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Subject: TH350 vac. modulator..adjust?

Original Message
Name: Shannon
Date: July 24, 1998 at 16:35:50
Subject: TH350 vac. modulator..adjust?
Going on a post from July pt. 1, I tried to adjust the vaccuum modulator on my TH 350 in my 79 Z28.......yes, i realize its not a Chevelle, but It is a bowtie and I do own a Chevelle.....trying to get it to shift at higher R's and firmer. I did read Schurkeys response...and appreciate the tip that this isnt the preferred method of accomplishing what I was trying to do.....BUT...I have a feeling the trans is headed south anyway, so i felt it worth a try. The 1st to 2nd shift is so quick and soft, you dont even realize it sometimes. SO.....i pulled the vaccuum line off, but could not find the adjustment screw.......even up in the nipple. Do some not have the adjustment screw?

The trans is the original 79 unit.....and to my knowlege has never been rebuilt....about 5 years ago...when we put the big block in...we drilled the stock plate for a homemade shift kit and installed a converter but no rebuild. When manually shifting it does fine.....the shifts do seem to be getting softer though. BTW.....the trans has 140 thousand miles on it.....w/ the past 10 thousand or so standing behind a lot of rompin and stompin big block torque........so if it goes it goes.

Is it possible I am overlooking the adjustment screw on the modulator......I poked a small screwdrive up in the nipple, but didnt seem to come in contact w/ anything. Also....is trans fluid in the modulator normal?.......probably not right?.......when I pulled the hose off, some came out......like I said.......140k miles, its due I guess.
Any tips or suggestions?.........are the B&M or TCI street trannys any good......they seem to be priced pretty reasonable........between $500. and $600.....give or take?.......would i be better off having mine built.......I have a spare TH 400......wanna stay w/ the 350 for the lower first gear ratio.
Thanx for suggestions and tips.
Let me know about the missing or hidden adj. screw.
Thanks again

Response Number 1
Name: steve balderson
Date: July 24, 1998 at 17:24:18
Subject: TH350 vac. modulator..adjust?
Sounds like your modulator is shot. In my opinion this could greatly contribute to your shifting problems. Get a new one, with the adjustable feature if so desired. As I recall if you get a 350 trans rebuild manual, or even possibly the factory service manual, you can set the spring tension up to match your engine vacuum to give you the kind of shift you want. I have had luck jacking up the BACK of the car to access this device so it can be R&R without losing a lot of fluid. I did not get dramatic results from adjusting the modulator. I read on post that suggested just unhooking the vacuum at the track for a firmer shift - this seems like a great lowcostnocost idea! Good luck. STEVE

Response Number 2
Name: Jeff
Date: July 24, 1998 at 17:55:41
Subject: TH350 vac. modulator..adjust?
Not all turbo350's had an adjustable vacumm modulator. Even if it does, it only adjust your shift's by about 3 to 5 Mph, one way or another. Doesn't really effect the firmness of the shift.
If I was you I'd rebuild (if needed) the turbo 400. It is designed much better to handle the torque of your big block, and you shouldn't notice much difference with it's higher 1st gear ratio.

Response Number 3
Name: Scooter
Date: July 25, 1998 at 07:29:50
Subject: TH350 vac. modulator..adjust?

There should be NO fluid in the modulator hose! It is sucking tranny fluid into your intake through the diaphragm that is broken and you need a new one. As previuosly stated they did not all come with an adjustable one. How is your trans fluid level? My guess is it would be low, or you've had to add some lately. Change your modulator and you may be ok. You are lucky that thing had held up as long as it has. Good Luck!

Response Number 4
Name: Chris Martin
Date: July 25, 1998 at 08:18:33
Subject: TH350 vac. modulator..adjust?

Flexible shift points can be accomplished by changing or modifying the governor. This takes some and expereince with the unit. I had mine done where the car shifts exactly at 5000RPM @ WOT.


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