C6 Trouble (posting for a friend)

[4x4TruckinGirl]

Friend of mine has an 88 f350 dually 7.3L 2x4 truck with a C6...... He's getting hard shifts and has no reverse......... Does this sound like a vacuum modulator switch (or regulator switch, i forget the right name) Or does this sound like the tranny is toast? He's been getting hard shifts for a while, tranny fluid is good and has been changed regularly...... doesn't leak or anything either. Any input would be greatly appreciated!!!!

 

[david85]

I think you are refering to the vacuum regulator valve. It varries the line pressure and moves the shift points relative to throttle position using vacuum signal to the transmission. Its the black plastic thing on the driver's side of the injector pump with two vacuum hoses goint to it.

Can it cause hard shifting? yes, but usually will also cause late shifting if its shifting hard. This happens with a loss of vacuum to the system, its like a failsafe mode. The transmission acts like the engine is at full throttle all the time (its better than low throttle assumption because that could allow slipping)

Will it cause a no reverse condition? Nope, thats an internal problem, possibly a stuck valve, or leaking seals, or slipping clutches depending on what the symptoms are like.

Did reverse just quit working out of the blue?
or
Did it gradually get worse over time?

when reverse did still work, was there slipping, or would it slam into gear and hold?

 

[gonecrazyi]

I have seen this on a few trucks, the hard shifts may be due to the kickdown on the back of the motor but its been awhile since I had a c6. Has he tried manually crawling under the truck and putting it into reverse, it could be possible that the linkage has gotten messed up.

My biggest problem with a c6 was the vaccum hose on the passenger side of the tranny kept coming off. It would begin to leak and shift hard and would then completely come of and not shift into 3rd, what a pita.

 

[4x4TruckinGirl]

He said he replaced something sort of on top of the tranny, below the motor (I dont know what he was referring to?) But hasn't swapped the vacuum regulator out...... I don't know for sure, but I think the reverse just quit working from what I can tell....

 

[david85]

He said he replaced something sort of on top of the tranny, below the motor (I dont know what he was referring to?) But hasn't swapped the vacuum regulator out...... I don't know for sure, but I think the reverse just quit working from what I can tell....

So, it quit working after the part was replaced?

I'm not sure what part to think of from your description. There is a vacuum diaphragm near the rear of the transmission that is sort of up on top of the valve body/pan section of the case. Maybe thats it??

If the kickdown rod is suspect, it can be disconected at the lower (transmission) end to see if thats the problem. Some guys opt to delete that part in these trucks because the vacuum control works so well already.

 

[typ4]

Does low gear work or is it starting out in second? There is a low reverse clutch and a reverse high clutch, second gear is the band. forward clutch is used in all forward gears, obviously. The roller clutch holds in low iirc but uses the frictions in reverse. either way a lack of reverse is an internal problem not the vacuum valve or modulator. Very unlikely a stuck valve body. If it starts out in second ,put it in manual low then try it, make sure it has all three gears if not yank it a get it rebuilt. If you were closer

 

[Diesel JD]

I know where he can get a working C6 for next to nothing if it comes to that Sam 2wd

 

[funnyman06]

Not to steal the thread, but my truck shifts out of 1st really fast, not sure if its ever actully in first. I know first works, but i swear its always going from 2nd to third and thats it. Its not a huge fan of downshifting either, seems to be content with where it is. Could this be the Type F fluid im running with it not having any friction modifiers in it?

 

[4x4TruckinGirl]

Hey JD..... let me know about that tranny if you could..... location, price? Was is rebuilt? Pulled off another truck? I'm gunna research a little more on this truck today.....


I was just trying to save him some money before he spent a bunch on a new tranny.... but it sounds like its headed south...... :-\

 

[DeepRoots]

still make sure all the vacuum is what it needs to be prior to pulling the trans.

 

[LCAM-01XA]

don't need vacuum for the reverse tho, my old mud truck had the vacuum line fall off the trans and it would still go in reverse.

 

[typ4]

Not to steal the thread, but my truck shifts out of 1st really fast, not sure if its ever actully in first. I know first works, but i swear its always going from 2nd to third and thats it. Its not a huge fan of downshifting either, seems to be content with where it is. Could this be the Type F fluid im running with it not having any friction modifiers in it?

They will shift out of first really soon under light load, you can adjust the modulator and the valve on the IP to time shifting.

mod. screw, in later shifts, earlier, out.

 

[trackspeeder]

Reverse is mechanical. No reverse is an internal issue. Time to play inside.

 

[wolfman]

Hey chris - when I first got my 86 f250, transmission shifted exactly the same - seemed like it changed to second at about 3 miles per hour - changed modulator which didn't help. Found that the vacum hoses at the VRV at the pump were crossed. Switched them and the trans shifted perfect.

 

[funnyman06]

That could be it, maybe i just drive it like an old man, idk. 20 years old and the only thing i can think about is how to get the best mileage outa the truck that i can. what has the world come to...