I have a 84 1 ton CUCV with a 6.2 diesel and a 400 turbo. The tranny shifts great when its WOT but with just normal driving it wont stay in second no time and it shifts right into 3rd. Its kinda annoying, I usually will manualy pull it into second so it will actually use it. I checked the vacume line from the vacuum pump down to the modulator and its fine. I know turbo 400 use no kick down and shift off of vacuum. My question is there a way to adjust the modulator, or do I need to adjust something else?

I have a 84 1 ton CUCV with a 6.2 diesel and a 400 turbo. The tranny shifts great when its WOT but with just normal driving it wont stay in second no time and it shifts right into 3rd. Its kinda annoying, I usually will manualy pull it into second so it will actually use it. I checked the vacume line from the vacuum pump down to the modulator and its fine. I know turbo 400 use no kick down and shift off of vacuum. My question is there a way to adjust the modulator, or do I need to adjust something else?

You have a bad vacum regulator.

Thanks Mike L! I knew there was a good tranny man around here.:ro)

You can test by bypassing the Vaccum Regulater up front. With my old 85 the tranny would stay in first and SLAM into 2nd. I bypassed Vregulator and it shifted smoothly.

Ill give that a try, thanks azzkicker.

You can test by bypassing the Vaccum Regulater up front. With my old 85 the tranny would stay in first and SLAM into 2nd. I bypassed Vregulator and it shifted smoothly.

Bad idea.:rolleyes:
Bypassing vacum regulator will drop mainline pressure severly enough to burn up the trans.

I didnt say to leave it like that. I said that was a good way to test it. I don't see how Trans would burn up anyway if it shifted normally. I bypassed VR and the truck shifted better than my newer 2006 2500HD w/ 4l80e out of the dealer.

THM400 does use a downshift switch. Mounted above throttle pedal, adjustable only to control full throttle or forced downshifts. Operates the downshift solenoid mounted on the valve body, inside the pan.

THM400 does use a downshift switch. Mounted above throttle pedal, adjustable only to control full throttle or forced downshifts. Operates the downshift solenoid mounted on the valve body, inside the pan.

That has nothing to do with the problem.

Who said it did, the comment above said there was no kick down and "uses vacuum". There is a kickdown switch. I am fully aware of the problem, I've been repairing them since '62! Mike, you're very knowledgable, you could be a little less offensive in your answers, we're all here trying to help.

The VRV is a controlled vacuum leak. If the switch warps, too much vacuum bleeds off causing high and late upshifts. Heat will cause the VRV to warp over time. Replacing and adjusting the VRV will help in most cases. Often, it's necessary to install an aftermarket adjustable vacuum modulator and back the screw out to attain the proper shifts.

I've installed new VRV's and still not been able to get the correct vacuum signal to the stock modulator, even with a new vacuum pump. I seen new VRV's read 27" hg @ inlet and less than 11'' at the modulator line. Sometimes, depending on the transmission calibration, that's not enough. The adjustable modulator allows for further adjustments to tailor the shift.

Fred482
I don't see how my answer was offensive in stating that the kickdown switch was not the problem. Seems like some of you guys are very thinned skinned. I apologize. I will stay out of this section in the future and leave the diagnosing to you guys. :D

Fred....That answer from Mike wasn't offensive....

NOW, if you want to see some of his replies that WERE offensive, I'll dig them up for you....):h

Fred482
I don't see how my answer was offensive in stating that the kickdown switch was not the problem. Seems like some of you guys are very thinned skinned. I apologize. I will stay out of this section in the future and leave the diagnosing to you guys. :D

Mike, No apology necessary, as I stated, you are a very knowledgeable guy and have helped many people on this sight. I'm not offended, I'm a Chief Officer in a fairly large volunteer Fire Dept., Thin skin is not one of my faults! ):h