

I backed it up going down my street (flat) a couple of times and it worked fine. I'm still looking for a hill today to backup (safely .

[Donnie]

Hello, yes, the pin that connects the modulator "can" to the valve in the case should be visable when you remove the modulator.
Even though it is only 1/8 inch in dia. & about 1 1/2 inches long,, it can fall out if you are not looking for it when you remove the mod.
I would remove the mod & check for it's presence....that is easier than dropping the pan....If the pin is missing, you have found your problem.
The pin is selectable in about 5 different lengths...If it is missing, post back & I will tell you how to select & make the correct pin..Donnie

[F100 Dad]

Hello, yes, the pin that connects the modulator "can" to the valve in the case should be visable when you remove the modulator.
Even though it is only 1/8 inch in dia. & about 1 1/2 inches long,, it can fall out if you are not looking for it when you remove the mod.
I would remove the mod & check for it's presence....that is easier than dropping the pan....If the pin is missing, you have found your problem.
The pin is selectable in about 5 different lengths...If it is missing, post back & I will tell you how to select & make the correct pin..Donnie

I pulled the modulator and the pin is in there.

Where should I look for the spring you mentioned?

Other thoughts?

[F100 Dad]

It's been shifting almost perfect with single digit temps we have been having lately.