My tranny was working fine until I made a big drive of about 1000 miles, and after that it started doing really weird things.

I've got a C6, and supposedly it's supposed to be a good tranny, NOT. It's very selective when it wants to shift into 2nd gear. It shifts to 3rd no problem, but making the transition from 1st to second is starting to really get on my nerves. The only way it shifts is if I let off the accelerator, and then it shifts into gear. Sometimes it'll shift on its own and act normally, or sometimes it becomes a real pain in the A**. I obviously checked the tranny fluid and it was full, but I went and bought some and added it to the tranny and it seems to have improved a bit, but it still has its moments where it doesn't want to shift. I don't reef on my truck when I'm at a stop sign or light, I drive it properly.

So any ideas guys?

Thanks!

 

Check the vacuum line going to the tranny. From what I have been reading, the c6 for the diesel has the same vacuum modulator control as the gas engines, but of course the diesel has no vacuum so they must be using the vacuum pump, and I have read they use a little gizmo on the throttle linkage to vary the vacuum and thus control how the tranny shifts.

I would hunt around and see if you can recognize any of this and see if all the lines are in good shape. Also check down on the passenger side rear of the tranny, since they usually have a short rubber line going to the modulator down there.

 

Check the rubber line where it connects to the modulator. Pull the line off and make sure that there is no ATF leaking through the modulator. If there is any fluid leaking through the modulator replace it immediately. DO NOT DRIVE IT IF THE MODULATOR IS LEAKING. It will suck ATF to the vacuum pump and ruin it. If you find the modulator leaking you need to replace all the rubber vacuum lines from the modulator to the vacuum pump. The ATF will destroy the rubber hoses and the diaphragm in the vacuum pump. It doesn't take much to do it either.

 

Thanks very much guys! I really appreciate it. But can one of you provide a step by step on how to take these lines off, precautions, and what's it gonna cost to fix this?

 

And also...what the hell does the modulator look like, and where's it bolted to?

 

The VRV mounted on the IP has a vacuum line that runs down to the tranny.
Just follow that line and the vacuum modulator is the thing on the tranny end of the line.

If I remember right it was on the passenger side, but it has been years since I looked at an automatic.

 

The modulator is on the passenger side rear of the transmission just above the pan mounting rail. It is held in by only one bolt but be careful when and if you remove it as it is know to break. As to the lines just pull the lines off as they are rubber.

 

and when you remove it theres a pin that may come out with the modulator dont lose it you will need it for the new one it will just slide into the end of the modulator before you install it

 

Alright thanks guys, will do.

 

Agemenon sorry to hear about your tranny problemes I HAVE THE SAME THING GOING ON WITH MINE AS WELL. I talked to a trans. shop and they said it might be the governer is sticking. to fix it take the gov. out and to clean it real good. 9 times out of 10 it will fix it. don't know how much tranny fluid you will lose? I have not worked on mine yet low on fundes@to dam hot!!!
ken

 

To remove the governor you will have to remove the tail housing from the transmission. The governor isn't that bad of a job to clean. The only thing that gets me is that it shoudl act up on the 2-3 shift also if it was the governor.

 

Ok well some people are saying its the governor, and some are saying it's the modulator...uh...help lol? Don't you have to take off the drive shaft to get to it?

 

You have to take the driveshaft off. If you have a four wheel drive you need to take the transfer case off also. Start with the modulator ad go from there. I would pull the vacuum line off the modulator and check it first. After that if the hose is not mushy and there is no fluid leaking through the modulator hook a vacuum gauge to make sure the lines are allowing vacuum through them. After you have checked all the quirks in the modulators we will worry about the governor. I can walk you through working on the governor if you need me too but I think the issue is mainly with your modulator assembly. There is a old time trick to free the governor up but you have to be careful doing it. First do a fluid change. The find a chemical call "TAP MAGIC" specify you want "TAP MAGIC FOR ALUMINUM". Pour about a 1/4 cup into the transmission. Drive it around for about 10 minutes then get the truck up to about 30 MPH no higher. Throw the truck into park for no more than 15-20 seconds at a time before pulling it back into drive. DO NOT LET IT DROP UNDER 25MPH. What this does is use the park pawl to repeatedly tap on the governor mounting surface in the back of the transmission 9 times out of ten this will free the governor up. We used to use this trick on Chrysler three speed front-wheel drives when they had a governor stick. If I have been confusing let me knwo and I'll try to figure how to better explain it.

 

Yes, check the easy stuff first. The vacuum modulator system is not hard to work on. Suddenly lifting the accelerator to make it shift is a sure sign of a vacuum problem(leak in a line somewhere).

 

Ok thanks so much guys, where can I find a vacuum gauge? I don't have 4x4, and I'm familiar with taking off the drive shaft. I had to change the hanger bearing a little while ago. Does anybody have a digi camera to take a picture of this modulator? Also...I've read that upshifting requires a low vacuum to shift?