Vacuum Lines/Transmission Relationship? - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 08-11-2006, 10:01 PM   #1
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Default Vacuum Lines/Transmission Relationship?

This quote is from another post and it got me thinking...
http://www.bonnevilleclub.com/forum/...r=asc&start=20

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Originally Posted by zeroburnz
Hey guys. Replaced all the vacuum connections down by the evap. canister and some others that looked like they needed replacing. Have noticed that my rpms at idle have smoothed out a little and no more jerking when the transmission shifts. I am however still getting the misfire code on cylinder 3. Other suggestions?
I know the rubber connections on my vac lines are starting to dry out and crack - I'll be replacing them soon regardless. Not sure if it'* related, but at light to medium light throttle the shifts have started to become a little rougher - nothing major but definitely not silky like they were just a few months ago. I don't pound on the car whatsoever and I did the trans fluid and filter less than 12k ago. Fluid is still nice and pink. I have a 4T60-E I believe and the "E" would lead me to think it'* totally electronically controlled but that comment above made me wonder! zeroburnz has a '97 SE as well.
What'* the deal? Should I also check the transmission solenoid for it sticking? Thanks
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Old 08-11-2006, 11:36 PM   #2
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Vac leaks effect your performance and that could effect how a shift feels..

The original author was probably a 4T60 transmission that is controlled by vacuum, whereas your 97 is fully electronic controlled.
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Old 08-12-2006, 01:48 AM   #3
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He'* got a '97 L36 too though; I thought I've got the same motor tranny combo...
I guess I'm still sort of confused.
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Old 08-13-2006, 06:00 AM   #4
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Same combo.

The vacuum lines affect shift because of (that brass looking can on the front of the tranny) having a vacuum connection.
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Old 08-13-2006, 06:48 AM   #5
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Dame is right...

The 4T60-E is mostly controlled by the PCM. However, vacuum controls the firmness of the shifts (I'm not sure if "line pressure" is the correct terminology, but we'll go with it). If you completely disconnect your vacuum lines the transmission will shift really hard, which is the same thing that happens if you have a vacuum leak or have the vacuum modulator adjusted to allow only slight vacuum pressure (like mine ). If your vac lines are compromised, your shifting line pressure will go up, causing more "jerky" shifts.

This is the reason why aftermarket parts companies advertise "higher shifting line pressure" adjustments to their PCMs, but the effect isn't felt in '97 L36 Bonnies (L67 Bonnies from '97 had the 4T65-E). The only way to change shift firmness in a 4T60-E is to reduce vacuum or install a shift kit. Line pressure is totally independent of the PCM, unlike a 4T65-E, which is fully electronic.
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Old 08-13-2006, 10:22 AM   #6
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Awesome explanation!! Thanks.
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