Nature of C6 Transmission Questions

[Hydro-idi]

Hey everyone I need a little input here. For a while, I've been noticing my truck shifts different when my truck sits overnight and engine is cold. Since winter is approaching and temps are decreasing, it seems to be more evident and shifts even harder when cold but smooths out once truck is warmed up. It also has delayed shifts.....rpm's go up more than normal and wait for it to shift. Around 2,200-2,500 rpm it will clunk into gear. Often times I will just let of throttle and it drops right into gear when cold. It does this for a few miles. My question is, is this just the nature of these transmissions or does something need to be adjusted? Again, when engine hits operating temp, transmission shifts just fine.
The transmission has been rebuilt approximately 70k ago, fresh atf about a month ago, and band adjusted properly.
Any help would be greatly appreciated guys thanks !

 

[Clayton]

Mine holds first and second longer when cold as well, After a few miles it's fine.

 

[dgr]

Mine does the same. I wish it would behave the same when warm as I prefer hard shifts and being able to control the shifts with the throttle.

 

[Wicked97]

Thats just a normal c6.

 

[RLDSL]

If concerned, it wouldnt hurt to give the tranny a good cleaning to free up the valve body to where it is running at maximum efficiency and deglaze the clutch plates . At the very least some transx, but since you dont have an immidiate problem, some Auto Rx in there would be better, and would do much better for the internal seals. ( trans x works a little better if you have a serious hard glaze problem preventing movement or causing very slow engagement from a stop, or hard slippage on a hill that requires a very fast fix, slow shifts are generally valve body related, and can have a LOT to do with fluid viscosity in colder weather, just the recent change to non worn out fluid could have caused that. high end Synthetic tranny fluids generally wont give you the cold shift delays until it gets REALLY cold )

 

[Hydro-idi]

If concerned, it wouldnt hurt to give the tranny a good cleaning to free up the valve body to where it is running at maximum efficiency and deglaze the clutch plates . At the very least some transx, but since you dont have an immidiate problem, some Auto Rx in there would be better, and would do much better for the internal seals. ( trans x works a little better if you have a serious hard glaze problem preventing movement or causing very slow engagement from a stop, or hard slippage on a hill that requires a very fast fix, slow shifts are generally valve body related, and can have a LOT to do with fluid viscosity in colder weather, just the recent change to non worn out fluid could have caused that. high end Synthetic tranny fluids generally wont give you the cold shift delays until it gets REALLY cold )

Thanks RLDSL I have 2 bottles of Auto Rx left but will be using that in the near future to put in the crankcase. I have used trans-x many times and have been very pleased with results. However, I did not add any to the transmission this time around when I put a new trans pan....thanks to idi-econoline! When I first bought the truck, I added 2 bottles of trans-x just for the heck of it and like the fact that it cleans well then evaporates from the system when it has done it's job.
I kind of figured that this was somewhat normal because trans fluid that is thicker may cause increased pump pressure and harder delayed shifts. Thanks for everyone's help

 

[Black dawg]

A weak vac. pump can also cause this issue.

 

[Hydro-idi]

A weak vac. pump can also cause this issue.

Ironically enough, this was going to be my next question. My vacuum pump is the original. I do know symptoms of a worn vacuum pump may be a lack of performance in regards to brakes.....if I am not misunderstood. However, my brakes work very well but guess it could still be worn out and cause shifting complications . I haven't really dug into the vacuum system to get a better understanding of how it works yet.

 

[mariner45]

As a matter of routine and suspecting the modulator to be original, I would recommend replacing the modulator on the transmission housing. They are about $40 or so and when changed, you know it is good and the replacement ones are adjustable. I thought my tranny was going but finally decided to renew the modulator and boy what an improvment. It was like a different transmission. Worth a try and not too expensive.

cheers

 

[blackflag]

Not to hi jack this thread but since it's on a C-6 I do have a question regarding the transmission.....and forgive me if it's a stupid question but here goes....can a fourth gear be added to a C-6 without having to add an overdrive unit? I wasn't sure if someone such as Brian's Truck Shop, a shop that several on here have used, can even do something like that. I've never heard anyone say either way so I just wanted to toss that out there. Thanks.

 

[icanfixall]

Not to hi jack this thread but since it's on a C-6 I do have a question regarding the transmission.....and forgive me if it's a stupid question but here goes....can a fourth gear be added to a C-6 without having to add an overdrive unit? I wasn't sure if someone such as Brian's Truck Shop, a shop that several on here have used, can even do something like that. I've never heard anyone say either way so I just wanted to toss that out there. Thanks.

If that could have been done.. It would have been done years ago. The C6 trans dates back to the early 60's. there just is not any extra room in the trans case to fit another gear.. But this has been a good question BTW... Nothing silly or stupid about it at all...

 

[PwrSmoke]

Cold oil also causes a harsher or delayed shift. ATF is like any other oil, it gets thicker when it's cold. Thick oil creates higher pressures. Higher pressures change the way the trans shifts to a small degree. On the other side of the coin, sometimes a cold trans has more internal leakage.. the seals are hard, the metal not yet expanded, shifts are delayed. Or it could be a combo of both plus issues with the vacuum system and modulator. Or gummy valves

 

[Hydro-idi]

I know it isn't really anything to worry about and wasn't sure if cold oil could cause it to shift to the extent it does when cold. Anyways, thanks guys.

 

[IDIDieselJohn]

All 3 of my C6's also do this.

 

[Hydro-idi]

Although the c6 transmissions aren't very well liked because of their mileage suckers, I like them because they are cheap to rebuild and are a darn near bullet proof transmission. And no electronics! I sold my previous truck because of that 4L60E GMC junk transmission I dumped thousands of dollars into.
Before I bought my idi, I called a guy I know who owns a transmission shop and told him I found a truck but wanted to know if the c6 was a good reliable transmission.....since I didn't know much about them at the time. He told me I probably wouldn't be seeing him because they are a stout trans that uses a simple design and will likely last a long time. An hour later I bought it lol.
Just figure I would post a new thread in regards to the harsh shifting because I don't remember it shifting that hard. Again, thanks for the help.