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Discussion Starter #1
So my 67 390 w/c6 has always had intermittant trans slipping type issues, but I pull away from a red light and give it about half throttle and the engine is struggling (shifter in drive and trans slipping pretty good). All of a sudden, massive smoke screen out the tailpipes. Rest of the way home, the trans would sorta try to slip (again, intermittant).
So, I figured bad transmission modulator valve allowing seal to be overcome by vacuum and pulled some trans fluid into motor and burnt it out (car running rough, big cloud of smoke, all cleared up in about 30 seconds).
Replacement solenoid is physically about 1/2 longer, but from where the little rod pokes out is same with both old and new modulator solenoid valve (whatever it's called). I've tried it as came from box (said already adjusted) and have backed out on the little middle pin in the vacuum port at the rear 6 full turns and still it acts like trans not getting any vacuum. If left in drive, it stays in first. I have to manually shift from 1st to 2nd and when I go to drive, it's still in 2nd and revving pretty good.
Is there any transmission modulator valve adjustment involving actually physically backing off the modulator valve back out of the transmission casing? The stock one I had to use vise grips but with little pressure, it came off. So, I did replacement one hand tight and tweaked a extra tad with vise grips (not hard, no case markings).
So, after replacing what was suspected bad modulator valve, I have manual 1st and 2nd and no drive gear at all. The vacuums are properly hooked up and engine runs like a champ.
 

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I don't know anything about transmissions even with all the issues with different ones over the years. But i would think the replacement modulator should be identical to the one you took off there. But then that could be a difference in brands. I had a tranny years ago that i would have to put into neutral then into drive again to get it to shift to 3rd. Have no idea what the issue was though. Then it was parked for 5 yrs and the trans quit completely. Might be the pump is dieing on it? I'm sure someone that knows these beasts have some thoughts. I'm curious to hear them myself.
 

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You need good vacuum signal to the modulator. Did you put the pin back in when you pulled the modulator?
 

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I too would try a modulator befrer a $1,000 rebuild, though I think we would just be running the bill up to $1,015.

Make sure the pin is in and it is the right length. With the modulator diaphram adjusted all the way back, you can suck on the thing to measure this, the pin should just exactly fit between it and the valve inside the transmission. Measuring is tricky because you have to subtract the length of the threads.

You can trim the ends on a finishing nail as a pin substitute. Then adjust the modulator halfway forward and reinstall.


Good luck
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Puttster hit on the key here...it's gonna take some measuring and comparisons between pin depths of both old and new modulators to come up with a fix. The modulator allows the valve in the tranny to move depending on the vacuum at the modulator. The pin simply presses into/against that valve to position the valve where the modulator wants it. There is a figure posted somewhere on the net(maybe even this site) of what the pin legth is supposed to be...with the CORRECT modulator, that is. The measurement is 1.75" or there abouts, from memory.

I can tell you if there is NO pin at all, the tranny will basically skip 2nd gear(or have a very short-term engagement), but still shift out of 1st and up to third while in "D". At least it did in my car. My guess is that the tranny "thinks" your pin is too long, since your tranny will not shift at all, so I would make up a couple of pins of shorter lengths than you have now and try slightly shorter and shorter ones till the problem is solved or changed. At least you'll know if you are on the right track.

Also, there are/were different types of modulators...is yours the push-in type or the thread-in type?
 

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Discussion Starter #7
mine is screw in type. I used the pin (didn't forget it or drop it) and oddly, have measured with calipers how long it was in stock one and replacement and it measures fine. So, I'd think it'd work? The replacement looks very similar but is 1/2" or so longer, but it's on the backside, the threaded portion looks same. The replacement has white stripe, I soaked the stock one in Purple Power to get the grunge off to see if I could read a color coded stripe, but if it did have one, it's long gone. I guess,for the heck of it, I'll make a few different length pins to see if any improvement. Probably put stock one back in to see if my automatic shifting comes back, etc.
Struggling hard due to herniated discs and working with ramps on the dirt makes it tough, so I appreciate the help.
Yes, vacuum source is great, tracked my line, installed new rubber at modulator as old rubber vacuum line was swollen. Given the pin pokes out the same in stock modulator as replacement, I'm just shocked it's not working.
 

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I can tell you if there is NO pin at all, the tranny will basically skip 2nd gear(or have a very short-term engagement), but still shift out of 1st and up to third while in "D".

Man I hope that was not the problem with mine. Spent $1,000 on a rebuild because Tranny skipped over second. That made $1,015 total :mad:
 

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Were the mechanics snickering as you wrote the check, Puttster? To the OP, assuming you pushed on the pin to make sure the valve was all the way forward before you measured the stick-out length, the pin's contact surface inside the new modulator may be deeper or shallower that the old mod. And there are "plain" mods which have no stripe at all. I've seen plain, white, yellow, green and black stripes on modulators...all having different springs and/or spring rates inside. Keep working at it.
 

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I would of checked to see if they had another that was identical to your old one. Took a couple times of ordering at the local parts store to get the right one for mine way back. Then you'd know for sure if it was the modulator or something else.
 

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Discussion Starter #13
Well, ShotRod64, I screwed with the Oreilly Pioneer modulator and was under the car about 20 times at least (under ramps), dodging getting burnt from exhaust (only once), and gave up and ordered from Dearborn Classic and it came with factory ford part number on box, so I knew I was in business. Sure enough, it even went on easier and shifted with zero adjustment. It cost twice as much, but I knew when I saw the factory part number, I was going to have much better luck.
I was all prepared to be upset if I opened the $35 box and it was the same $15 modulator I already had, but that was it. Because I appreciate everyones time to help, I try to type the fix for others who run searches like I do. So, the factory 67 Ford C-6 screw in modulator part number is C1AZ-7A377-B. Shipped to your door from Dearborn Classics I guess as long as they have factory ones left. I guess then you have to start taking pot-luck with the cheapo knockoff parts that hit and miss sometimes as far as working properly.
I am so thankful to have a shifting transmission, I'm going to try to get on ramps early tomorrow pm and drop the trans pan and do a fluid change and find out if I have a replaceable filter or the mesh screen ones like I've seen before. Thank you again for your suggestions and help.
 

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Man I hope that was not the problem with mine. Spent $1,000 on a rebuild because Tranny skipped over second. :mad:

No way can youget a whole gear back with just a modulator :mad::mad::mad:
 

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If it wasnt "missing" there is....
 
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