TH400 trans vacuum modulators - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Robert1970C20rstbukt · 12-14-2004, 12:46 AM

A couple weeks ago I replaced the old, huge vacuum modulator on the TH400 in my truck and now it shifts really fast, I'm talking 1st to 2nd at like right below 10MPH. The old style ones have been discontinued and the replacements are friggin small, like 1/3 the size of the old ones. The bad thing is the shift points on these aren't adjustable, they don't have the screw sticking out of the end. Can the old style ones still be bought somewhere even though they're discontinued? Every dam* place I checked at only has the stupid small ones. The tranny is a '67 out of a Chevy Caprice if that matters.

cableguy0 · 12-14-2004, 01:00 AM

the screw doesnt stick out the end of the modulator its inside the nipple you need a small screwdriver to turn it in.

JimKshortstep4x4 · 12-14-2004, 11:36 AM

If your modulator does not have the little screw in the nipple, then you can install a valve in the vacuum line to adjust the shift point. Here is a pic of what has been done to adjust the vacuum.

Jim

Robert1970C20rstbukt · 12-14-2004, 11:10 PM

Jim I've never seen a valve like that before, where'd you get that? At least it goes to third when I'm on the highway, that's all that matters right now. I did manage to find an old NOS one in my Granddad's garage, at least I'll have an original one when I need it. Thanks for the help cableguy, I'll check if I have a screwdriver small enough and I'll try and see if I can adjust it sometime this week.

Hooter · 12-15-2004, 12:02 AM

hmm technically you could use any valve for that line, even something from a fish tank would work, just needs to adjust the amount of vacum going to the tranny

JimKshortstep4x4 · 12-15-2004, 08:30 AM

Robert,

Hooter is right just about any valve will work. The nice features of the valve shown is that it is small, well made, and it has a locking feature on it. There are several manufacturers of these small valves. They are available at industrial supply stores as they are used for controlling pneumatic cylinders in air assembly machines among other uses.

If you need any other information on them I will be glad to get it for you.

Jim

12-14-2004, 12:46 AM	#1
Robert1970C20rstbukt Registered User Join Date: Jun 2004 Location: Beeville, Texas Posts: 1,943	TH400 trans vacuum modulators A couple weeks ago I replaced the old, huge vacuum modulator on the TH400 in my truck and now it shifts really fast, I'm talking 1st to 2nd at like right below 10MPH. The old style ones have been discontinued and the replacements are friggin small, like 1/3 the size of the old ones. The bad thing is the shift points on these aren't adjustable, they don't have the screw sticking out of the end. Can the old style ones still be bought somewhere even though they're discontinued? Every dam* place I checked at only has the stupid small ones. The tranny is a '67 out of a Chevy Caprice if that matters. __________________ RJD 1984 Chevrolet C30 welding truck 454/SM465 1977 GMC C15 pickup 350/TH350 1970 Chevrolet C20 pickup 350/TH400 1970 Chevrolet K20 Suburban 454/TH350 1969 GMC 8500 478M V6/5 spd winch truck

12-14-2004, 01:00 AM	#2
cableguy0 Senior Member Join Date: Apr 2000 Location: Delta,Pa Posts: 14,950	the screw doesnt stick out the end of the modulator its inside the nipple you need a small screwdriver to turn it in. __________________ Owner of North Point Car Care in Dundalk Md. We specialize in custom exhaust on both modern and classic vehicles. We are a full service auto shop from classics to modern vehicles. Feel free to contact me with questions. I will give a 10% discount to any board member.

12-14-2004, 11:36 AM	#3
JimKshortstep4x4 Senior Member Join Date: Jul 2001 Location: Muskegon,MI,USA Posts: 6,026	If your modulator does not have the little screw in the nipple, then you can install a valve in the vacuum line to adjust the shift point. Here is a pic of what has been done to adjust the vacuum. Jim Attached Images

12-14-2004, 11:10 PM	#4
Robert1970C20rstbukt Registered User Join Date: Jun 2004 Location: Beeville, Texas Posts: 1,943	Jim I've never seen a valve like that before, where'd you get that? At least it goes to third when I'm on the highway, that's all that matters right now. I did manage to find an old NOS one in my Granddad's garage, at least I'll have an original one when I need it. Thanks for the help cableguy, I'll check if I have a screwdriver small enough and I'll try and see if I can adjust it sometime this week. __________________ RJD 1984 Chevrolet C30 welding truck 454/SM465 1977 GMC C15 pickup 350/TH350 1970 Chevrolet C20 pickup 350/TH400 1970 Chevrolet K20 Suburban 454/TH350 1969 GMC 8500 478M V6/5 spd winch truck

12-15-2004, 12:02 AM	#5
Hooter My other Love Join Date: Aug 2001 Location: Castlegar B.C. Canada Posts: 4,085	hmm technically you could use any valve for that line, even something from a fish tank would work, just needs to adjust the amount of vacum going to the tranny __________________ Castlegar B.C.The great white North (Canada Eh!) Hooter_5@hotmail.com First generation Monte Carlo club pictures of my life

12-15-2004, 08:30 AM	#6
JimKshortstep4x4 Senior Member Join Date: Jul 2001 Location: Muskegon,MI,USA Posts: 6,026	Robert, Hooter is right just about any valve will work. The nice features of the valve shown is that it is small, well made, and it has a locking feature on it. There are several manufacturers of these small valves. They are available at industrial supply stores as they are used for controlling pneumatic cylinders in air assembly machines among other uses. If you need any other information on them I will be glad to get it for you. Jim



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