Rebuilt C6 WOT Shift Problem

[Hydro-idi]

Hey fellas,

I recently got my c6 transmission rebuilt at a reputable shop in Lodi. Transmission has been shifting perfect at 1/4, 1/2, and 3/4 throttle in all gears. It actually has never shifted this nicely before. Kickdown seems to be working properly as well. However, last week I was at a stop light and decided to try a WOT pass. The engine hit 3800 rpm and stayed there for a few seconds before shifting into second gear. If I let off the pedal, it will immediately shift into second. WOT from 2nd to third is spot on and does not have any problem shifting.
Truck is back in transmission shop and he is using a calibration tool to adjust modulator and VRV (vacuum regulator valve). He said that he has got it to shift a little sooner but there is still a delay from first to second gear ONLY under WOT. I gave him my spare VRV and he is going to try that this afternoon.
NOTE: He installed a green band vacuum modulator on transmission. Do you guys think this could be causing the symptoms?
Any help would be greatly appreciated. Thanks again

 

[Hydro-idi]

To add, the vacuum pump has been replaced recently and seems to be working properly.

 

[Clb]

A loooong time ago' in a gasser i had the same deal,,,, we did some voodo with the modulator valves to cure it... The idi shifted under load perfect( unknown modulator) hope this helps.

 

[Hydro-idi]

Doing some research on the internet, it looks like the C6 trans is supposed to have a black striped modulator for diesel applications. But I'm not sure if this is correct

 

[OLDBULL8]

IIRC The black strip modulator for Diesel's.

 

[Hydro-idi]

Thanks for the diagram Oldbull. This is what the trans builder is using to adjust the VRV. However, I am not so sure this is the problem. Might give him a followup call tomorrow and see if he can install a black band modulator and see if that solves issue. Maybe it's that or the internal governor in the tailshaft housing .

 

[Hydro-idi]

Clb, what exactly did you do with the modulators? Did you just swap color codes until the right one worked for you or did you modify the adjustment pin?

 

[Clb]

Initially swapped band color,cured my problem.... later down the line i seem to recall someone had an adjustable one??? Like the turbo 400?? A loooong time ago.
Vrv adj. Made a big difference on the 88.

 

[Hydro-idi]

Thanks for the info. Hoping the transmission shop will be able to get er going properly by tomorrow.

 

[Hydro-idi]

Transmission shop mechanic just called and said that they are still working on it. The used VRV I gave him pulls a little bit more vacuum apparently which has helped the shifting points. They are adjusting the spring tension behind the modulator and have almost got it dialed in. He is hoping to have it done by this afternoon. Didn't really understand what he was talking about...since my knowledge for auto transmissions is very limited.....but it sure sounded good lol.

 

[Hydro-idi]

Just picked up truck. All fixed! I guess they had to shorten the modulator pin to get it to shift properly. It seemed to be shifting a little earlier and softer than before so I pulled over on side of road and turned the trans modulator screw 1/2 turn clockwise to make it shift a little later and firmer. Seems to work perfect now. Thanks for the help guys