C6 Modulator

Clutch, transmission, rear axle

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C6 Modulator

Postby coop on Sun Jan 03, 2010 11:30 pm

What would the symptoms be for a C6 trans be if the modulator valve isn't working? Is there a way to test the modulator valve?

I have a 1970 F250 just recently the truck when you romp on it would stay at 40 mph and not shift and on another occasion I was climbing a hill and the truck wouldn't go faster than 50 mph. When before it would pull 70 on it. If you just drive it normally it seems ok. It seems to happen when it kicks down.

I did have a bad dist vac modulator it was ruptured and the timing was off I fixed both. It seems a little better but it still doesn't seem to be shifting the same as before.

It is like the engine is hauling ass but the truck isn't. Does this sound like a trans prob it just started about a couple of weeks ago.
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Re: C6 Modulator

Postby Dragon on Mon Jan 04, 2010 2:42 am

It won't shift smooth and at full throttle it might not shift at all until the governor forces a shift.  The modulator positions a valve so the fluid at high vacuum does not jerk the tranny into gear. At low vacuum it allows the full force of the shift pressure to kick the tranny from one gear to the next.  At high vacuum another part of the same valve will cause the tranny to shift by being in the position to allow fluid to move to the shift valves.

Get a hand vacuum pump and take the modulator out and pump the handle the valve spool will move into the modulator body.

It sounds like the tranny is not getting several signals. The governor is not forcing a shift nor is the modulator allowing a shift.  How clean is the fluid.
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Re: C6 Modulator

Postby ArizonaDan on Mon Jan 04, 2010 11:51 pm

Congratulations on the distributor resolution,

Transmission shifting issues can be tough to isolate because there are numerous components associated with the single action of shifting.
In no particular order I would humbly suggest checking the following items:

Transmission fluid level.
Transmission fluid temperature (if possible) Too hot will cause problems.
Transmission fluid cleanliness.
Transmission fluid filter condition.
Transmission modulator operation (diaphragm integrity as well as actuator movement)
Vacuum leaks at ALL components AND HOSES (I know I emphasized this for your timing issue but vacuum is too often overlooked when troubleshooting transmission shifting issues. I'm not hung up on vacuum, it's jusy really important.) It could very well be that the vacuum signal from your carburetor port to your transmission modulator is intermittant or weak for some reason.
Internal wear of transmission clutches (or bands) I do not know which terminology Ford uses for their transmissions.
Torque converter operation by determining transmission fluid pressure with an auxilary pressure gauge. I don't know what proper pressure would be but someone else here most assuradely does. Keith seems to know more about these Fords than even old Henry himself.

Unfortunately some of these troubleshooting processes will require you to spend money for an indeterminate outcome. Interal wear of transmission components and transmission filter condition requiure you to replace suspect parts with known good parts in order for defective parts to be identified. Not fun I know.

Good luck
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Re: C6 Modulator

Postby coop on Tue Jan 05, 2010 4:23 pm

Thanks Dragon and Arizona Dan for your advise and suggestions.

I did change the fluid and filter about a 1000 miles ago also the oil in the torque converter. I will double check the fluid condition and level that's a easy place to start and go from there. I will keep you posted.

Thanks again.
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Re: C6 Modulator

Postby ArizonaDan on Tue Jan 05, 2010 9:12 pm

A lightbulb just flashed on over my head! :eek:

One additional possibility on your transmission shifting problem: Fluid incompatability with internal clutches/bands.

It just occured to me that you mentioned recently changing the fluid. Not insinuating that you did anything wrong but if your transmission has never been rebuilt and has the original clutches/bands then it requires the exclusive use of, I believe, Type F transmission fluid only. The wrong transmission fluid can definitely cause shifting problems.

Or, if at any point in the transmission's long history an improper transmission fluid was added the possibility is that insufficient lubrication has occured that over a long period of time could lead to problematic transmission operation you are now experiencing.

I wouldn't consider this possibility to be the most likely one but it is just another potential factor that could be looked into.

I'm just throwing any potential scenarios at you that may not otherwise be considered.

Good luck, again.
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Re: C6 Modulator

Postby Dragon on Tue Jan 05, 2010 9:30 pm

Dexron (GM) Fluid is not slippery like Type F it will make the Ford Tranny shift hard.
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Re: C6 Modulator

Postby Bullitt390 on Tue Jan 05, 2010 9:56 pm

Type F has the friction modifiers and makes the tranny shift harder.  That is why many GM guys either use Type F or B&M Trick Shift to "Firm" up their shifting.

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Re: C6 Modulator

Postby ArizonaDan on Thu Jan 07, 2010 6:34 pm

Clarification please,

Dragon says Type F is "slippery" and that Dexron will make a Ford transmission shift hard, if I correctly interpret his posting.

Bullitt390 says Type F has friction modifiers, which makes a Ford transmission shift hard.

Your respective claims are perfectly contradictory! It is impossible that both of your are correct.
Last edited by ArizonaDan on Fri Jan 08, 2010 9:08 am, edited 1 time in total.
1968 F100 -360, 2BBL, C6, 3.25, ugly? yeah, she's ugly.
1997 Expedition -4.6L, 4WD, 14MPG, gutless? yeah, she's gutless.
2001 Kawasaki ZRX1200R, fast? yeah, she's fast.
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Re: C6 Modulator

Postby FORDification on Thu Jan 07, 2010 6:54 pm

In this case, Dragon is correct and Bullitt390 is (half) mistaken. Type F had no friction modifiers...Dexron does. But Bullitt390 is correct in stating that if you use Type F in a transmission designed for Dexron/Mercon, the shifts will be firmer.
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Re: C6 Modulator

Postby Dragon on Mon Feb 15, 2010 9:50 pm

Harder for towing and racing softer for light car cruising.
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Re: C6 Modulator

Postby coop on Wed Feb 17, 2010 8:46 pm

Cool thanks
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Re: C6 Modulator

Postby rjewkes on Fri Feb 19, 2010 6:57 pm

Leave it there as long as t isn't problematic, and you should be fine.
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Re: C6 Modulator

Postby ToughOldFord on Sun Feb 21, 2010 11:18 am

Type F is not 'slippery'. It is a high friction fluid:

All Ford Motor Company transmissions built prior to 1977, and certain models that continued in use during 1977-1980, were designed for a high-friction fluid that allowed the shifting clutches to lock up or engage quickly. Ford specification ESW-M2C33-F covers this type of product, which is commonly referred to as "Type F" fluid.


http://www.mobiloil.com/USA-English/Mot ... F_ATF.aspx
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