# Question regarding the vaccum modulator for the transmission.



## Hoofer814 (Apr 11, 2020)

Hi all. I am wondering about adjusting the transmission vacuum modulator valve as i have to ease off the gas to get the transmission to shift from 2nd to 3rd gear. The engine winds up and won't shift on it's own so at around 40 mph i have to ease off the gas and it will shift. This started last year when i changed the hose from the engine to the modulator as the old one was cracked and had a hole in it causing it not to shift. Do i just need to turn the screw slightly to the right or left and if yes, what direction in order to make it shift on it's own? I hope this makes sense. Thanks for any help you can provide!


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## BLK69JUDGE (Jun 10, 2010)

sounds like the hose is collapsing maybe

are you sure your using a good vacuum hose ??

there should be a 4' metal tube and a 3" hose at the modulator and a couple inch at the motor


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## ylwgto (Oct 27, 2009)

BLK69JUDGE said:


> sounds like the hose is collapsing maybe
> 
> are you sure your using a good vacuum hose ??
> 
> there should be a 4' metal tube and a 3" hose at the modulator and a couple inch at the motor



maybe a collapsing hose is my problem too? although, my issue is shifting way too soon rather than not shifting. no adjustment to the screw has made any difference. this also started when I replaced my modulator, which makes me believe it is not the governor.

I have 3" of rubber from the modulator to the hard line, hard line up to the rear of the passenger valve cover, and rubber to the carb port. Not sure if it is vacuum line...


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## BLK69JUDGE (Jun 10, 2010)

sometimes a 90 degree bend in the hose or less can limit vacuum also ,,,, had that on a chebby truck th350
quality hose,,, the lower hoses at the modulator are available pre formed also...
any signs of tranny fluid at the hose to modulator connection ??
correct vacuum fittings at the carb and location of the vacuum source can effect the shifting also

keep us informed,,,,


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## armyadarkness (Dec 7, 2020)

As ya'll know, I just went through this mess. I bent a nice metal line up and then ran rubber at the ends. However, I also bought a brand new modulator, which failed in 2 days, so unless your spending $12 and trying a new modulator, you might be missing an easy fix.

At $12 and 2 minutes to swap, it's a worthy investment.


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## Hoofer814 (Apr 11, 2020)

Thanks everyone. I will check the length of the lines, that might be the issue. and there may be more of a bend in the hose at the modulator than needed. the preformed at the modulator sounds like a good idea as well as more metal pipe than hose. I'll keep everyone posted.


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## BearGFR (Aug 25, 2008)

FYI, the trans governor controls shift points based on RPM - (actually transmission output shaft RPM), so it's going to be calibrated for wide open throttle. The vacuum modulator "overrides" the governor and causes what amounts to early upshifts based on engine load, which it senses from manifold vacuum. The higher the vacuum, the lower the engine load. 

If you change to a 'hot' cam that lowers vacuum, it can result in higher RPM shift points because the modulator 'thinks' the engine is more heavily loaded than it actually is. 

A bad modulator can also do weird things, as can a low vacuum signal due to a leaking hose, etc.

Bear


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## Hoofer814 (Apr 11, 2020)

Hoofer814 said:


> Hi all. I am wondering about adjusting the transmission vacuum modulator valve as i have to ease off the gas to get the transmission to shift from 2nd to 3rd gear. The engine winds up and won't shift on it's own so at around 40 mph i have to ease off the gas and it will shift. This started last year when i changed the hose from the engine to the modulator as the old one was cracked and had a hole in it causing it not to shift. Do i just need to turn the screw slightly to the right or left and if yes, what direction in order to make it shift on it's own? I hope this makes sense. Thanks for any help you can provide!


Hello again everyone. I wanted to update you on my issue. It's amazing what an $18 part can do and what a difference it can make! I changed out the really old vacuum modulator and the difference was night and day. Super smooth shifts and it shifts on it's own without lifting off of the gas. It's like butter! Thanks for the input and i really appreciate this forum! Have a great week!


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