My c-6 won't kickdown from 3rd to 2nd above 55mph. is there a way to raise that to say 65mph and is there any harm if i do so. my 99 gmc will kickdown up to 70mph.

 

sound about right 55 same as mine sounds pretty scary when it kicks down at 54 hate to see 65!! but it can be adjusted goto local tranny guy and talk to him as far as i know it can be done

 

There wouldn't be any point in it kicking down at 55, the rato is too steep, in other words your engine will be screeming and out of power. If you want 2nd gear ,just drop down on your selector. The kick-down is manual, there is a adjustment on the carb, but it just adjusts it to kick down only at full throtle .

 

Your 99 GMC has a 4 speed automatic, with overdrive, and a lockup torque converter. That's why it'll still drop a gear at 70 MPH, it drops from overdrive to third...
Your C6 is only a three speed, with no overdrive.
You DO NOT want a C6 dropping to second at 65 MPH, unless your engine will rev to the moon.

 

thanks guys. i suspected that 55 of 60 might be its limit but i do not have a tack on it so i do not know what rpm it is at at 55. sounds like i should leave good enough alone. thanks for the help

 

My C-6 in my 82 will downshift at 55, not at 60 though. Its stays in until about 70 mph.

I have 3.54 gears. You can adjust the downshift point also. You will see a set screw on the carburetor where the kick down rod sets. Study it and you will see how it works. Turning the screw clockwise will make it kick down at a higher speed. Don't get it too high though.

Your gear ratio will play a part in how much to adjust it. Yeah, if you have low gears, you could be in trouble if you turn it very much.

 

My truck will downshift to second all the way up to about 75-80. I know it will run almost 90 in second. Of course I'm not even close to stock. Also I have 3.27 gears.

The first thing you want to do is check the kickdown rod and make sure it is adjusted properly. Then you can tweak the shift points by adjusting the vacuum modulator in the tranny. It is located on the bottom side near the back of the pan and should have a vacuum hose going to it. Take of the vaccuum hose and use an alan wrench (I think some took a flat head screwdriver) and turn it to adjust it. I think clockwise raises shift points but I can't remember. Also you can buy a replacement modulator with a different range. They are all adjustable but they are available in different ranges. I am going from memory here, but I think I went with one with the black stripe. Something like that, anyway the parts store should know. I chose the one with the highest shift points and then I adjusted it up even more. You can also take out the vacuum modulator and take out the rod that it controls and make a longer one to raise shift points.

 

how do measure if the kickrod is adjusted right? my tranny shifts well and under 55mph downshifts smoothly. does the adjustment of the kickdown effect the highest speed it will downshift? or just adjusting the vacumn modulator do that?

 

The kickdown is basically a forced downshift. If the rod is not adjusted properly, you may not be forcing it to kick down, or at the very least you are having to push the pedal further than you should in order to make it kick down. The vacuum modulator is more for partial throttle shifts, although adjusting it WILL change WOT shifts as well. I can attest to this personally. I had been told that it would not, but after looking at the schematic for the C6, I was sure that it would affect all shifts no matter the position of the throttle, whether part throttle or WOT. I was right. Adjusting the modulator does indeed change the shift points.

 

I remember seeing some where an actual measurement for this.

I think it was you manually hold the carb linkage wide open and the downshift rod back as far as it will go, then adjust the screw on the carburetor until there is 60 thousands of an inch clearance between the two.

I always just adjust it until it satisfies me where it downshifts.

My cousin and an old Ford truck, we had to put some loctite on the adjustment screw, the vibration of the truck would keep backing the screw off until it would not downshift at all.

I adjust mine to downshift at no more than 60 mph.

 

thanks! i will give it a try