Transmission Vacuum Modulator

[kdml]

Anyone know a good source for a new transmission vacuum modulator for my '65.  Everything I see online does not look like what I currently have.  I have the original BR transmission. 

 

Thanks

[RivNut]

Your transmission is basically a GM Turbo Hydramatic 400 and I would think the vacuum modulators should be the same for all TH400's.  

 

[Hans3]

You're correct that the "new" vacuum modulators don't look the same--they're physically smaller than the older units.  Rest assured they all function the same AND have an adjustment screw inside the vacuum line port for changing the shift points.  Your local Napa will have the best brick&mortar price, but you can see the full range available new today on Rock Auto.com.  Their price will be more expensive than Napa's after factoring their sky-high shipping charges.

[telriv]

Hans3,

 

    Your original modulator DOES have an adjustment on it.   DON'T DRIVE YOURSELF CRAZY TRYING TO FIND AN ORIGINAL. The smaller one functions just as well or maybe even better than the original.

 

Tom T.

[kdml]

Thanks all

 

i figured the replacements would work.  Just thought it best to confirm first. 

[Bloo]

Modulators have a diaphragm inside that occasionally fails, and should be viewed as consumables like tires, batteries, and hoses. Look for an original only if it's for show, otherwise take Hans3 and telriv's advice and get the new one.

 

Be sure to verify the adjustment. It affects the shift points as well as the internal pressures. I don't have one in front of me, but I believe the shop manual will tell you when it should shift.

[telriv]

With very light throttle the trans. will shift from 1st. to 2nd.  at around 5-8MPH.  2nd. to 3rd. with light throttle should be at around 15-18MPH.  Turn the internal screw out for sooner shifts 1/2 turn at a time. AND in for later shifts  1/2-1 turn at a time.

IF it has 3.42 gears the shifts will be slightly/little sooner.

[EmTee]

Delayed upshifts on a Turbo 400 is your queue to inspect the vacuum plumbing to the modulator.  I've never actually had a modulator fail, but have had several instances where the vacuum hose at one end or the other of the steel line has disintegrated.

Edited July 19, 2020 by EmTee (see edit history)

[Rafz66riv]

On 7/19/2020 at 6:07 AM, EmTee said:

Delayed upshifts on a Turbo 400 is your queue to inspect the vacuum plumbing to the modulator.  I've never actually had a modulator fail, but have had several instances where the vacuum hose at one end or the other of the steel line has disintegrated.

What size is the hose going to the modulator? Mines keeps coming loose and won’t stay connect to the modulator on my 66 riv 

[EmTee]

I believe the hose is 1/4-inch.  Yours sounds like it has disintegrated due to age and exposure to oil.  90° formed hoses are available for a better fit.  My '67 has a metal line with the bend in it and only uses a short, straight piece of hose to join the metal line to the modulator nipple.

 

Example: https://www.monstertransmission.com/TH400-3L80-Hose-Modulator-Elbow-51-Up-MERCH_p_17292.html

 

 

Edited January 8 by EmTee (see edit history)