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Old 08-19-2006, 04:26 PM   #1 (permalink)
icer97
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400 turbo guys....quick 2-3 shift

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?
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Old 08-19-2006, 08:46 PM   #2 (permalink)
Mike L.
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Quote:
Originally Posted by icer97;1250584;
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.
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Old 08-19-2006, 10:53 PM   #3 (permalink)
icer97
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Thanks Mike L! I knew there was a good tranny man around here.
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2004 GMC SLT Crew Cab LB7 Duramax 4x4
Hot Juice with Attitude, Predator stack, Stock Tranny holdin up for now but for how long??
Soon to come (or atleast saving up for) EFI Live with McRat Tune, Suncoast V and TC

1976 Chevy shortbed converted 1 ton, 383 stroker, 400 trans, 205 gear drive, 14 bolt,dana 60 both with 4:56 gears and detroit and powertrax lockers sitting on 40's its my mud toy!

1984 Chevy CUCV 1 1/4, 6.2 diesel, 400 trans, 208 transfer 14 bolt, dana 60 4:56 gears, blackout lights work and everything!

2006 Polaris Sportsman 800 efi for a play toy
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Old 08-20-2006, 01:09 AM   #4 (permalink)
Ruben Z
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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.
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Old 08-20-2006, 10:39 AM   #5 (permalink)
icer97
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Ill give that a try, thanks azzkicker.
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Hot Juice with Attitude, Predator stack, Stock Tranny holdin up for now but for how long??
Soon to come (or atleast saving up for) EFI Live with McRat Tune, Suncoast V and TC

1976 Chevy shortbed converted 1 ton, 383 stroker, 400 trans, 205 gear drive, 14 bolt,dana 60 both with 4:56 gears and detroit and powertrax lockers sitting on 40's its my mud toy!

1984 Chevy CUCV 1 1/4, 6.2 diesel, 400 trans, 208 transfer 14 bolt, dana 60 4:56 gears, blackout lights work and everything!

2006 Polaris Sportsman 800 efi for a play toy
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Old 08-20-2006, 06:06 PM   #6 (permalink)
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Quote:
Originally Posted by AzzKicker;1251288;
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.
Bypassing vacum regulator will drop mainline pressure severly enough to burn up the trans.
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Old 08-20-2006, 06:16 PM   #7 (permalink)
Ruben Z
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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.
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Old 09-15-2006, 03:35 PM   #8 (permalink)
Fred482`
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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.
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Old 09-15-2006, 10:31 PM   #9 (permalink)
Mike L.
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Quote:
Originally Posted by Fred482`;1301669;
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.
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Old 09-16-2006, 12:18 PM   #10 (permalink)
Fred482`
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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.
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