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Old 12-21-2005, 06:31 PM
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T-400 Shifts Hard

I have a 350 with a TH400 when i'm in gear driving the tranny shifts hard from 1st to 2nd and even from 2nd to 3rd but not as hard as 1st to 2nd. Is this a converter problem or something different. i'm not too good with trannys so any and all advice on this would be helpful. Thanks

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Old 12-21-2005, 07:21 PM
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Does it tend to stay in 1st to long...? Possibly a vacume modulator...? (or lack of vacume source...?)
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Old 12-21-2005, 11:25 PM
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Yep. Sounds like a vac modulator to me.
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Old 12-21-2005, 11:40 PM
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yeah it stays in 1st sometimes for a bit then shifts, if it's the vac modulator how do i fix it?
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Old 12-21-2005, 11:51 PM
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well first ensure that you are getting engine vacuum to the vacuum modulor. The vacuum line will run from the intake down to the passager side of the transmission. Get Under the car and find your vacuum mondulor it will be on the passager side with a rubber or metal line(with a rubber tip) attached to it.
Pull that line off and use your figure to test for vacuum or use a vacuum gage to test it. If there is vacuum in the line then your diaphram in the vacuum mondular may have failed. To remove the modular you will find a bolt beside it that holds a tank against it. Once you remove this bolt it will just pull out.
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Old 12-22-2005, 12:39 AM
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If you have to replace it, its about a $20 part that's held on the exterior of the case by one bolt. Should be easier than changing your oil.
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Old 12-22-2005, 12:34 PM
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Ok thanks I'll get on that as soon as possible the weather here is bad right now when it clears up for a bit i'll try it. What if there is no vac on the line then what do I do.
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Old 12-22-2005, 03:06 PM
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ok weather cleared up for a bit I went out and checked the vac there was good vac on the line so I replaced the vac modulator and if shifts MUCH BETTER!!!!! thanks for the help guys.
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Old 12-23-2005, 03:02 AM
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ok guys the trans shifts good when u give it alittle gas but when you romp on it it shifts hard from 1st to 2nd but not so hard 2nd to 3rd. I was guessing it's because of the torq converter on the trans. Could that be why it shifts like it does. I don't mind the hard shift like that I was just wondering if that was it or if it was something that needed to be fixed.
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Old 12-23-2005, 03:12 AM
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The torque converter has nothing to do with the firmness of the shift. Count your blessings... The harder the shift, the less wear is occuring on the internal clutches of the tranny. I buy expensive shift kits to make my shifts firmer

If you bought an aftermarket vacuum modulator, chances are its an adjustable unit, or calibrated for a bit firmer shifts. If its adjustable, you can change the firmness and shift threshold. It may be different because the tranny is showing some age or because the vacuum modulator is calibrated slightly differently than the original. Either way, drive it. Its shifting, its not slipping, and your part replacement showed a positive change.
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Old 12-23-2005, 08:14 PM
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Are you sure the vacuum modulator determine the firmness of the shifts?
I thought it determines when the trans shift, not the firmness of the shift.
Thanks.
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Old 12-23-2005, 08:19 PM
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Has it always done this?

Maybe a previous owner put a shift kit in it.
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Old 12-24-2005, 02:05 AM
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It has done this since I bought the truck. Now that I put a new vac modulator on which by the way is bigger than the one that was on there but thats the one auto zone says goes on my truck anyways it has always done that but now that I put that new part on when I romp on it it shifts hard into 2nd and sometimes churps the tires. i like it i just didn't want the trans to break because of something, so if it's ok then thats great.
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Old 12-24-2005, 05:23 AM
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Probably has a shift kit.....like was said, better to shift hard and fast than slow and sloppy.
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Old 12-28-2005, 08:42 AM
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The torque converter DOES affect shift quality.

If you are in freezing weather, the modulator could have condensation build up and be frozen. Does the problem get better when the vehicle warms up?
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