I'm not as familiar with a C-6 as I am a TH400 or a TF727, so I'm hoping someone here can help......The shift points are too early for me. I have a factory service manual, but it doesn't tell me where the shift points should be. (It says they are printed in some TSB but I have already spent way too much money on service manuals for this truck, I'm not buying a service bulletin) My 1-2 shift is at around 15-20 mph and my 2-3 shift is around 25-30 mph. It doesn't matter if I'm going easy on the gas or have it floored, or whether I'm empty or towing 7000 lbs. It makes it a real dog when towing. I have checked and adjusted the VRV to specs and that made no difference. I checked the vacuum diaphram at the trans and that was good. The vacuum is getting to the diaphram o.k., also. I then drove the truck with the vac diaphram disconnected and the shifts were exactly the same. My understanding of these transmissions is that with no vacuum signal my shifts should have been late and rather harsh. Can anyone tell me why they didn't change? Is there something else on a C-6 that determines the shift points? I know the trans was rebuilt, I've contacted the company that did it, but they aren't able to tell me how it was rebuilt.( They lost all their records to a Y2K bug) They did tell me that they would have done a heavy duty rebuild for a diesel 1-ton, though. Other than the shift points, the trans works fine. I have taken to doing some of the shifts manually lately and praying I'm not killing it. As always, any help or suggestions is appreciated.
C-6 trans. questions
- Thread starter flybear
- Start date
Michael Fowler
Registered User
How much vacuum are you getting the tranny modulator? My guess is that you might have the wrong modulator. Some of them are adjustable. Pull off the vac hose, and insert a small screwdriver and see if there is an adjusting screw in that nipple. You are correct, with no vac, shifts are as if WOT on a gasser, late and harsh.
Sounds to me like the shifts are right on. Most C6's are done shifting by 30mph. It doesn't sound to me like the modulator is working if disconnecting the vacuum yeilds no changes, and your sure you have good vacuum and a good adjusted VRV. Your used to gas engines winding out, and these diesels don't really go for that. Working vacuum will only cause the shifts to occur earlier.
Michael Fowler
Registered User
I can hold 2nd gear to 55 mph or so at WOT
If you can get to 50 mph in 2nd gear, please tell me how!!!! The vac modulator is getting the vacuum to it. I also put a vac pump on the modulator and it works correctly. It is an adjustable one, but it makes no difference in the shifts. That's why I decided to run it with the modulator disconnected, to make it think I was at WOT with a gas motor. I can't imagine that in that situation it would shift into 3rd at 30 mph.
flybear,
A couple of things to check: what color is the painted band on the vacuum modulator? There are 2 or three available for the C6 and only one is diesel specific. I'll need to crawl under my truck and check because I can't remember the color (purple or green??? Must be getting old...
) Does your truck still have the kickdown linkage attached and is it working freely? Still very puzzling why there was no change with the modulator disconnected which makes me wonder about the modulator itself 
. When you tested it where did you disconnect the modulator?
Bruce
A couple of things to check: what color is the painted band on the vacuum modulator? There are 2 or three available for the C6 and only one is diesel specific. I'll need to crawl under my truck and check because I can't remember the color (purple or green??? Must be getting old...
) Does your truck still have the kickdown linkage attached and is it working freely? Still very puzzling why there was no change with the modulator disconnected which makes me wonder about the modulator itself . When you tested it where did you disconnect the modulator?Bruce
When I removed the modulator I looked for the paint color on it as well as the rod, but could find no paint on either. I tested the modulator with a vac pump while it was removed. It moved smoothly and held vacuum fine.
The kickdown linkage is hooked up and moves freely. I'm in the process of getting it adjusted right, but so far that hasn't made a difference either.
There has to be a reason why the shift points don't change when there is no vacuum signal. I may end up at a trans shop to try to nail this down, I guess.
The kickdown linkage is hooked up and moves freely. I'm in the process of getting it adjusted right, but so far that hasn't made a difference either.
There has to be a reason why the shift points don't change when there is no vacuum signal. I may end up at a trans shop to try to nail this down, I guess.
A couple of other things to check: It sounds to me like the valve that the modulator controls is not operating correctly. It could be stuck and not moving, or the pin that goes between the valve and the modulator could be missing. When you pull the modulator off, you should see a steel pin sticking out of the hole - it's about 1/8 inch in diameter and maybe 1 1/2 to 2 inches long. It can fall out easily. If the pin is there, try taking it out and reaching in with a pair of needle-nosed pliers and see if you can pull the valve out. It should move freely. If it doesn't, work it out and polish the outer diameters where it contacts the case until it will move freely in the bore, put the rod back in and the modulator back on and try that. If the rod is there, and the valve moves freely, then your problem may be in the governor. That's a little harder to get to and a little more complicated, but can be worked on also, without removing the transmission. Check the modulator valve first and let us know what you find.
Dieselguy123
Full Access Member
I have had this same problem, I did just about everything and in the end in a last bit of hope i pulled the valve body out and resealed it, not sure what the issue was but it worked, There may have been a little metal shaving or somthing in there holding the check balls from moving (didnt really care since it was working) I understand the main guts of an automatic but dont know much about the valve body. -Brad-
The pin is there. I measured it, it's (1.705 in.) It should have a white paint spot according to the book. The book lists green, black and purple stripes as options on the modulator. I will look again on both for a color. I will also try to move the valve that the pin is supposed to be moving. I wasn't sure if that was something I should be able to move or pull out.
On another note, I'm getting my kickdown linkage adjusted to where it seems to be starting to work better. Still no change in the shift points, but with the help I'm getting here, I'm starting to get some hope.
If I end up removing the valve body, this thing will get a shift improver kit installed, I know that much. Thanks for the help folks, and keep the suggestions coming- I'm listening!!
On another note, I'm getting my kickdown linkage adjusted to where it seems to be starting to work better. Still no change in the shift points, but with the help I'm getting here, I'm starting to get some hope.
If I end up removing the valve body, this thing will get a shift improver kit installed, I know that much. Thanks for the help folks, and keep the suggestions coming- I'm listening!!
Diesel JD
Full Access Member
Don't remove the pin in the modulator or subject the tranny to continuous vaccum in any manner, you very well could do severe damage to the hard parts and seals...found this out the hard way. I think its just how the C6 is...pretty reliable but quite inefficient. Mine acts about like yours does.
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